16 лет назад · 03f0f816dc
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -20,6 +20,9 @@ Backward compatibile changes:
 
															   BUGFIXES: 
														
 
															+  ZOOKEEPER-181. Some Source Forge Documents did not get moved over: 
														
 
															+  javaExample, zookeeperTutorial, zookeeperInternals (robbie via phunt)
														
 
															+
														
 
															   ZOOKEEPER-180. Placeholder sections needed in document for new topics that
														
 
															   the umbrella jira discusses (robbie via phunt)
														
--- a/docs/images/2pc.png
+++ b/docs/images/2pc.png
--- a/docs/index.html
+++ b/docs/index.html
@@ -123,6 +123,15 @@ document.write("Last Published: " + document.lastModified);
 
															 <a href="zookeeperAdmin.html">Administrator's Guide</a>
														
 
															 </div>
														
 
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															+<a href="javaExample.html">Java Example</a>
														
 
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															 <a href="api/index.html">API Docs</a>
														
 
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@@ -189,6 +198,15 @@ The following documents provide concepts and procedures to get you started using
 
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															+<a href="javaExample.html">ZooKeeper Java Example</a> - A simple Zookeeer Client, written in Java</li>
														
 
															+      
														
 
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															+<a href="zookeeperTutorial.html">Barrier and Queue Tutorial</a> - Simple Implementations of Barriers and Queues</li>
														
 
															+      
														
 
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															+<a href="zookeeperInternals.html">ZooKeeper Internals</a> - Assorted Topics on the Inner Workings of ZooKeeper</li>
														
 
															+      
														
 
															 <li>
														
 
															 <a href="api/index.html">API Docs</a> - the technical reference to ZooKeeper APIs</li>
														
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+++ b/docs/index.pdf
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															+<h1>ZooKeeper Java Example</h1>
														
 
															+<div id="minitoc-area">
														
 
															+<ul class="minitoc">
														
 
															+<li>
														
 
															+<a href="#ch_Introduction">A Simple Watch Client</a>
														
 
															+<ul class="minitoc">
														
 
															+<li>
														
 
															+<a href="#sc_requirements">Requirements</a>
														
 
															+</li>
														
 
															+<li>
														
 
															+<a href="#sc_design">Program Design</a>
														
 
															+</li>
														
 
															+</ul>
														
 
															+</li>
														
 
															+<li>
														
 
															+<a href="#sc_executor">The Executor Class</a>
														
 
															+</li>
														
 
															+<li>
														
 
															+<a href="#sc_DataMonitor">The DataMonitor Class</a>
														
 
															+</li>
														
 
															+<li>
														
 
															+<a href="#sc_completeSourceCode">Complete Source Listings</a>
														
 
															+</li>
														
 
															+</ul>
														
 
															+</div>
														
 
															+  
														
 
															+
														
 
															+  
														
 
															+
														
 
															+  
														
 
															+<a name="N10009"></a><a name="ch_Introduction"></a>
														
 
															+<h2 class="h3">A Simple Watch Client</h2>
														
 
															+<div class="section">
														
 
															+<p>To introduce you to the ZooKeeper Java API, we develop here a very simple 
														
 
															+    watch client. This ZooKeeper client watches a ZooKeeper node for changes 
														
 
															+    and responds to by starting or stopping a program.</p>
														
 
															+<a name="N10012"></a><a name="sc_requirements"></a>
														
 
															+<h3 class="h4">Requirements</h3>
														
 
															+<p>The client has four requirements:</p>
														
 
															+<ul>
														
 
															+<li>
														
 
															+<p>It takes as parameters:</p>
														
 
															+    	
														
 
															+<ul>
														
 
															+		
														
 
															+<li>
														
 
															+<p>the address of the ZooKeeper service</p>
														
 
															+</li>
														
 
															+		
														
 
															+<li> 
														
 
															+<p> a znode, by name</p>
														
 
															+</li>
														
 
															+		
														
 
															+<li>
														
 
															+<p> an executable with arguments.</p>
														
 
															+</li>
														
 
															+</ul>
														
 
															+</li>
														
 
															+	
														
 
															+<li>
														
 
															+<p>It fetches the data associated with the znode and starts the executable.</p>
														
 
															+</li>
														
 
															+	
														
 
															+<li>
														
 
															+<p>If the znode changes, the client refetches the contents and restarts the executable.</p>
														
 
															+</li>
														
 
															+	
														
 
															+<li>
														
 
															+<p>If the znode disappears, the client kills the executable.</p>
														
 
															+</li>
														
 
															+</ul>
														
 
															+<a name="N1003B"></a><a name="sc_design"></a>
														
 
															+<h3 class="h4">Program Design</h3>
														
 
															+<p>Conventionally, ZooKeeper applications are broken into two units, one which maintains the connection, 
														
 
															+   and the other which monitors data.  In this application, the class called the <strong>Executor</strong> 
														
 
															+   maintains the ZooKeeper connection, and the class called the  <strong>DataMonitor</strong> monitors the data
														
 
															+   in the ZooKeeper tree. Also, Executor contains the main thread and contains the execution logic.
														
 
															+   It is responsible for what little user interaction there is, as well as interaction with the exectuable program you
														
 
															+   pass in as an argument and which the sample (per the requirements) shuts down and restarts, according to the 
														
 
															+   state of the znode.</p>
														
 
															+</div>
														
 
															+
														
 
															+   
														
 
															+<a name="N1004C"></a><a name="sc_executor"></a>
														
 
															+<h2 class="h3">The Executor Class</h2>
														
 
															+<div class="section">
														
 
															+<p>The Executor object is the primary container of the sample application. It contains 
														
 
															+    both the <strong>ZooKeeper</strong> object, <strong>DataMonitor</strong>, as described above in 
														
 
															+    <a href="#sc_design">Program Design</a>.  </p>
														
 
															+<pre class="code">
														
 
															+// from the Executor class...
														
 
															+    
														
 
															+public static void main(String[] args) {
														
 
															+    if (args.length &lt; 4) {
														
 
															+        System.err
														
 
															+                .println("USAGE: Executor hostPort znode filename program [args ...]");
														
 
															+        System.exit(2);
														
 
															+    }
														
 
															+    String hostPort = args[0];
														
 
															+    String znode = args[1];
														
 
															+    String filename = args[2];
														
 
															+    String exec[] = new String[args.length - 3];
														
 
															+    System.arraycopy(args, 3, exec, 0, exec.length);
														
 
															+    try {
														
 
															+        Executor theExectutor = new Executor(hostPort, znode, filename, exec);
														
 
															+	theExectutor.run();
														
 
															+    } catch (Exception e) {
														
 
															+        e.printStackTrace();
														
 
															+    }
														
 
															+}
														
 
															+    
														
 
															+public Executor(String hostPort, String znode, String filename,
														
 
															+    String exec[]) throws KeeperException, IOException {
														
 
															+    this.filename = filename;
														
 
															+    this.exec = exec;
														
 
															+    
														
 
															+    //create a new zookeeper object, passing a self-reference in a the Watcher
														
 
															+    zk = new ZooKeeper(hostPort, 3000, this);
														
 
															+    dm = new DataMonitor(zk, znode, null, this);
														
 
															+}
														
 
															+   
														
 
															+public void run() {
														
 
															+    try {
														
 
															+        synchronized (this) {
														
 
															+            while (!dm.dead) {
														
 
															+                wait();
														
 
															+            }
														
 
															+        }
														
 
															+    } catch (InterruptedException e) {    
														
 
															+    }
														
 
															+}
														
 
															+</pre>
														
 
															+<p>
														
 
															+    Recall that the Executor's job is to starts and stop the executable whose name you pass in on the command line. 
														
 
															+    It does this in response to events fired by the ZooKeeper object. As you can see in the code above, the Executor passes
														
 
															+    a reference to itself as the Watcher argument in the ZooKeeper constructor. It also passes a reference to itself
														
 
															+    as DataMonitorListener argument to the DataMonitor constructor. Per the Executor's definition, it implements both these
														
 
															+    interfaces:
														
 
															+    </p>
														
 
															+<pre class="code">
														
 
															+public class Executor implements Watcher, Runnable, DataMonitorListener {
														
 
															+...
														
 
															+    </pre>
														
 
															+<p>The <strong>Watcher</strong> interface is defined by the ZooKeeper Java API.
														
 
															+    ZooKeeper uses it to communicate back to its container. It supports only one method, <span class="codefrag command">process()</span>, and ZooKeeper uses 
														
 
															+    it to communciates generic events that the main thread would be intersted in, such as the state of the ZooKeeper connection or the ZooKeeper session.The Executor 
														
 
															+    in this example simply forwards those events down to the DataMonitor to decide what to do with them. It does this simply to illustrate
														
 
															+    the point that, by convention, the Executor or some Executor-like object "owns" the ZooKeeper connection, but it is free to delegate the events to other
														
 
															+    events to other objects. It also uses this as the default channel on which to fire watch events. (More on this later.)</p>
														
 
															+<pre class="code">
														
 
															+public void process(WatcherEvent event) {
														
 
															+    dm.process(event);
														
 
															+}
														
 
															+</pre>
														
 
															+<p>The <strong>DataMonitorListener</strong> 
														
 
															+    interface, on the other hand, is not part of the the ZooKeeper API. It is a completely custom interface, 
														
 
															+    designed for this sample application. The DataMonitor object uses it to communicate back to its container, which
														
 
															+    is also the the Executor object.The DataMonitorListener interface looks like this:</p>
														
 
															+<pre class="code">
														
 
															+public interface DataMonitorListener {
														
 
															+    /**
														
 
															+    * The existence status of the node has changed.
														
 
															+    */
														
 
															+    void exists(byte data[]);
														
 
															+
														
 
															+    /**
														
 
															+    * The ZooKeeper session is no longer valid.
														
 
															+    * 
														
 
															+    * @param rc
														
 
															+    * the ZooKeeper reason code
														
 
															+    */
														
 
															+    void closing(int rc);
														
 
															+}
														
 
															+</pre>
														
 
															+<p>This interface is defined in the DataMonitor class and implemented in the Executor class. 
														
 
															+    When <span class="codefrag command">Executor.exists()</span> is invoked,
														
 
															+    the Executor decides whether to start up or shut down per the requirements. Recall that the requires say to kill the executable when the 
														
 
															+    znode ceases to <em>exist</em>. </p>
														
 
															+<p>When <span class="codefrag command">Executor.closing()</span>
														
 
															+    is invoked, the Executor decides whether or not to shut itself down in response to the ZooKeeper connection permanently disappearing.</p>
														
 
															+<p>As you might have guessed, DataMonitor is the object that invokes 
														
 
															+    these methods, in response to changes in ZooKeeper's state.</p>
														
 
															+<p>Here are Executor's implementation of 
														
 
															+    <span class="codefrag command">DataMonitorListener.exists()</span> and <span class="codefrag command">DataMonitorListener.closing</span>:
														
 
															+    </p>
														
 
															+<pre class="code">
														
 
															+public void exists( byte[] data ) {
														
 
															+    if (data == null) {
														
 
															+        if (child != null) {
														
 
															+            System.out.println("Killing process");
														
 
															+            child.destroy();
														
 
															+            try {
														
 
															+                child.waitFor();
														
 
															+            } catch (InterruptedException e) {
														
 
															+            }
														
 
															+        }
														
 
															+        child = null;
														
 
															+    } else {
														
 
															+        if (child != null) {
														
 
															+            System.out.println("Stopping child");
														
 
															+            child.destroy();
														
 
															+            try {
														
 
															+               child.waitFor();
														
 
															+            } catch (InterruptedException e) {
														
 
															+            e.printStackTrace();
														
 
															+            }
														
 
															+        }
														
 
															+        try {
														
 
															+            FileOutputStream fos = new FileOutputStream(filename);
														
 
															+            fos.write(data);
														
 
															+            fos.close();
														
 
															+        } catch (IOException e) {
														
 
															+            e.printStackTrace();
														
 
															+        }
														
 
															+        try {
														
 
															+            System.out.println("Starting child");
														
 
															+            child = Runtime.getRuntime().exec(exec);
														
 
															+            new StreamWriter(child.getInputStream(), System.out);
														
 
															+            new StreamWriter(child.getErrorStream(), System.err);
														
 
															+        } catch (IOException e) {
														
 
															+            e.printStackTrace();
														
 
															+        }
														
 
															+    }
														
 
															+}
														
 
															+
														
 
															+public void closing(int rc) {
														
 
															+    synchronized (this) {
														
 
															+        notifyAll();
														
 
															+    }
														
 
															+}
														
 
															+</pre>
														
 
															+</div>
														
 
															+
														
 
															+<a name="N100A0"></a><a name="sc_DataMonitor"></a>
														
 
															+<h2 class="h3">The DataMonitor Class</h2>
														
 
															+<div class="section">
														
 
															+<p>
														
 
															+The DataMonitor class has the meat of the ZooKeeper logic. It is mostly 
														
 
															+asynchronous and event driven. DataMonitor kicks things off in the constructor with:</p>
														
 
															+<pre class="code">
														
 
															+public DataMonitor(ZooKeeper zk, String znode, Watcher chainedWatcher,
														
 
															+        DataMonitorListener listener) {
														
 
															+    this.zk = zk;
														
 
															+    this.znode = znode;
														
 
															+    this.chainedWatcher = chainedWatcher;
														
 
															+    this.listener = listener;
														
 
															+    
														
 
															+    // Get things started by checking if the node exists. We are going
														
 
															+    // to be completely event driven
														
 
															+    <strong>zk.exists(znode, true, this, null);</strong>
														
 
															+}
														
 
															+</pre>
														
 
															+<p>The call to <span class="codefrag command">ZooKeeper.exists()</span> checks for the existence of the znode, 
														
 
															+sets a watch, and passes a reference to itself (<span class="codefrag command">this</span>)
														
 
															+as the completion callback object. In this sense, it kicks things off, since the
														
 
															+real processing happens when the watch is triggered.</p>
														
 
															+<div class="note">
														
 
															+<div class="label">Note</div>
														
 
															+<div class="content">
														
 
															+
														
 
															+<p>Don't confuse the completion callback with the watch callback. The <span class="codefrag command">ZooKeeper.exists()</span> 
														
 
															+completion callback, which happens to be the method <span class="codefrag command">StatCallback.processResult()</span> implemented 
														
 
															+in the DataMonitor object, is invoked when the asynchronous <em>setting of the watch</em> operation 
														
 
															+(by <span class="codefrag command">ZooKeeper.exists()</span>) completes on the server. </p>
														
 
															+
														
 
															+<p>
														
 
															+The triggering of the watch, on the other hand, sends an event to the <em>Executor</em> object, since
														
 
															+the Executor registered as the Watcher of the ZooKeeper object.</p>
														
 
															+
														
 
															+
														
 
															+<p>As an aside, you might note that the DataMonitor could also register itself as the Watcher
														
 
															+for this particular watch event. This is new to ZooKeeper 3.0.0 (the support of multiple Watchers). In this
														
 
															+example, however, DataMonitor does not register as the Watcher.</p>
														
 
															+
														
 
															+</div>
														
 
															+</div>
														
 
															+<p>When the <span class="codefrag command">ZooKeeper.exists()</span> operation completes on the server, the ZooKeeper API invokes this completion callback on 
														
 
															+the client:</p>
														
 
															+<pre class="code">
														
 
															+public void processResult(int rc, String path, Object ctx, Stat stat) {
														
 
															+    boolean exists;
														
 
															+    switch (rc) {
														
 
															+    case Code.Ok:
														
 
															+        exists = true;
														
 
															+        break;
														
 
															+    case Code.NoNode:
														
 
															+        exists = false;
														
 
															+        break;
														
 
															+    case Code.SessionExpired:
														
 
															+    case Code.NoAuth:
														
 
															+        dead = true;
														
 
															+        listener.closing(rc);
														
 
															+        return;
														
 
															+    default:
														
 
															+        // Retry errors
														
 
															+        zk.exists(znode, true, this, null);
														
 
															+        return;
														
 
															+    }
														
 
															+ 
														
 
															+    byte b[] = null;
														
 
															+    if (exists) {
														
 
															+        try {
														
 
															+            <strong>b = zk.getData(znode, false, null);</strong>
														
 
															+        } catch (KeeperException e) {
														
 
															+            // We don't need to worry about recovering now. The watch
														
 
															+            // callbacks will kick off any exception handling
														
 
															+            e.printStackTrace();
														
 
															+        } catch (InterruptedException e) {
														
 
															+            return;
														
 
															+        }
														
 
															+    }     
														
 
															+    if ((b == null &amp;&amp; b != prevData)
														
 
															+            || (b != null &amp;&amp; !Arrays.equals(prevData, b))) {
														
 
															+        <strong>listener.exists(b);</strong>
														
 
															+        prevData = b;
														
 
															+    }
														
 
															+}
														
 
															+</pre>
														
 
															+<p>
														
 
															+The code first checks the error codes for znode existence, fatal errors, and 
														
 
															+recoverable errors. If the file (or znode) exists, it gets the data from the znode, and 
														
 
															+then invoke the exists() callback of Executor if the state has changed. Note, 
														
 
															+it doesn't have to do any Exception processing for the getData call because it 
														
 
															+has watches pending for anything that could cause an error: if the node is deleted 
														
 
															+before it calls <span class="codefrag command">ZooKeeper.getData()</span>, the watch event set by 
														
 
															+the <span class="codefrag command">ZooKeeper.exists()</span> triggers a callback; 
														
 
															+if there is a communication error, a connection watch event fires when 
														
 
															+the connection comes back up.
														
 
															+</p>
														
 
															+<p>Finally, notice how DataMonitor processes watch events: </p>
														
 
															+<pre class="code">
														
 
															+public void process(WatcherEvent event) {
														
 
															+    String path = event.getPath();
														
 
															+    if (event.getType() == Watcher.Event.EventNone) {
														
 
															+        // We are are being told that the state of the
														
 
															+        // connection has changed
														
 
															+        switch (event.getState()) {
														
 
															+        case Event.KeeperStateSyncConnected:
														
 
															+            // Everything is happy. Lets kick things off
														
 
															+            // again by checking the existence of the znode
														
 
															+            zk.exists(znode, true, this, null);
														
 
															+            break;
														
 
															+        case Event.KeeperStateExpired:
														
 
															+            // It's all over
														
 
															+            dead = true;
														
 
															+            listener.closing(KeeperException.Code.SessionExpired);
														
 
															+            break;
														
 
															+        }
														
 
															+    } else {
														
 
															+        if (path != null &amp;&amp; path.equals(znode)) {
														
 
															+            // Something has changed on the node, let's find out
														
 
															+            zk.exists(znode, true, this, null);
														
 
															+        }
														
 
															+   }
														
 
															+   if (chainedWatcher != null) {
														
 
															+        chainedWatcher.process(event);
														
 
															+   }
														
 
															+}
														
 
															+</pre>
														
 
															+<p>
														
 
															+If the client-side ZooKeeper libraries can reestablish the communication channel to ZooKeeper, DataMonitor simply kicks
														
 
															+everything off again with the call to <span class="codefrag command">ZooKeeper.exists()</span>. 
														
 
															+If it gets an event for a znode, it calls <span class="codefrag command">ZooKeeper.exists()</span> to find out what has changed.
														
 
															+</p>
														
 
															+</div>
														
 
															+
														
 
															+
														
 
															+<a name="N10104"></a><a name="sc_completeSourceCode"></a>
														
 
															+<h2 class="h3">Complete Source Listings</h2>
														
 
															+<div class="section">
														
 
															+<div class="note example">
														
 
															+<div class="label">Executor.java</div>
														
 
															+<div class="content">
														
 
															+<title>Executor.java</title>
														
 
															+<pre class="code">
														
 
															+/**
														
 
															+ * A simple example program to use DataMonitor to start and
														
 
															+ * stop executables based on a znode. The program watches the
														
 
															+ * specified znode and saves the data that corresponds to the
														
 
															+ * znode in the filesystem. It also starts the specified program
														
 
															+ * with the specified arguments when the znode exists and kills
														
 
															+ * the program if the znode goes away.
														
 
															+ */
														
 
															+package com.yahoo.zk.executor;
														
 
															+
														
 
															+import java.io.FileOutputStream;
														
 
															+import java.io.IOException;
														
 
															+import java.io.InputStream;
														
 
															+import java.io.OutputStream;
														
 
															+
														
 
															+import com.yahoo.zk.executor.DataMonitor.DataMonitorListener;
														
 
															+import com.yahoo.zookeeper.KeeperException;
														
 
															+import com.yahoo.zookeeper.Watcher;
														
 
															+import com.yahoo.zookeeper.ZooKeeper;
														
 
															+import com.yahoo.zookeeper.proto.WatcherEvent;
														
 
															+
														
 
															+public class Executor implements Watcher, Runnable, DataMonitorListener {
														
 
															+    String znode;
														
 
															+
														
 
															+    DataMonitor dm;
														
 
															+
														
 
															+    ZooKeeper zk;
														
 
															+
														
 
															+    String filename;
														
 
															+
														
 
															+    String exec[];
														
 
															+
														
 
															+    Process child;
														
 
															+
														
 
															+    public Executor(String hostPort, String znode, String filename,
														
 
															+            String exec[]) throws KeeperException, IOException {
														
 
															+        this.filename = filename;
														
 
															+        this.exec = exec;
														
 
															+        zk = new ZooKeeper(hostPort, 3000, this);
														
 
															+        dm = new DataMonitor(zk, znode, null, this);
														
 
															+    }
														
 
															+
														
 
															+    /**
														
 
															+     * @param args
														
 
															+     */
														
 
															+    public static void main(String[] args) {
														
 
															+        if (args.length &lt; 4) {
														
 
															+            System.err
														
 
															+                    .println("USAGE: Executor hostPort znode filename program [args ...]");
														
 
															+            System.exit(2);
														
 
															+        }
														
 
															+        String hostPort = args[0];
														
 
															+        String znode = args[1];
														
 
															+        String filename = args[2];
														
 
															+        String exec[] = new String[args.length - 3];
														
 
															+        System.arraycopy(args, 3, exec, 0, exec.length);
														
 
															+        try {
														
 
															+            new Executor(hostPort, znode, filename, exec).run();
														
 
															+        } catch (Exception e) {
														
 
															+            e.printStackTrace();
														
 
															+        }
														
 
															+    }
														
 
															+
														
 
															+    /***************************************************************************
														
 
															+     * We do process any events ourselves, we just need to forward them on.
														
 
															+     * 
														
 
															+     * @see com.yahoo.zookeeper.Watcher#process(com.yahoo.zookeeper.proto.WatcherEvent)
														
 
															+     */
														
 
															+    @Override
														
 
															+    public void process(WatcherEvent event) {
														
 
															+        dm.process(event);
														
 
															+    }
														
 
															+
														
 
															+    @Override
														
 
															+    public void run() {
														
 
															+        try {
														
 
															+            synchronized (this) {
														
 
															+                while (!dm.dead) {
														
 
															+                    wait();
														
 
															+                }
														
 
															+            }
														
 
															+        } catch (InterruptedException e) {
														
 
															+        }
														
 
															+    }
														
 
															+
														
 
															+    @Override
														
 
															+    public void closing(int rc) {
														
 
															+        synchronized (this) {
														
 
															+            notifyAll();
														
 
															+        }
														
 
															+    }
														
 
															+
														
 
															+    static class StreamWriter extends Thread {
														
 
															+        OutputStream os;
														
 
															+
														
 
															+        InputStream is;
														
 
															+
														
 
															+        StreamWriter(InputStream is, OutputStream os) {
														
 
															+            this.is = is;
														
 
															+            this.os = os;
														
 
															+            start();
														
 
															+        }
														
 
															+
														
 
															+        public void run() {
														
 
															+            byte b[] = new byte[80];
														
 
															+            int rc;
														
 
															+            try {
														
 
															+                while ((rc = is.read(b)) &gt; 0) {
														
 
															+                    os.write(b, 0, rc);
														
 
															+                }
														
 
															+            } catch (IOException e) {
														
 
															+            }
														
 
															+
														
 
															+        }
														
 
															+    }
														
 
															+
														
 
															+    @Override
														
 
															+    public void exists(byte[] data) {
														
 
															+        if (data == null) {
														
 
															+            if (child != null) {
														
 
															+                System.out.println("Killing process");
														
 
															+                child.destroy();
														
 
															+                try {
														
 
															+                    child.waitFor();
														
 
															+                } catch (InterruptedException e) {
														
 
															+                }
														
 
															+            }
														
 
															+            child = null;
														
 
															+        } else {
														
 
															+            if (child != null) {
														
 
															+                System.out.println("Stopping child");
														
 
															+                child.destroy();
														
 
															+                try {
														
 
															+                    child.waitFor();
														
 
															+                } catch (InterruptedException e) {
														
 
															+                    e.printStackTrace();
														
 
															+                }
														
 
															+            }
														
 
															+            try {
														
 
															+                FileOutputStream fos = new FileOutputStream(filename);
														
 
															+                fos.write(data);
														
 
															+                fos.close();
														
 
															+            } catch (IOException e) {
														
 
															+                e.printStackTrace();
														
 
															+            }
														
 
															+            try {
														
 
															+                System.out.println("Starting child");
														
 
															+                child = Runtime.getRuntime().exec(exec);
														
 
															+                new StreamWriter(child.getInputStream(), System.out);
														
 
															+                new StreamWriter(child.getErrorStream(), System.err);
														
 
															+            } catch (IOException e) {
														
 
															+                e.printStackTrace();
														
 
															+            }
														
 
															+        }
														
 
															+    }
														
 
															+}
														
 
															+</pre>
														
 
															+	
														
 
															+
														
 
															+</div>
														
 
															+</div>
														
 
															+<div class="note example">
														
 
															+<div class="label">DataMonitor.java</div>
														
 
															+<div class="content">
														
 
															+	
														
 
															+<title>DataMonitor.java</title>
														
 
															+	
														
 
															+<pre class="code">
														
 
															+/**
														
 
															+ * A simple class that monitors the data and existence of a ZooKeeper
														
 
															+ * node. It uses asynchronous ZooKeeper APIs.
														
 
															+ */
														
 
															+package com.yahoo.zk.executor;
														
 
															+
														
 
															+import java.util.Arrays;
														
 
															+
														
 
															+import com.yahoo.zookeeper.KeeperException;
														
 
															+import com.yahoo.zookeeper.Watcher;
														
 
															+import com.yahoo.zookeeper.ZooKeeper;
														
 
															+import com.yahoo.zookeeper.AsyncCallback.StatCallback;
														
 
															+import com.yahoo.zookeeper.KeeperException.Code;
														
 
															+import com.yahoo.zookeeper.data.Stat;
														
 
															+import com.yahoo.zookeeper.proto.WatcherEvent;
														
 
															+
														
 
															+public class DataMonitor implements Watcher, StatCallback {
														
 
															+
														
 
															+    ZooKeeper zk;
														
 
															+
														
 
															+    String znode;
														
 
															+
														
 
															+    Watcher chainedWatcher;
														
 
															+
														
 
															+    boolean dead;
														
 
															+
														
 
															+    DataMonitorListener listener;
														
 
															+
														
 
															+    byte prevData[];
														
 
															+
														
 
															+    public DataMonitor(ZooKeeper zk, String znode, Watcher chainedWatcher,
														
 
															+            DataMonitorListener listener) {
														
 
															+        this.zk = zk;
														
 
															+        this.znode = znode;
														
 
															+        this.chainedWatcher = chainedWatcher;
														
 
															+        this.listener = listener;
														
 
															+        // Get things started by checking if the node exists. We are going
														
 
															+        // to be completely event driven
														
 
															+        zk.exists(znode, true, this, null);
														
 
															+    }
														
 
															+
														
 
															+    /**
														
 
															+     * Other classes use the DataMonitor by implementing this method
														
 
															+     */
														
 
															+    public interface DataMonitorListener {
														
 
															+        /**
														
 
															+         * The existence status of the node has changed.
														
 
															+         */
														
 
															+        void exists(byte data[]);
														
 
															+
														
 
															+        /**
														
 
															+         * The ZooKeeper session is no longer valid.
														
 
															+         * 
														
 
															+         * @param rc
														
 
															+         *                the ZooKeeper reason code
														
 
															+         */
														
 
															+        void closing(int rc);
														
 
															+    }
														
 
															+
														
 
															+    @Override
														
 
															+    /**
														
 
															+     * This is a watch event callback. The node we were watching has changed or
														
 
															+     * something happened to our connection to ZooKeeper.
														
 
															+     */
														
 
															+    public void process(WatcherEvent event) {
														
 
															+        String path = event.getPath();
														
 
															+        if (event.getType() == Watcher.Event.EventNone) {
														
 
															+            // We are are being told that the state of the
														
 
															+            // connection has changed
														
 
															+            switch (event.getState()) {
														
 
															+            case Event.KeeperStateSyncConnected:
														
 
															+                // Everything is happy. Lets kick things off
														
 
															+                // again by checking the existence of the znode
														
 
															+                zk.exists(znode, true, this, null);
														
 
															+                break;
														
 
															+            case Event.KeeperStateExpired:
														
 
															+                // It's all over
														
 
															+                dead = true;
														
 
															+                listener.closing(KeeperException.Code.SessionExpired);
														
 
															+                break;
														
 
															+            }
														
 
															+        } else {
														
 
															+            if (path != null &amp;&amp; path.equals(znode)) {
														
 
															+                // Something has changed on the node, let's find out
														
 
															+                zk.exists(znode, true, this, null);
														
 
															+            }
														
 
															+        }
														
 
															+        if (chainedWatcher != null) {
														
 
															+            chainedWatcher.process(event);
														
 
															+        }
														
 
															+    }
														
 
															+
														
 
															+    @Override
														
 
															+    public void processResult(int rc, String path, Object ctx, Stat stat) {
														
 
															+        boolean exists;
														
 
															+        switch (rc) {
														
 
															+        case Code.Ok:
														
 
															+            exists = true;
														
 
															+            break;
														
 
															+        case Code.NoNode:
														
 
															+            exists = false;
														
 
															+            break;
														
 
															+        case Code.SessionExpired:
														
 
															+        case Code.NoAuth:
														
 
															+            dead = true;
														
 
															+            listener.closing(rc);
														
 
															+            return;
														
 
															+        default:
														
 
															+            // Retry errors
														
 
															+            zk.exists(znode, true, this, null);
														
 
															+            return;
														
 
															+        }
														
 
															+
														
 
															+        byte b[] = null;
														
 
															+        if (exists) {
														
 
															+            try {
														
 
															+                b = zk.getData(znode, false, null);
														
 
															+            } catch (KeeperException e) {
														
 
															+                // We don't need to worry about recovering now. The watch
														
 
															+                // callbacks will kick off any exception handling
														
 
															+                e.printStackTrace();
														
 
															+            } catch (InterruptedException e) {
														
 
															+                return;
														
 
															+            }
														
 
															+        }
														
 
															+        if ((b == null &amp;&amp; b != prevData)
														
 
															+                || (b != null &amp;&amp; !Arrays.equals(prevData, b))) {
														
 
															+            listener.exists(b);
														
 
															+            prevData = b;
														
 
															+        }
														
 
															+    }
														
 
															+}
														
 
															+</pre>
														
 
															+
														
 
															+</div>
														
 
															+</div>
														
 
															+</div>
														
 
															+
														
 
															+
														
 
															+
														
 
															+
														
 
															+<p align="right">
														
 
															+<font size="-2"></font>
														
 
															+</p>
														
 
															+</div>
														
 
															+<!--+
														
 
															+    |end content
														
 
															+    +-->
														
 
															+<div class="clearboth">&nbsp;</div>
														
 
															+</div>
														
 
															+<div id="footer">
														
 
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															+    |start bottomstrip
														
 
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															+<div class="lastmodified">
														
 
															+<script type="text/javascript"><!--
														
 
															+document.write("Last Published: " + document.lastModified);
														
 
															+//  --></script>
														
 
															+</div>
														
 
															+<div class="copyright">
														
 
															+        Copyright &copy;
														
 
															+         2008 <a href="http://www.apache.org/licenses/">The Apache Software Foundation.</a>
														
 
															+</div>
														
 
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															+</body>
														
 
															+</html>
														
--- a/docs/javaExample.pdf
+++ b/docs/javaExample.pdf
--- a/docs/linkmap.html
+++ b/docs/linkmap.html
@@ -123,6 +123,15 @@ document.write("Last Published: " + document.lastModified);
 
															 <a href="zookeeperAdmin.html">Administrator's Guide</a>
														
 
															 </div>
														
 
															 <div class="menuitem">
														
 
															+<a href="javaExample.html">Java Example</a>
														
 
															+</div>
														
 
															+<div class="menuitem">
														
 
															+<a href="zookeeperTutorial.html">Barrie and Queue Tutorial</a>
														
 
															+</div>
														
 
															+<div class="menuitem">
														
 
															+<a href="zookeeperInternals.html">ZooKeeper Internals</a>
														
 
															+</div>
														
 
															+<div class="menuitem">
														
 
															 <a href="api/index.html">API Docs</a>
														
 
															 </div>
														
 
															 <div class="menuitem">
														
@@ -210,6 +219,24 @@ document.write("Last Published: " + document.lastModified);
 
															 </li>
														
 
															 </ul>
														
 
															+<ul>
														
 
															+<li>
														
 
															+<a href="javaExample.html">Java Example</a>&nbsp;&nbsp;___________________&nbsp;&nbsp;<em>javaEx</em>
														
 
															+</li>
														
 
															+</ul>
														
 
															+    
														
 
															+<ul>
														
 
															+<li>
														
 
															+<a href="zookeeperTutorial.html">Barrie and Queue Tutorial</a>&nbsp;&nbsp;___________________&nbsp;&nbsp;<em>barTutor</em>
														
 
															+</li>
														
 
															+</ul>
														
 
															+    
														
 
															+<ul>
														
 
															+<li>
														
 
															+<a href="zookeeperInternals.html">ZooKeeper Internals</a>&nbsp;&nbsp;___________________&nbsp;&nbsp;<em>internals</em>
														
 
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															+</ul>
														
 
															+    
														
 
															 <ul>
														
 
															 <li>
														
 
															 <a href="api/index.html">API Docs</a>&nbsp;&nbsp;___________________&nbsp;&nbsp;<em>api</em>
														
--- a/docs/linkmap.pdf
+++ b/docs/linkmap.pdf
@@ -5,10 +5,10 @@
 
															 /Producer (FOP 0.20.5) >>
														
 
															 endobj
														
 
															 5 0 obj
														
 
															-<< /Length 840 /Filter [ /ASCII85Decode /FlateDecode ]
														
 
															+<< /Length 955 /Filter [ /ASCII85Decode /FlateDecode ]
														
 
															  >>
														
 
															 stream
														
 
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--- a/docs/recipes.html
+++ b/docs/recipes.html
@@ -123,6 +123,15 @@ document.write("Last Published: " + document.lastModified);
 
															 <a href="zookeeperAdmin.html">Administrator's Guide</a>
														
 
															 </div>
														
 
															 <div class="menuitem">
														
 
															+<a href="javaExample.html">Java Example</a>
														
 
															+</div>
														
 
															+<div class="menuitem">
														
 
															+<a href="zookeeperTutorial.html">Barrie and Queue Tutorial</a>
														
 
															+</div>
														
 
															+<div class="menuitem">
														
 
															+<a href="zookeeperInternals.html">ZooKeeper Internals</a>
														
 
															+</div>
														
 
															+<div class="menuitem">
														
 
															 <a href="api/index.html">API Docs</a>
														
 
															 </div>
														
 
															 <div class="menuitem">
														
--- a/docs/skin/profile.css
+++ b/docs/skin/profile.css
@@ -156,8 +156,13 @@ a:hover { color:#6587ff}
 
															       font-family: monospace;
														
 
															     }
														
 
															-    .code {
														
 
															-      font-family: "Courier New", Courier, monospace;
														
 
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															+    pre.code {
														
 
															+      margin-left: 0em;
														
 
															+      padding: 0.5em;
														
 
															+      background-color: #f0f0f0;
														
 
															+      font-family: monospace;
														
 
															     }
														
 
															+
														
 
															+
														
 
															+
														
--- a/docs/zookeeperAdmin.html
+++ b/docs/zookeeperAdmin.html
@@ -123,6 +123,15 @@ document.write("Last Published: " + document.lastModified);
 
															 <div class="menupagetitle">Administrator's Guide</div>
														
 
															 </div>
														
 
															 <div class="menuitem">
														
 
															+<a href="javaExample.html">Java Example</a>
														
 
															+</div>
														
 
															+<div class="menuitem">
														
 
															+<a href="zookeeperTutorial.html">Barrie and Queue Tutorial</a>
														
 
															+</div>
														
 
															+<div class="menuitem">
														
 
															+<a href="zookeeperInternals.html">ZooKeeper Internals</a>
														
 
															+</div>
														
 
															+<div class="menuitem">
														
 
															 <a href="api/index.html">API Docs</a>
														
 
															 </div>
														
 
															 <div class="menuitem">
														
@@ -341,7 +350,6 @@ document.write("Last Published: " + document.lastModified);
 
															 <p>Install the ZooKeeper Server Package. It can be downloaded
														
 
															             from:
														
 
															           </p>
														
 
															-
														
 
															 <p>
														
@@ -474,7 +482,7 @@ server.3=zoo3:2888</span>
 
															 <p>The steps to setting up a single server instance are the similar
														
 
															       to the above, except the configuration file is simpler. You can find the
														
 
															       complete instructions in the <a href="zookeeperStarted.html#sc_InstallingSingleMode">Installing and
														
 
															-      Running ZooKeeper in SIngle Server Mode</a> section of the <a href="zookeeperStarted.html">ZooKeeper Getting Started
														
 
															+      Running ZooKeeper in Single Server Mode</a> section of the <a href="zookeeperStarted.html">ZooKeeper Getting Started
														
 
															       Guide</a>.</p>
														
 
															 <p>For information on installing the client side libraries, refer to
														
 
															       the <a href="zookeeperProgrammers.html#Bindings">Bindings</a>
														
@@ -615,11 +623,18 @@ server.3=zoo3:2888</span>
 
															 <p></p>
														
 
															 <a name="N1018A"></a><a name="sc_logging"></a>
														
 
															 <h3 class="h4">Logging</h3>
														
 
															-<p></p>
														
 
															-<a name="N10193"></a><a name="sc_troubleshooting"></a>
														
 
															+<p>ZooKeeper uses <strong>log4j</strong> version 1.2 as 
														
 
															+      its logging infrastructure. The  ZooKeeper default <span class="codefrag filename">log4j.properties</span> 
														
 
															+      file resides in the <span class="codefrag filename">conf</span> directory. Log4j requires that 
														
 
															+      <span class="codefrag filename">log4j.properties</span> either be in the working directory 
														
 
															+      (the directory from which ZooKeeper is run) or be accessible from the classpath.</p>
														
 
															+<p>For more information, see 
														
 
															+      <a href="http://logging.apache.org/log4j/1.2/manual.html#defaultInit">Log4j Default Initialization Procedure</a> 
														
 
															+      of the log4j manual.</p>
														
 
															+<a name="N101AA"></a><a name="sc_troubleshooting"></a>
														
 
															 <h3 class="h4">Troubleshooting</h3>
														
 
															 <p></p>
														
 
															-<a name="N1019C"></a><a name="sc_configuration"></a>
														
 
															+<a name="N101B3"></a><a name="sc_configuration"></a>
														
 
															 <h3 class="h4">Configuration Parameters</h3>
														
 
															 <p>ZooKeeper's behavior is governed by the ZooKeeper configuration
														
 
															       file. This file is designed so that the exact same file can be used by
														
@@ -627,7 +642,7 @@ server.3=zoo3:2888</span>
 
															       layouts are the same. If servers use different configuration files, care
														
 
															       must be taken to ensure that the list of servers in all of the different
														
 
															       configuration files match.</p>
														
 
															-<a name="N101A5"></a><a name="sc_minimumConfiguration"></a>
														
 
															+<a name="N101BC"></a><a name="sc_minimumConfiguration"></a>
														
 
															 <h4>Minimum Configuration</h4>
														
 
															 <p>Here are the minimum configuration keywords that must be defined
														
 
															         in the configuration file:</p>
														
@@ -674,7 +689,7 @@ server.3=zoo3:2888</span>
 
															 </dd>
														
 
															 </dl>
														
 
															-<a name="N101CC"></a><a name="sc_advancedConfiguration"></a>
														
 
															+<a name="N101E3"></a><a name="sc_advancedConfiguration"></a>
														
 
															 <h4>Advanced Configuration</h4>
														
 
															 <p>The configuration settings in the section are optional. You can
														
 
															         use them to further fine tune the behaviour of your ZooKeeper servers.
														
@@ -765,7 +780,7 @@ server.3=zoo3:2888</span>
 
															 </dd>
														
 
															 </dl>
														
 
															-<a name="N1022C"></a><a name="sc_clusterOptions"></a>
														
 
															+<a name="N10243"></a><a name="sc_clusterOptions"></a>
														
 
															 <h4>Cluster Options</h4>
														
 
															 <p>The options in this section are designed for use in quorums --
														
 
															         that is, when deploying clusters of servers.</p>
														
@@ -868,7 +883,7 @@ server.3=zoo3:2888</span>
 
															 </dl>
														
 
															 <p></p>
														
 
															-<a name="N10298"></a><a name="Unsafe+Options"></a>
														
 
															+<a name="N102AF"></a><a name="Unsafe+Options"></a>
														
 
															 <h4>Unsafe Options</h4>
														
 
															 <p>The following options can be useful, but be careful when you use
														
 
															         them. The risk of each is explained along with the explanation of what
														
@@ -913,7 +928,7 @@ server.3=zoo3:2888</span>
 
															 </dd>
														
 
															 </dl>
														
 
															-<a name="N102CA"></a><a name="sc_zkCommands"></a>
														
 
															+<a name="N102E1"></a><a name="sc_zkCommands"></a>
														
 
															 <h3 class="h4">ZooKeeper Commands: The Four Letter Words</h3>
														
 
															 <p>ZooKeeper responds to a small set of commands. Each command is
														
 
															       composed of four letters. You issue the commands to ZooKeeper via telnet
														
@@ -962,7 +977,7 @@ server.3=zoo3:2888</span>
 
															 <pre class="code">$ echo ruok | nc 127.0.0.1 5111
														
 
															 imok
														
 
															 </pre>
														
 
															-<a name="N102FC"></a><a name="sc_dataFileManagement"></a>
														
 
															+<a name="N10313"></a><a name="sc_dataFileManagement"></a>
														
 
															 <h3 class="h4">Data File Management</h3>
														
 
															 <p>ZooKeeper stores its data in a data directory and its transaction
														
 
															       log in a transaction log directory. By default these two directories are
														
@@ -970,7 +985,7 @@ imok
 
															       transaction log files in a separate directory than the data files.
														
 
															       Throughput increases and latency decreases when transaction logs reside
														
 
															       on a dedicated log devices.</p>
														
 
															-<a name="N10305"></a><a name="The+Data+Directory"></a>
														
 
															+<a name="N1031C"></a><a name="The+Data+Directory"></a>
														
 
															 <h4>The Data Directory</h4>
														
 
															 <p>This directory has two files in it:</p>
														
 
															 <ul>
														
@@ -1016,14 +1031,14 @@ imok
 
															         idempotent nature of its updates. By replaying the transaction log
														
 
															         against fuzzy snapshots ZooKeeper gets the state of the system at the
														
 
															         end of the log.</p>
														
 
															-<a name="N10341"></a><a name="The+Log+Directory"></a>
														
 
															+<a name="N10358"></a><a name="The+Log+Directory"></a>
														
 
															 <h4>The Log Directory</h4>
														
 
															 <p>The Log Directory contains the ZooKeeper transaction logs.
														
 
															         Before any update takes place, ZooKeeper ensures that the transaction
														
 
															         that represents the update is written to non-volatile storage. A new
														
 
															         log file is started each time a snapshot is begun. The log file's
														
 
															         suffix is the first zxid written to that log.</p>
														
 
															-<a name="N1034B"></a><a name="File+Management"></a>
														
 
															+<a name="N10362"></a><a name="File+Management"></a>
														
 
															 <h4>File Management</h4>
														
 
															 <p>The format of snapshot and log files does not change between
														
 
															         standalone ZooKeeper servers and different configurations of
														
@@ -1040,7 +1055,7 @@ imok
 
															         needs the latest complete fuzzy snapshot and the log files from the
														
 
															         start of that snapshot. The PurgeTxnLog utility implements a simple
														
 
															         retention policy that administrators can use.</p>
														
 
															-<a name="N1035C"></a><a name="sc_commonProblems"></a>
														
 
															+<a name="N10373"></a><a name="sc_commonProblems"></a>
														
 
															 <h3 class="h4">Things to Avoid</h3>
														
 
															 <p>Here are some common problems you can avoid by configuring
														
 
															       ZooKeeper correctly:</p>
														
@@ -1094,7 +1109,7 @@ imok
 
															 </dd>
														
 
															 </dl>
														
 
															-<a name="N10380"></a><a name="sc_bestPractices"></a>
														
 
															+<a name="N10397"></a><a name="sc_bestPractices"></a>
														
 
															 <h3 class="h4">Best Practices</h3>
														
 
															 <p>For best results, take note of the following list of good
														
 
															       Zookeeper practices. <em>[tbd...]</em>
														
--- a/docs/zookeeperAdmin.pdf
+++ b/docs/zookeeperAdmin.pdf
--- a/docs/zookeeperInternals.html
+++ b/docs/zookeeperInternals.html
@@ -0,0 +1,729 @@
 
															+<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
														
 
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															+<meta name="Forrest-version" content="0.8">
														
 
															+<meta name="Forrest-skin-name" content="pelt">
														
 
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															+<a href="http://www.apache.org/">Apache</a> &gt; <a href="http://hadoop.apache.org/">Hadoop</a> &gt; <a href="http://hadoop.apache.org/zookeeper/">ZooKeeper</a><script src="skin/breadcrumbs.js" language="JavaScript" type="text/javascript"></script>
														
 
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															+<h1>ZooKeeper Internals</h1>
														
 
															+<div id="minitoc-area">
														
 
															+<ul class="minitoc">
														
 
															+<li>
														
 
															+<a href="#ch_Introduction">Introduction</a>
														
 
															+</li>
														
 
															+<li>
														
 
															+<a href="#sc_atomicBroadcast">Atomic Broadcast</a>
														
 
															+<ul class="minitoc">
														
 
															+<li>
														
 
															+<a href="#sc_guaranteesPropertiesDefinitions">Guarantees, Properties, and Definitions</a>
														
 
															+</li>
														
 
															+<li>
														
 
															+<a href="#sc_leaderElection">Leader Activation</a>
														
 
															+</li>
														
 
															+<li>
														
 
															+<a href="#sc_activeMessaging">Active Messaging</a>
														
 
															+</li>
														
 
															+<li>
														
 
															+<a href="#sc_summary">Summary</a>
														
 
															+</li>
														
 
															+<li>
														
 
															+<a href="#sc_comparisons">Comparisons</a>
														
 
															+</li>
														
 
															+</ul>
														
 
															+</li>
														
 
															+<li>
														
 
															+<a href="#sc_logging">Logging</a>
														
 
															+<ul class="minitoc">
														
 
															+<li>
														
 
															+<a href="#sc_developerGuidelines">Developer Guidelines</a>
														
 
															+<ul class="minitoc">
														
 
															+<li>
														
 
															+<a href="#sc_rightLevel">Logging at the Right Level</a>
														
 
															+</li>
														
 
															+<li>
														
 
															+<a href="#sc_log4jIdioms">Use of Standard log4j Idioms</a>
														
 
															+</li>
														
 
															+</ul>
														
 
															+</li>
														
 
															+</ul>
														
 
															+</li>
														
 
															+</ul>
														
 
															+</div>
														
 
															+  
														
 
															+
														
 
															+  
														
 
															+
														
 
															+  
														
 
															+<a name="N10009"></a><a name="ch_Introduction"></a>
														
 
															+<h2 class="h3">Introduction</h2>
														
 
															+<div class="section">
														
 
															+<p>This document contains information on the inner workings of ZooKeeper. 
														
 
															+    So far, it discusses these topics:
														
 
															+    </p>
														
 
															+<ul>    
														
 
															+
														
 
															+<li>
														
 
															+<p>
														
 
															+<a href="#sc_atomicBroadcast">Atomic Broadcast</a>
														
 
															+</p>
														
 
															+</li>
														
 
															+
														
 
															+<li>
														
 
															+<p>
														
 
															+<a href="#sc_logging">Logging</a>
														
 
															+</p>
														
 
															+</li>
														
 
															+
														
 
															+</ul>
														
 
															+</div>
														
 
															+
														
 
															+
														
 
															+<a name="N10022"></a><a name="sc_atomicBroadcast"></a>
														
 
															+<h2 class="h3">Atomic Broadcast</h2>
														
 
															+<div class="section">
														
 
															+<p>
														
 
															+At the heart of ZooKeeper is an atomic messaging system that keeps all of the servers in sync.</p>
														
 
															+<a name="N1002B"></a><a name="sc_guaranteesPropertiesDefinitions"></a>
														
 
															+<h3 class="h4">Guarantees, Properties, and Definitions</h3>
														
 
															+<p>
														
 
															+The specific guarantees provided by the messaging system used by ZooKeeper are the following:</p>
														
 
															+<dl>
														
 
															+
														
 
															+
														
 
															+<dt>
														
 
															+<term>
														
 
															+<em>Reliable delivery</em>
														
 
															+</term>
														
 
															+</dt>
														
 
															+<dd>
														
 
															+<p>If a message, m, is delivered 
														
 
															+by one server, it will be eventually delivered by all servers.</p>
														
 
															+</dd>
														
 
															+
														
 
															+
														
 
															+<dt>
														
 
															+<term>
														
 
															+<em>Total order</em>
														
 
															+</term>
														
 
															+</dt>
														
 
															+<dd>
														
 
															+<p> If a message is 
														
 
															+delivered before message b by one server, a will be delivered before b by all 
														
 
															+servers. If a and b are delivered messages, either a will be delivered before b 
														
 
															+or b will be delivered before a.</p>
														
 
															+</dd>
														
 
															+
														
 
															+
														
 
															+<dt>
														
 
															+<term>
														
 
															+<em>Causal order</em> 
														
 
															+</term>
														
 
															+</dt>
														
 
															+<dd>
														
 
															+<p>
														
 
															+If a message b is sent after a message a has been delivered by the sender of b, 
														
 
															+a must be ordered before b. If a sender sends c after sending b, c must be ordered after b.
														
 
															+</p>
														
 
															+</dd>
														
 
															+
														
 
															+
														
 
															+</dl>
														
 
															+<p>
														
 
															+The ZooKeeper messaging system also needs to be efficient, reliable, and easy to 
														
 
															+implement and maintain. We make heavy use of messaging, so we need the system to 
														
 
															+be able to handle thousands of requests per second. Although we can require at 
														
 
															+least k+1 correct servers to send new messages, we must be able to recover from 
														
 
															+correlated failures such as power outages. When we implemented the system we had 
														
 
															+little time and few engineering resources, so we needed a protocol that is 
														
 
															+accessible to engineers and is easy to implement. We found that our protocol 
														
 
															+satisfied all of these goals.
														
 
															+
														
 
															+</p>
														
 
															+<p>
														
 
															+Our protocol assumes that we can construct point-to-point FIFO channels between 
														
 
															+the servers. While similar services usually assume message delivery that can 
														
 
															+lose or reorder messages, our assumption of FIFO channels is very practical 
														
 
															+given that we use TCP for communication. Specifically we rely on the following property of TCP:</p>
														
 
															+<dl>
														
 
															+
														
 
															+
														
 
															+<dt>
														
 
															+<term>
														
 
															+<em>Ordered delivery</em>
														
 
															+</term>
														
 
															+</dt>
														
 
															+<dd>
														
 
															+<p>Data is delivered in the same order it is sent and a message m is 
														
 
															+delivered only after all messages sent before m have been delivered. 
														
 
															+(The corollary to this is that if message m is lost all messages after m will be lost.)</p>
														
 
															+</dd>
														
 
															+
														
 
															+
														
 
															+<dt>
														
 
															+<term>
														
 
															+<em>No message after close</em>
														
 
															+</term>
														
 
															+</dt>
														
 
															+<dd>
														
 
															+<p>Once a FIFO channel is closed, no messages will be received from it.</p>
														
 
															+</dd>
														
 
															+
														
 
															+
														
 
															+</dl>
														
 
															+<p>
														
 
															+FLP proved that consensus cannot be achieved in asynchronous distributed systems 
														
 
															+if failures are possible. To ensure we achieve consensus in the presence of failures 
														
 
															+we use timeouts. However, we rely on times for liveness not for correctness. So, 
														
 
															+if timeouts stop working (clocks malfunction for example) the messaging system may 
														
 
															+hang, but it will not violate its guarantees.</p>
														
 
															+<p>When describing the ZooKeeper messaging protocol we will talk of packets, 
														
 
															+proposals, and messages:</p>
														
 
															+<dl>
														
 
															+
														
 
															+<dt>
														
 
															+<term>
														
 
															+<em>Packet</em>
														
 
															+</term>
														
 
															+</dt>
														
 
															+<dd>
														
 
															+<p>a sequence of bytes sent through a FIFO channel</p>
														
 
															+</dd>
														
 
															+<dt>
														
 
															+<term>
														
 
															+<em>Proposal</em>
														
 
															+</term>
														
 
															+</dt>
														
 
															+<dd>
														
 
															+<p>a unit of agreement. Proposals are agreed upon by exchanging packets 
														
 
															+with a quorum of ZooKeeper servers. Most proposals contain messages, however the 
														
 
															+NEW_LEADER proposal is an example of a proposal that does not correspond to a message.</p>
														
 
															+</dd>
														
 
															+<dt>
														
 
															+<term>
														
 
															+<em>Message</em>
														
 
															+</term>
														
 
															+</dt>
														
 
															+<dd>
														
 
															+<p>a sequence of bytes to be atomically broadcast to all ZooKeeper 
														
 
															+servers. A message put into a proposal and agreed upon before it is delivered.</p>
														
 
															+</dd>
														
 
															+
														
 
															+
														
 
															+</dl>
														
 
															+<p>
														
 
															+As stated above, ZooKeeper guarantees a total order of messages, and it also 
														
 
															+guarantees a total order of proposals. ZooKeeper exposes the total ordering using
														
 
															+a ZooKeeper transaction id (<em>zxid</em>). All proposals will be stamped with a zxid when 
														
 
															+it is proposed and exactly reflects the total ordering. Proposals are sent to all 
														
 
															+ZooKeeper servers and committed when a quorum of them acknowledge the proposal. 
														
 
															+If a proposal contains a message, the message will be delivered when the proposal 
														
 
															+is committed. Acknowledgement means the server has recorded the proposal to persistent storage. 
														
 
															+Our quorums have the requirement that any pair of quorum must have at least one server 
														
 
															+in common. We ensure this by requiring that all quorums have size (<em>n/2+1</em>) where 
														
 
															+n is the number of servers that make up a ZooKeeper service.
														
 
															+</p>
														
 
															+<p>
														
 
															+The zxid has two parts: the epoch and a counter. In our implementation the zxid 
														
 
															+is a 64-bit number. We use the high order 32-bits for the epoch and the low order 
														
 
															+32-bits for the counter. Because it has two parts represent the zxid both as a 
														
 
															+number and as a pair of integers, (<em>epoch, count</em>). The epoch number represents a 
														
 
															+change in leadership. Each time a new leader comes into power it will have its 
														
 
															+own epoch number. We have a simple algorithm to assign a unique zxid to a proposal: 
														
 
															+the leader simply increments the zxid to obtain a unique zxid for each proposal. 
														
 
															+<em>Leadership activation will ensure that only one leader uses a given epoch, so our 
														
 
															+simple algorithm guarantees that every proposal will have a unique id.</em>
														
 
															+
														
 
															+</p>
														
 
															+<p>
														
 
															+ZooKeeper messaging consists of two phases:</p>
														
 
															+<dl>
														
 
															+
														
 
															+<dt>
														
 
															+<term>
														
 
															+<em>Leader activation</em>
														
 
															+</term>
														
 
															+</dt>
														
 
															+<dd>
														
 
															+<p>In this phase a leader establishes the correct state of the system 
														
 
															+and gets ready to start making proposals.</p>
														
 
															+</dd>
														
 
															+
														
 
															+
														
 
															+<dt>
														
 
															+<term>
														
 
															+<em>Active messaging</em>
														
 
															+</term>
														
 
															+</dt>
														
 
															+<dd>
														
 
															+<p>In this phase a leader accepts messages to propose and coordinates message delivery.</p>
														
 
															+</dd>
														
 
															+
														
 
															+</dl>
														
 
															+<p>
														
 
															+ZooKeeper is a holistic protocol. We do not focus on individual proposals, rather 
														
 
															+look at the stream of proposals as a whole. Our strict ordering allows us to do this 
														
 
															+efficiently and greatly simplifies our protocol. Leadership activation embodies 
														
 
															+this holistic concept. A leader becomes active only when a quorum of followers 
														
 
															+(The leader counts as a follower as well. You can always vote for yourself ) has synced 
														
 
															+up with the leader, they have the same state. This state consists of all of the 
														
 
															+proposals that the leader believes have been committed and the proposal to follow 
														
 
															+the leader, the NEW_LEADER proposal. (Hopefully you are thinking to 
														
 
															+yourself, <em>Does the set of proposals that the leader believes has been committed 
														
 
															+included all the proposals that really have been committed?</em> The answer is <em>yes</em>. 
														
 
															+Below, we make clear why.)
														
 
															+</p>
														
 
															+<a name="N100B9"></a><a name="sc_leaderElection"></a>
														
 
															+<h3 class="h4">Leader Activation</h3>
														
 
															+<p>
														
 
															+Leader activation includes leader election. We currently have two leader election 
														
 
															+algorithms in ZooKeeper: LeaderElection and FastLeaderElection (AuthFastLeaderElection 
														
 
															+is a variant of FastLeaderElection). ZooKeeper messaging doesn't care about the 
														
 
															+exact method of electing a leader has long as the following holds:
														
 
															+</p>
														
 
															+<ul>
														
 
															+
														
 
															+
														
 
															+<li>
														
 
															+<p>The leader has seen the highest zxid of all the followers.</p>
														
 
															+</li>
														
 
															+
														
 
															+<li>
														
 
															+<p>A quorum of servers have committed to following the leader.</p>
														
 
															+</li>
														
 
															+
														
 
															+
														
 
															+</ul>
														
 
															+<p>
														
 
															+Of these two requirements only the first, the highest zxid amoung the followers 
														
 
															+needs to hold for correct operation. The second requirement, a quorum of followers, 
														
 
															+just needs to hold with high probability. We are going to recheck the second requirement, 
														
 
															+so if a failure happens during or after the leader election and quorum is lost, 
														
 
															+we will recover by abandoning leader activation and running another election.
														
 
															+</p>
														
 
															+<p>
														
 
															+After leader election a single server will be designated as a leader and start 
														
 
															+waiting for followers to connect. The rest of the servers will try to connect to 
														
 
															+the leader. The leader will sync up with followers by sending any proposals they 
														
 
															+are missing, or if a follower is missing too many proposals, it will send a full 
														
 
															+snapshot of the state to the follower.
														
 
															+</p>
														
 
															+<p>
														
 
															+There is a corner cases that a follower arrives that has proposals, U, not seen 
														
 
															+by a leader. Proposals are seen in order, so the proposals of U will have a zxids 
														
 
															+higher than zxids seen by the leader. The follower must have arrived after the 
														
 
															+leader election, otherwise the follower would have been elected leader given that 
														
 
															+it has seen a higher zxid. Since committed proposals must be seen by a quorum of 
														
 
															+servers, and a quorum of servers that elected the leader did not see U, the proposals 
														
 
															+of you have not been committed, so they can be discarded. When the follower connects 
														
 
															+to the leader, the leader will tell the follower to discard U.
														
 
															+</p>
														
 
															+<p>
														
 
															+A new leader establishes a zxid to start using for new proposals by getting the 
														
 
															+epoch, e, of the highest zxid it has seen and setting the next zxid to use to be 
														
 
															+(e+1, 0), fter the leader syncs with a follower, it will propose a NEW_LEADER 
														
 
															+proposal. Once the NEW_LEADER proposal has been committed, the leader will activate 
														
 
															+and start receiving and issuing proposals.
														
 
															+</p>
														
 
															+<p>
														
 
															+It all sounds complicated but here are the basic rules of operation during leader 
														
 
															+activation:
														
 
															+</p>
														
 
															+<ul>
														
 
															+
														
 
															+<li>
														
 
															+<p>A follower will ACK the NEW_LEADER proposal after it has synced with the leader.</p>
														
 
															+</li>
														
 
															+
														
 
															+<li>
														
 
															+<p>A follower will only ACK a NEW_LEADER proposal with a given zxid from a single server.</p>
														
 
															+</li>
														
 
															+
														
 
															+<li>
														
 
															+<p>A new leader will COMMIT the NEW_LEADER proposal when a quorum of followers have ACKed it.</p>
														
 
															+</li>
														
 
															+
														
 
															+<li>
														
 
															+<p>A follower will commit any state it received from the leader when the NEW_LEADER proposal is COMMIT.</p>
														
 
															+</li>
														
 
															+
														
 
															+<li>
														
 
															+<p>A new leader will not accept new proposals until the NEW_LEADER proposal has been COMMITED.</p>
														
 
															+</li>
														
 
															+
														
 
															+</ul>
														
 
															+<p>
														
 
															+If leader election terminates erroneously, we don't have a problem since the 
														
 
															+NEW_LEADER proposal will not be committed since the leader will not have quorum. 
														
 
															+When this happens, the leader and any remaining followers will timeout and go back 
														
 
															+to leader election.
														
 
															+</p>
														
 
															+<a name="N100F7"></a><a name="sc_activeMessaging"></a>
														
 
															+<h3 class="h4">Active Messaging</h3>
														
 
															+<p>
														
 
															+Leader Activation does all the heavy lifting. Once the leader is coronated he can 
														
 
															+start blasting out proposals. As long as he remains the leader no other leader can 
														
 
															+emerge since no other leader will be able to get a quorum of followers. If a new 
														
 
															+leader does emerge, 
														
 
															+it means that the leader has lost quorum, and the new leader will clean up any 
														
 
															+mess left over during her leadership activation.
														
 
															+</p>
														
 
															+<p>ZooKeeper messaging operates similar to a classic two-phase commit.</p>
														
 
															+<img alt="" src="images/2pc.png"><p>
														
 
															+All communication channels are FIFO, so everything is done in order. Specifically 
														
 
															+the following operating constraints are observed:</p>
														
 
															+<ul>
														
 
															+
														
 
															+
														
 
															+<li>
														
 
															+<p>The leader sends proposals to all followers using 
														
 
															+the same order. Moreover, this order follows the order in which requests have been 
														
 
															+received. Because we use FIFO channels this means that followers also receive proposals in order.
														
 
															+</p>
														
 
															+</li>
														
 
															+
														
 
															+
														
 
															+<li>
														
 
															+<p>Followers process messages in the order they are received. This 
														
 
															+means that messages will be ACKed in order and the leader will receive ACKs from 
														
 
															+followers in order, due to the FIFO channels. It also means that if message $m$ 
														
 
															+has been written to non-volatile storage, all messages that were proposed before 
														
 
															+$m$ have been written to non-volatile storage.</p>
														
 
															+</li>
														
 
															+
														
 
															+
														
 
															+<li>
														
 
															+<p>The leader will issue a COMMIT to all followers as soon as a 
														
 
															+quorum of followers have ACKed a message. Since messages are ACKed in order, 
														
 
															+COMMITs will be sent by the leader as received by the followers in order.</p>
														
 
															+</li>
														
 
															+
														
 
															+
														
 
															+<li>
														
 
															+<p>COMMITs are processed in order. Followers deliver a proposals 
														
 
															+message when that proposal is committed.</p>
														
 
															+</li>
														
 
															+
														
 
															+
														
 
															+</ul>
														
 
															+<a name="N1011E"></a><a name="sc_summary"></a>
														
 
															+<h3 class="h4">Summary</h3>
														
 
															+<p>So there you go. Why does it work? Specifically, why does is set of proposals 
														
 
															+believed by a new leader always contain any proposal that has actually been committed? 
														
 
															+First, all proposals have a unique zxid, so unlike other protocols, we never have 
														
 
															+to worry about two different values being proposed for the same zxid; followers 
														
 
															+(a leader is also a follower) see and record proposals in order; proposals are 
														
 
															+committed in order; there is only one active leader at a time since followers only 
														
 
															+follow a single leader at a time; a new leader has seen all committed proposals 
														
 
															+from the previous epoch since it has seen the highest zxid from a quorum of servers; 
														
 
															+any uncommited proposals from a previous epoch seen by a new leader will be committed 
														
 
															+by that leader before it becomes active.</p>
														
 
															+<a name="N10127"></a><a name="sc_comparisons"></a>
														
 
															+<h3 class="h4">Comparisons</h3>
														
 
															+<p>
														
 
															+Isn't this just Multi-Paxos? No, Multi-Paxos requires some way of assuring that 
														
 
															+there is only a single coordinator. We do not count on such assurances. Instead 
														
 
															+we use the leader activation to recover from leadership change or old leaders 
														
 
															+believing they are still active.
														
 
															+</p>
														
 
															+<p>
														
 
															+Isn't this just Paxos? Your active messaging phase looks just like phase 2 of Paxos? 
														
 
															+Actually, to us active messaging looks just like 2 phase commit without the need to 
														
 
															+handle aborts. Active messaging is different from both in the sense that it has 
														
 
															+cross proposal ordering requirements. If we do not maintain strict FIFO ordering of 
														
 
															+all packets, it all falls apart. Also, our leader activation phase is different from 
														
 
															+both of them. In particular, our use of epochs allows us to skip blocks of uncommitted
														
 
															+proposals and to not worry about duplicate proposals for a given zxid.
														
 
															+</p>
														
 
															+</div>
														
 
															+
														
 
															+
														
 
															+
														
 
															+<a name="N10134"></a><a name="sc_logging"></a>
														
 
															+<h2 class="h3">Logging</h2>
														
 
															+<div class="section">
														
 
															+<p>
														
 
															+ZooKeeper uses 
														
 
															+<a href="http://logging.apache.org/log4j">log4j</a>
														
 
															+version 1.2 as its logging infrastructure. For information on configuring log4j for
														
 
															+ZooKeeper, see the <a href="zookeeperAdmin.html#sc_logging">Logging</a> section 
														
 
															+of the <a href="zookeeperAdmin.html">ZooKeeper Administrator's Guide.</a>
														
 
															+
														
 
															+</p>
														
 
															+<a name="N10149"></a><a name="sc_developerGuidelines"></a>
														
 
															+<h3 class="h4">Developer Guidelines</h3>
														
 
															+<p>Please follow these guidelines when submitting code. Patch reviewers will look for the following:</p>
														
 
															+<a name="N10151"></a><a name="sc_rightLevel"></a>
														
 
															+<h4>Logging at the Right Level</h4>
														
 
															+<p>
														
 
															+There are <a href="http://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/Level.html#FATAL">6 levels of logging in log4j</a>. 
														
 
															+It's important to pick the right one. In order of higher to lower severity:</p>
														
 
															+<ol>
														
 
															+   
														
 
															+<li>
														
 
															+<p> FATAL level designates very severe error events that will presumably lead the application to abort</p>
														
 
															+</li>
														
 
															+   
														
 
															+<li>
														
 
															+<p>ERROR level designates error events that might still allow the application to continue running.</p>
														
 
															+</li>
														
 
															+   
														
 
															+<li>
														
 
															+<p>WARN level designates potentially harmful situations.</p>
														
 
															+</li>
														
 
															+   
														
 
															+<li>
														
 
															+<p>INFO level designates informational messages that highlight the progress of the application at coarse-grained level.</p>
														
 
															+</li>
														
 
															+   
														
 
															+<li>
														
 
															+<p>EBUG Level designates fine-grained informational events that are most useful to debug an application.</p>
														
 
															+</li>
														
 
															+   
														
 
															+<li>
														
 
															+<p>TRACE Level designates finer-grained informational events than the DEBUG.</p>
														
 
															+</li>
														
 
															+
														
 
															+</ol>
														
 
															+<p>
														
 
															+ZooKeeper is typically run in production such that log messages of INFO level 
														
 
															+severity and higher (more severe) are output to the log.</p>
														
 
															+<a name="N1017C"></a><a name="sc_log4jIdioms"></a>
														
 
															+<h4>Use of Standard log4j Idioms</h4>
														
 
															+<p>
														
 
															+<em>Static Message Logging</em>
														
 
															+</p>
														
 
															+<pre class="code">
														
 
															+LOG.debug("process completed successfully!");
														
 
															+</pre>
														
 
															+<p>However when creating a message from a number of components (string 
														
 
															+concatenation), the log call should be wrapped with a "isXEnabled()" call. this 
														
 
															+eliminates the string concatenation overhead when debug level logging is not enabled.
														
 
															+</p>
														
 
															+<pre class="code">
														
 
															+if (LOG.isDebugEnabled()) {
														
 
															+    LOG.debug("got " + count + " messages in " + time + " minutes");
														
 
															+}
														
 
															+</pre>
														
 
															+<p>
														
 
															+<em>Naming</em>
														
 
															+</p>
														
 
															+<p>
														
 
															+Loggers should be named after the class in which they are used. (See the 
														
 
															+<a href="http://logging.apache.org/log4j/1.2/faq.html#2.4">log4j faq</a> 
														
 
															+for reasons why this is a good idea.)
														
 
															+</p>
														
 
															+<pre class="code">
														
 
															+public class Foo {
														
 
															+    private static final Logger LOG = Logger.getLogger(Foo.class);
														
 
															+    ....
														
 
															+    public Foo() {
														
 
															+       LOG.info("constructing Foo");
														
 
															+</pre>
														
 
															+<p>
														
 
															+<em>Exception handling</em>
														
 
															+</p>
														
 
															+<pre class="code">
														
 
															+try {
														
 
															+  // code
														
 
															+} catch (XYZException e) {
														
 
															+  // do this
														
 
															+  LOG.error("Something bad happened", e);
														
 
															+  // don't do this (generally)
														
 
															+  // LOG.error(e);
														
 
															+  // why? because "don't do" case hides the stack trace
														
 
															+ 
														
 
															+  // continue process here as you need... recover or (re)throw
														
 
															+}
														
 
															+</pre>
														
 
															+</div>
														
 
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--- a/docs/zookeeperInternals.pdf
+++ b/docs/zookeeperInternals.pdf
--- a/docs/zookeeperOtherInfo.html
+++ b/docs/zookeeperOtherInfo.html
@@ -123,6 +123,15 @@ document.write("Last Published: " + document.lastModified);
 
															 <a href="zookeeperAdmin.html">Administrator's Guide</a>
														
 
															 </div>
														
 
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															 <a href="api/index.html">API Docs</a>
														
 
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--- a/docs/zookeeperOver.html
+++ b/docs/zookeeperOver.html
@@ -123,6 +123,15 @@ document.write("Last Published: " + document.lastModified);
 
															 <a href="zookeeperAdmin.html">Administrator's Guide</a>
														
 
															 </div>
														
 
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															 <a href="api/index.html">API Docs</a>
														
 
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--- a/docs/zookeeperProgrammers.html
+++ b/docs/zookeeperProgrammers.html
@@ -123,6 +123,15 @@ document.write("Last Published: " + document.lastModified);
 
															 <a href="zookeeperAdmin.html">Administrator's Guide</a>
														
 
															 </div>
														
 
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															+<a href="javaExample.html">Java Example</a>
														
 
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															+<a href="zookeeperTutorial.html">Barrie and Queue Tutorial</a>
														
 
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															+<a href="zookeeperInternals.html">ZooKeeper Internals</a>
														
 
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															 <a href="api/index.html">API Docs</a>
														
 
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--- a/docs/zookeeperStarted.html
+++ b/docs/zookeeperStarted.html
@@ -123,6 +123,15 @@ document.write("Last Published: " + document.lastModified);
 
															 <a href="zookeeperAdmin.html">Administrator's Guide</a>
														
 
															 </div>
														
 
															 <div class="menuitem">
														
 
															+<a href="javaExample.html">Java Example</a>
														
 
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															+<a href="zookeeperTutorial.html">Barrie and Queue Tutorial</a>
														
 
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															+<a href="zookeeperInternals.html">ZooKeeper Internals</a>
														
 
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															 <a href="api/index.html">API Docs</a>
														
 
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@@ -216,18 +225,17 @@ document.write("Last Published: " + document.lastModified);
 
															 </div>
														
 
															 </div>
														
 
															 <p>Once you have downloaded the ZooKeeper source, cd to the root of
														
 
															-      your ZooKeeper source, and run "ant jar". For example:<span class="codefrag computeroutput">$ cd ~/dev/zookeeper
														
 
															-
														
 
															-$ ~/dev/zookeeper/: ant jar</span>
														
 
															-</p>
														
 
															+      your ZooKeeper source, and run "ant jar". For example:</p>
														
 
															+<pre class="code">
														
 
															+$ cd ~/dev/zookeeper
														
 
															+$ ~/dev/zookeeper/: ant jar
														
 
															+</pre>
														
 
															 <p>This should generate a JAR file called zookeeper.jar. To start
														
 
															-      Zookeeper, compile and run zookeeper.jar. <em>[tbd, some more
														
 
															-      instruction here. Perhaps a command line? Are these two steps or
														
 
															-      one?]</em>
														
 
															-</p>
														
 
															+      Zookeeper, compile and run zookeeper.jar.</p>
														
 
															 <p>To start ZooKeeper you need a configuration file. Here is a sample
														
 
															       file:</p>
														
 
															-<pre class="code">tickTime=2000
														
 
															+<pre class="code">
														
 
															+tickTime=2000
														
 
															 dataDir=/var/zookeeper/ 
														
 
															 clientPort=2181
														
 
															 </pre>
														
@@ -274,9 +282,7 @@ clientPort=2181
 
															 </dl>
														
 
															 <p>Now that you created the configuration file, you can start
														
 
															       ZooKeeper:</p>
														
 
															-<p>
														
 
															-<span class="codefrag computeroutput">java -cp zookeeper-dev.jar:java/lib/log4j-1.2.15.jar:conf org.apache.zookeeper.server.quorum.QuorumPeerMain zoo.cfg</span>
														
 
															-</p>
														
 
															+<pre class="code">java -cp zookeeper-dev.jar:java/lib/log4j-1.2.15.jar:conf org.apache.zookeeper.server.quorum.QuorumPeerMain zoo.cfg</pre>
														
 
															 <p>ZooKeeper logs messages using log4j -- more detail available in
														
 
															       the <a href="zookeeperProgrammers.html#Logging">Logging</a>
														
 
															       section of the Programmer's Guide.<em>[tbd:
														
@@ -288,7 +294,7 @@ clientPort=2181
 
															       replicated mode, please see <a href="#sc_RunningReplicatedZooKeeper">Running Replicated
														
 
															       Zookeeper</a>.</p>
														
 
															 <p></p>
														
 
															-<a name="N10076"></a><a name="sc_ConnectingToZooKeeper"></a>
														
 
															+<a name="N10072"></a><a name="sc_ConnectingToZooKeeper"></a>
														
 
															 <h3 class="h4">Connecting to ZooKeeper</h3>
														
 
															 <p>Once ZooKeeper is running, you have several option for connection
														
 
															       to it:</p>
														
@@ -312,7 +318,7 @@ clientPort=2181
 
															 <p>
														
 
															 <strong>C</strong>: compile cli_mt
														
 
															           (multi-threaded) or cli_st (single-threaded) by running
														
 
															-          <span class="codefrag command">_make cli_mt_</span> or <span class="codefrag command">_make cli_st_</span>
														
 
															+          <span class="codefrag command">make cli_mt</span> or <span class="codefrag command">make cli_st</span>
														
 
															           in the c subdirectory in the ZooKeeper sources.</p>
														
@@ -324,7 +330,7 @@ clientPort=2181
 
															 </li>
														
 
															 </ul>
														
 
															-<a name="N100A7"></a><a name="sc_ProgrammingToZooKeeper"></a>
														
 
															+<a name="N100A3"></a><a name="sc_ProgrammingToZooKeeper"></a>
														
 
															 <h3 class="h4">Programming to ZooKeeper</h3>
														
 
															 <p>ZooKeeper has a Java bindings and C bindings. They are
														
 
															       functionally equivalent. The C bindings exist in two variants: single
														
@@ -332,7 +338,7 @@ clientPort=2181
 
															       is done. For more information, see the <a href="zookeeperProgrammers.html#ch_programStructureWithExample.html">Programming
														
 
															       Examples in the Zookeeper Programmer's Guide</a> for
														
 
															       sample code using of the different APIs.</p>
														
 
															-<a name="N100B5"></a><a name="sc_RunningReplicatedZooKeeper"></a>
														
 
															+<a name="N100B1"></a><a name="sc_RunningReplicatedZooKeeper"></a>
														
 
															 <h3 class="h4">Running Replicated ZooKeeper</h3>
														
 
															 <p>Running ZooKeeper in standalone mode is convenient for evaluation,
														
 
															       some development, and testing. But in production, you should run
														
@@ -341,15 +347,16 @@ clientPort=2181
 
															       mode, all servers in the quorum have copies of the same configuration
														
 
															       file. The file is similar to the one used in standalone mode, but with a
														
 
															       few differences. Here is an example:</p>
														
 
															-<p>
														
 
															-<span class="codefrag computeroutput">tickTime=2000 
														
 
															+<pre class="code">
														
 
															+tickTime=2000 
														
 
															 dataDir=/var/zookeeper/ 
														
 
															 clientPort=2181 
														
 
															 initLimit=5 
														
 
															 syncLimit=2 
														
 
															-server.1=zoo1:2888 server.2=zoo2:2888 
														
 
															-server.3=zoo3:2888 </span>
														
 
															-</p>
														
 
															+server.1=zoo1:2888
														
 
															+server.2=zoo2:2888 
														
 
															+server.3=zoo3:2888 
														
 
															+</pre>
														
 
															 <p>The new entry, <strong>initLimit</strong> is
														
 
															       timeouts ZooKeeper uses to limit the length of time the Zookeeper
														
 
															       servers in quorum have to connect to a leader. The entry <strong>syncLimit</strong> limits how far out of date a server can
														
@@ -377,7 +384,7 @@ server.3=zoo3:2888 </span>
 
															 </div>
														
 
															 </div>
														
 
															-<a name="N100EC"></a><a name="Other+Optimizations"></a>
														
 
															+<a name="N100E6"></a><a name="Other+Optimizations"></a>
														
 
															 <h3 class="h4">Other Optimizations</h3>
														
 
															 <p>There are a couple of other configuration parameters that can
														
 
															       greatly increase performance:</p>
														
--- a/docs/zookeeperStarted.pdf
+++ b/docs/zookeeperStarted.pdf
--- a/docs/zookeeperTutorial.html
+++ b/docs/zookeeperTutorial.html
@@ -0,0 +1,857 @@
 
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															+<a class="dida" href="zookeeperTutorial.pdf"><img alt="PDF -icon" src="skin/images/pdfdoc.gif" class="skin"><br>
														
 
															+        PDF</a>
														
 
															+</div>
														
 
															+<h1>Programming with ZooKeeper - A basic tutorial</h1>
														
 
															+<div id="minitoc-area">
														
 
															+<ul class="minitoc">
														
 
															+<li>
														
 
															+<a href="#ch_Introduction">Introduction</a>
														
 
															+</li>
														
 
															+<li>
														
 
															+<a href="#sc_barriers">Barriers</a>
														
 
															+</li>
														
 
															+<li>
														
 
															+<a href="#sc_producerConsumerQueues">Producer-Consumer Queues</a>
														
 
															+</li>
														
 
															+<li>
														
 
															+<a href="#sc_sourceListing">Complete Source Listing</a>
														
 
															+</li>
														
 
															+</ul>
														
 
															+</div>
														
 
															+  
														
 
															+
														
 
															+  
														
 
															+
														
 
															+  
														
 
															+<a name="N10009"></a><a name="ch_Introduction"></a>
														
 
															+<h2 class="h3">Introduction</h2>
														
 
															+<div class="section">
														
 
															+<p>In this tutorial, we show simple implementations of barriers and 
														
 
															+    producer-consumer queues using ZooKeeper. We call the respective classes Barrier and Queue. 
														
 
															+    These examples assume that you have at least one ZooKeeper server running.</p>
														
 
															+<p>Both primitives use the following common excerpt of code:</p>
														
 
															+<pre class="code">
														
 
															+static ZooKeeper zk = null;
														
 
															+static Integer mutex;
														
 
															+
														
 
															+String root;
														
 
															+
														
 
															+SyncPrimitive(String address) {
														
 
															+	if(zk == null){
														
 
															+	    try {
														
 
															+		System.out.println("Starting ZK:");
														
 
															+		zk = new ZooKeeper(address, 3000, this);
														
 
															+		mutex = new Integer(-1);
														
 
															+		System.out.println("Finished starting ZK: " + zk);
														
 
															+	    } catch (KeeperException e) {
														
 
															+		System.out.println("Keeper exception when starting new session: "
														
 
															+			+ e.toString());
														
 
															+		zk = null;
														
 
															+	    } catch (IOException e) {
														
 
															+		System.out.println(e.toString());
														
 
															+		zk = null;
														
 
															+	    }
														
 
															+	}
														
 
															+}
														
 
															+
														
 
															+synchronized public void process(WatcherEvent event) {
														
 
															+	synchronized (mutex) {
														
 
															+	    mutex.notify();
														
 
															+	}
														
 
															+}
														
 
															+
														
 
															+</pre>
														
 
															+<p>Both classes extend SyncPrimitive. In this way, we execute steps that are 
														
 
															+common to all primitives in the constructor of SyncPrimitive. To keep the examples 
														
 
															+simple, we create a ZooKeeper object the first time we instantiate either a barrier 
														
 
															+object or a queue object, and we declare a static variable that is a reference 
														
 
															+to this object. The subsequent instances of Barrier and Queue check whether a 
														
 
															+ZooKeeper object exists. Alternatively, we could have the application creating a
														
 
															+ZooKeeper object and passing it to the constructor of Barrier and Queue.</p>
														
 
															+<p>
														
 
															+We use the process() method to process notifications triggered due to watches. 
														
 
															+In the following discussion, we present code that sets watches. A watch is internal 
														
 
															+structure that enables ZooKeeper to notify a client of a change to a node. For example, 
														
 
															+if a client is waiting for other clients to leave a barrier, then it can set a watch and 
														
 
															+wait for modifications to a particular node, which can indicate that it is the end of the wait. 
														
 
															+This point becomes clear once we go over the examples.
														
 
															+</p>
														
 
															+</div>
														
 
															+   
														
 
															+ 
														
 
															+<a name="N1001F"></a><a name="sc_barriers"></a>
														
 
															+<h2 class="h3">Barriers</h2>
														
 
															+<div class="section">
														
 
															+<p>
														
 
															+ A barrier is a primitive that enables a group of processes to synchronize the 
														
 
															+ beginning and the end of a computation. The general idea of this implementation 
														
 
															+ is to have a barrier node that serves the purpose of being a parent for individual 
														
 
															+ process nodes. Suppose that we call the barrier node "/b1". Each process "p" then 
														
 
															+ creates a node "/b1/p". Once enough processes have created their corresponding 
														
 
															+ nodes, joined processes can start the computation.
														
 
															+ </p>
														
 
															+<p>In this example, each process instantiates a Barrier object, and its constructor takes as parameters:</p>
														
 
															+<ul>
														
 
															+<li>
														
 
															+<p>the address of a ZooKeeper server (e.g., "zoo1.foo.com:2181")</p>
														
 
															+</li>
														
 
															+
														
 
															+<li>
														
 
															+<p>the path of the barrier node on ZooKeeper (e.g., "/b1")</p>
														
 
															+</li>
														
 
															+
														
 
															+<li>
														
 
															+<p>the size of the group of processes</p>
														
 
															+</li>
														
 
															+
														
 
															+</ul>
														
 
															+<p>The constructor of Barrier passes the address of the Zookeeper server to the 
														
 
															+constructor of the parent class. The parent class creates a ZooKeeper instance if 
														
 
															+one does not exist. The constructor of Barrier then creates a 
														
 
															+barrier node on ZooKeeper, which is the parent node of all process nodes, and 
														
 
															+we call root (<strong>Note:</strong> This is not the ZooKeeper root "/").</p>
														
 
															+<pre class="code">
														
 
															+ /**
														
 
															+ * Barrier constructor
														
 
															+ *
														
 
															+ * @param address
														
 
															+ * @param name
														
 
															+ * @param size
														
 
															+ */
														
 
															+Barrier(String address, String name, int size) {
														
 
															+    super(address);
														
 
															+    this.root = name;
														
 
															+    this.size = size;
														
 
															+
														
 
															+    // Create barrier node
														
 
															+    if (zk != null) {
														
 
															+	try {
														
 
															+	    Stat s = zk.exists(root, false);
														
 
															+	    if (s == null) {
														
 
															+		zk.create(root, new byte[0], Ids.OPEN_ACL_UNSAFE, 0);
														
 
															+	    }
														
 
															+	} catch (KeeperException e) {
														
 
															+	    System.out.println("Keeper exception when instantiating queue: " + e.toString());
														
 
															+	} catch (InterruptedException e) {
														
 
															+	    System.out.println("Interrupted exception");
														
 
															+	}
														
 
															+    }
														
 
															+
														
 
															+    // My node name
														
 
															+    try {
														
 
															+	name = new String(InetAddress.getLocalHost().getCanonicalHostName().toString());
														
 
															+    } catch (UnknownHostException e) {
														
 
															+	System.out.println(e.toString());
														
 
															+    }
														
 
															+
														
 
															+}
														
 
															+</pre>
														
 
															+<p>
														
 
															+To enter the barrier, a process calls enter(). The process creates a node under 
														
 
															+the root to represent it, using its host name to form the node name. It then wait 
														
 
															+until enough processes have entered the barrier. A process does it by checking 
														
 
															+the number of children the root node has with "getChildren()", and waiting for 
														
 
															+notifications in the case it does not have enough. To receive a notification when 
														
 
															+there is a change to the root node, a process has to set a watch, and does it 
														
 
															+through the call to "getChildren()". In the code, we have that "getChildren()" 
														
 
															+has two parameters. The first one states the node to read from, and the second is
														
 
															+a boolean flag that enables the process to set a watch. In the code the flag is true.
														
 
															+</p>
														
 
															+<pre class="code">
														
 
															+ /**
														
 
															+ * Join barrier
														
 
															+ *
														
 
															+ * @return
														
 
															+ * @throws KeeperException
														
 
															+ * @throws InterruptedException
														
 
															+ */
														
 
															+
														
 
															+boolean enter() throws KeeperException, InterruptedException{
														
 
															+    zk.create(root + "/" + name, new byte[0], Ids.OPEN_ACL_UNSAFE,
														
 
															+	    CreateFlags.EPHEMERAL);
														
 
															+    while (true) {
														
 
															+	synchronized (mutex) {
														
 
															+	    ArrayList&lt;String&gt; list = zk.getChildren(root, true);
														
 
															+
														
 
															+	    if (list.size() &lt; size) {
														
 
															+		mutex.wait();
														
 
															+	    } else {
														
 
															+		return true;
														
 
															+	    }
														
 
															+	}
														
 
															+    }
														
 
															+}
														
 
															+</pre>
														
 
															+<p>
														
 
															+Note that enter() throws both KeeperException and InterruptedException, so it is 
														
 
															+the reponsability of the application to catch and handle such exceptions.</p>
														
 
															+<p>
														
 
															+Once the computation is finished, a process calls leave() to leave the barrier. 
														
 
															+First it deletes its corresponding node, and then it gets the children of the root 
														
 
															+node. If there is at least one child, then it waits for a notification (obs: note 
														
 
															+that the second parameter of the call to getChildren() is true, meaning that 
														
 
															+ZooKeeper has to set a watch on the the root node). Upon reception of a notification, 
														
 
															+it checks once more whether the root node has any child.</p>
														
 
															+<pre class="code">
														
 
															+ /**
														
 
															+ * Wait until all reach barrier
														
 
															+ *
														
 
															+ * @return
														
 
															+ * @throws KeeperException
														
 
															+ * @throws InterruptedException
														
 
															+ */
														
 
															+
														
 
															+boolean leave() throws KeeperException, InterruptedException{
														
 
															+    zk.delete(root + "/" + name, 0);
														
 
															+    while (true) {
														
 
															+	synchronized (mutex) {
														
 
															+	    ArrayList&lt;String&gt; list = zk.getChildren(root, true);
														
 
															+		if (list.size() &gt; 0) {
														
 
															+		    mutex.wait();
														
 
															+		} else {
														
 
															+		    return true;
														
 
															+		}
														
 
															+	    }
														
 
															+	}
														
 
															+}
														
 
															+</pre>
														
 
															+</div>
														
 
															+
														
 
															+<a name="N10051"></a><a name="sc_producerConsumerQueues"></a>
														
 
															+<h2 class="h3">Producer-Consumer Queues</h2>
														
 
															+<div class="section">
														
 
															+<p>
														
 
															+A producer-consumer queue is a distributed data estructure thata group of processes 
														
 
															+use to generate and consume items. Producer processes create new elements and add 
														
 
															+them to the queue. Consumer processes remove elements from the list, and process them. 
														
 
															+In this implementation, the elements are simple integers. The queue is represented 
														
 
															+by a root node, and to add an element to the queue, a producer process creates a new node, 
														
 
															+a child of the root node.
														
 
															+</p>
														
 
															+<p>
														
 
															+The following excerpt of code corresponds to the constructor of the object. As 
														
 
															+with Barrier objects, it first calls the constructor of the parent class, SyncPrimitive, 
														
 
															+that creates a ZooKeeper object if one doesn't exist. It then verifies if the root 
														
 
															+node of the queue exists, and creates if it doesn't.
														
 
															+</p>
														
 
															+<pre class="code">
														
 
															+/**
														
 
															+ * Constructor of producer-consumer queue
														
 
															+ *
														
 
															+ * @param address
														
 
															+ * @param name
														
 
															+ */
														
 
															+Queue(String address, String name) {
														
 
															+    super(address);
														
 
															+    this.root = name;
														
 
															+    // Create ZK node name
														
 
															+    if (zk != null) {
														
 
															+	try {
														
 
															+	    Stat s = zk.exists(root, false);
														
 
															+	    if (s == null) {
														
 
															+		zk.create(root, new byte[0], Ids.OPEN_ACL_UNSAFE, 0);
														
 
															+	    }
														
 
															+	} catch (KeeperException e) {
														
 
															+	    System.out
														
 
															+		    .println("Keeper exception when instantiating queue: "
														
 
															+			    + e.toString());
														
 
															+	} catch (InterruptedException e) {
														
 
															+	    System.out.println("Interrupted exception");
														
 
															+	}
														
 
															+    }
														
 
															+}
														
 
															+ </pre>
														
 
															+<p>
														
 
															+A producer process calls "produce()" to add an element to the queue, and passes 
														
 
															+an integer as an argument. To add an element to the queue, the method creates a 
														
 
															+new node using "create()", and uses the SEQUENCE flag to instruct ZooKeeper to 
														
 
															+append the value of the sequencer counter associated to the root node. In this way, 
														
 
															+we impose a total order on the elements of the queue, thus guaranteeing that the 
														
 
															+oldest element of the queue is the next one consumed.
														
 
															+</p>
														
 
															+<pre class="code">
														
 
															+/**
														
 
															+ * Add element to the queue.
														
 
															+ *
														
 
															+ * @param i
														
 
															+ * @return
														
 
															+ */
														
 
															+
														
 
															+boolean produce(int i) throws KeeperException, InterruptedException{
														
 
															+    ByteBuffer b = ByteBuffer.allocate(4);
														
 
															+    byte[] value;
														
 
															+
														
 
															+    // Add child with value i
														
 
															+    b.putInt(i);
														
 
															+    value = b.array();
														
 
															+    zk.create(root + "/element", value, Ids.OPEN_ACL_UNSAFE,
														
 
															+		CreateFlags.SEQUENCE);
														
 
															+
														
 
															+    return true;
														
 
															+}
														
 
															+</pre>
														
 
															+<p>
														
 
															+To consume an element, a consumer process obtains the children of the root node, 
														
 
															+reads the node with smallest counter value, and returns the element. Note that 
														
 
															+if there is a conflict, then one of the two contending processes won't be able to 
														
 
															+delete the node and the delete operation will throw an exception.</p>
														
 
															+<p>
														
 
															+A call to getChildren() returns the list of children in lexicographic order. 
														
 
															+As lexicographic order does not necessary follow the numerical order of the counter 
														
 
															+values, we need to decide which element is the smallest. To decide which one has 
														
 
															+the smallest counter value, we traverse the list, and remove the prefix "element" 
														
 
															+from each one.</p>
														
 
															+<pre class="code">
														
 
															+/**
														
 
															+ * Remove first element from the queue.
														
 
															+ *
														
 
															+ * @return
														
 
															+ * @throws KeeperException
														
 
															+ * @throws InterruptedException
														
 
															+ */
														
 
															+int consume() throws KeeperException, InterruptedException{
														
 
															+    int retvalue = -1;
														
 
															+    Stat stat = null;
														
 
															+
														
 
															+    // Get the first element available
														
 
															+    while (true) {
														
 
															+	synchronized (mutex) {
														
 
															+	    ArrayList&lt;String&gt; list = zk.getChildren(root, true);
														
 
															+	    if (list.size() == 0) {
														
 
															+		System.out.println("Going to wait");
														
 
															+		mutex.wait();
														
 
															+	    } else {
														
 
															+		Integer min = new Integer(list.get(0).substring(7));
														
 
															+		for(String s : list){
														
 
															+		    Integer tempValue = new Integer(s.substring(7));
														
 
															+		    if(tempValue &gt; min) min = tempValue;
														
 
															+		}
														
 
															+		System.out.println("Temporary value: " + root + "/element" + min);
														
 
															+		byte[] b = zk.getData(root + "/element" + min, false, stat);
														
 
															+		zk.delete(root + "/element" + min, 0);
														
 
															+		ByteBuffer buffer = ByteBuffer.wrap(b);
														
 
															+		retvalue = buffer.getInt();
														
 
															+
														
 
															+		return retvalue;
														
 
															+	    }
														
 
															+	}
														
 
															+    }
														
 
															+}
														
 
															+</pre>
														
 
															+</div>
														
 
															+
														
 
															+<a name="N1006F"></a><a name="sc_sourceListing"></a>
														
 
															+<h2 class="h3">Complete Source Listing</h2>
														
 
															+<div class="section">
														
 
															+<div class="note example">
														
 
															+<div class="label">SyncPrimitive.Java</div>
														
 
															+<div class="content">
														
 
															+
														
 
															+<title>SyncPrimitive.Java</title>
														
 
															+
														
 
															+<pre class="code">
														
 
															+package com.yahoo.SyncPrimitive;
														
 
															+
														
 
															+import java.io.IOException;
														
 
															+import java.net.InetAddress;
														
 
															+import java.net.UnknownHostException;
														
 
															+import java.nio.ByteBuffer;
														
 
															+import java.lang.InterruptedException;
														
 
															+import java.util.ArrayList;
														
 
															+import java.util.Random;
														
 
															+
														
 
															+import com.yahoo.zookeeper.Watcher;
														
 
															+import com.yahoo.zookeeper.data.Stat;
														
 
															+import com.yahoo.zookeeper.proto.WatcherEvent;
														
 
															+import com.yahoo.zookeeper.KeeperException;
														
 
															+import com.yahoo.zookeeper.ZooDefs.Ids;
														
 
															+import com.yahoo.zookeeper.ZooDefs.CreateFlags;
														
 
															+import com.yahoo.zookeeper.ZooKeeper;
														
 
															+
														
 
															+public class SyncPrimitive implements Watcher {
														
 
															+
														
 
															+    static ZooKeeper zk = null;
														
 
															+    static Integer mutex;
														
 
															+    
														
 
															+    String root;
														
 
															+
														
 
															+    SyncPrimitive(String address) {
														
 
															+        if(zk == null){
														
 
															+            try {
														
 
															+                System.out.println("Starting ZK:");
														
 
															+                zk = new ZooKeeper(address, 3000, this);
														
 
															+                mutex = new Integer(-1);
														
 
															+                System.out.println("Finished starting ZK: " + zk);
														
 
															+            } catch (KeeperException e) {
														
 
															+                System.out.println("Keeper exception when starting new session: "
														
 
															+                        + e.toString());
														
 
															+                zk = null;
														
 
															+            } catch (IOException e) {
														
 
															+                System.out.println(e.toString());
														
 
															+                zk = null;
														
 
															+            }
														
 
															+        }
														
 
															+        //else mutex = new Integer(-1);
														
 
															+    }
														
 
															+
														
 
															+    synchronized public void process(WatcherEvent event) {
														
 
															+        synchronized (mutex) {
														
 
															+            //System.out.println("Process: " + event.getType());
														
 
															+            mutex.notify();
														
 
															+        }
														
 
															+    }
														
 
															+
														
 
															+    /**
														
 
															+     * Barrier
														
 
															+     */
														
 
															+    static public class Barrier extends SyncPrimitive {
														
 
															+        int size;
														
 
															+        String name;
														
 
															+
														
 
															+        /**
														
 
															+         * Barrier constructor
														
 
															+         * 
														
 
															+         * @param address
														
 
															+         * @param name
														
 
															+         * @param size
														
 
															+         */
														
 
															+        Barrier(String address, String name, int size) {
														
 
															+            super(address);
														
 
															+            this.root = name;
														
 
															+            this.size = size;
														
 
															+
														
 
															+            // Create barrier node
														
 
															+            if (zk != null) {
														
 
															+                try {
														
 
															+                    Stat s = zk.exists(root, false);
														
 
															+                    if (s == null) {
														
 
															+                        zk.create(root, new byte[0], Ids.OPEN_ACL_UNSAFE, 0);
														
 
															+                    }
														
 
															+                } catch (KeeperException e) {
														
 
															+                    System.out
														
 
															+                            .println("Keeper exception when instantiating queue: "
														
 
															+                                    + e.toString());
														
 
															+                } catch (InterruptedException e) {
														
 
															+                    System.out.println("Interrupted exception");
														
 
															+                }
														
 
															+            }
														
 
															+            
														
 
															+            // My node name
														
 
															+            try {
														
 
															+                name = new String(InetAddress.getLocalHost().getCanonicalHostName().toString());
														
 
															+            } catch (UnknownHostException e) {
														
 
															+                System.out.println(e.toString());
														
 
															+            }
														
 
															+            
														
 
															+        }
														
 
															+
														
 
															+        /**
														
 
															+         * Join barrier
														
 
															+         * 
														
 
															+         * @return
														
 
															+         * @throws KeeperException
														
 
															+         * @throws InterruptedException
														
 
															+         */
														
 
															+        
														
 
															+        boolean enter() throws KeeperException, InterruptedException{
														
 
															+            zk.create(root + "/" + name, new byte[0], Ids.OPEN_ACL_UNSAFE,
														
 
															+                    CreateFlags.EPHEMERAL);
														
 
															+            while (true) {
														
 
															+                synchronized (mutex) {                    
														
 
															+                    ArrayList&lt;String&gt; list = zk.getChildren(root, true);
														
 
															+       
														
 
															+                    if (list.size() &lt; size) {
														
 
															+                        mutex.wait();
														
 
															+                    } else {
														
 
															+                        return true;
														
 
															+                    }
														
 
															+                }
														
 
															+            }
														
 
															+        }
														
 
															+
														
 
															+        /**
														
 
															+         * Wait until all reach barrier
														
 
															+         * 
														
 
															+         * @return
														
 
															+         * @throws KeeperException
														
 
															+         * @throws InterruptedException
														
 
															+         */
														
 
															+        
														
 
															+        boolean leave() throws KeeperException, InterruptedException{
														
 
															+            zk.delete(root + "/" + name, 0);
														
 
															+            while (true) {
														
 
															+                synchronized (mutex) {
														
 
															+                    ArrayList&lt;String&gt; list = zk.getChildren(root, true);
														
 
															+                        if (list.size() &gt; 0) {
														
 
															+                            mutex.wait();
														
 
															+                        } else {
														
 
															+                            return true;
														
 
															+                        }
														
 
															+                    }
														
 
															+                }
														
 
															+        }
														
 
															+    }
														
 
															+
														
 
															+    /**
														
 
															+     * Producer-Consumer queue
														
 
															+     */
														
 
															+    static public class Queue extends SyncPrimitive {
														
 
															+
														
 
															+        /**
														
 
															+         * Constructor of producer-consumer queue
														
 
															+         * 
														
 
															+         * @param address
														
 
															+         * @param name
														
 
															+         */
														
 
															+        Queue(String address, String name) {
														
 
															+            super(address);
														
 
															+            this.root = name;
														
 
															+            // Create ZK node name
														
 
															+            if (zk != null) {
														
 
															+                try {
														
 
															+                    Stat s = zk.exists(root, false);
														
 
															+                    if (s == null) {
														
 
															+                        zk.create(root, new byte[0], Ids.OPEN_ACL_UNSAFE, 0);
														
 
															+                    }
														
 
															+                } catch (KeeperException e) {
														
 
															+                    System.out
														
 
															+                            .println("Keeper exception when instantiating queue: "
														
 
															+                                    + e.toString());
														
 
															+                } catch (InterruptedException e) {
														
 
															+                    System.out.println("Interrupted exception");
														
 
															+                }
														
 
															+            }
														
 
															+        }
														
 
															+
														
 
															+        /**
														
 
															+         * Add element to the queue.
														
 
															+         * 
														
 
															+         * @param i
														
 
															+         * @return
														
 
															+         */
														
 
															+        
														
 
															+        boolean produce(int i) throws KeeperException, InterruptedException{
														
 
															+            ByteBuffer b = ByteBuffer.allocate(4);
														
 
															+            byte[] value;
														
 
															+
														
 
															+            // Add child with value i
														
 
															+            b.putInt(i);
														
 
															+            value = b.array();
														
 
															+            zk.create(root + "/element", value, Ids.OPEN_ACL_UNSAFE,
														
 
															+                        CreateFlags.SEQUENCE);
														
 
															+            
														
 
															+            return true;
														
 
															+        }
														
 
															+
														
 
															+
														
 
															+        /**
														
 
															+         * Remove first element from the queue.
														
 
															+         * 
														
 
															+         * @return
														
 
															+         * @throws KeeperException
														
 
															+         * @throws InterruptedException
														
 
															+         */
														
 
															+        int consume() throws KeeperException, InterruptedException{
														
 
															+            int retvalue = -1;
														
 
															+            Stat stat = null;
														
 
															+            
														
 
															+            // Get the first element available
														
 
															+            while (true) {
														
 
															+                synchronized (mutex) {
														
 
															+                    ArrayList&lt;String&gt; list = zk.getChildren(root, true);
														
 
															+                    if (list.size() == 0) {
														
 
															+                        System.out.println("Going to wait");
														
 
															+                        mutex.wait();
														
 
															+                    } else {
														
 
															+                        Integer min = new Integer(list.get(0).substring(7));
														
 
															+                        for(String s : list){
														
 
															+                            Integer tempValue = new Integer(s.substring(7));
														
 
															+                            //System.out.println("Temporary value: " + tempValue);
														
 
															+                            if(tempValue &lt; min) min = tempValue;
														
 
															+                        }
														
 
															+                        System.out.println("Temporary value: " + root + "/element" + min);
														
 
															+                        byte[] b = zk.getData(root + "/element" + min,
														
 
															+                                    false, stat);
														
 
															+                        zk.delete(root + "/element" + min, 0);
														
 
															+                        ByteBuffer buffer = ByteBuffer.wrap(b);
														
 
															+                        retvalue = buffer.getInt();
														
 
															+                        
														
 
															+                        return retvalue;
														
 
															+                    }
														
 
															+                }
														
 
															+            }           
														
 
															+        }
														
 
															+    }
														
 
															+
														
 
															+    public static void main(String args[]) {
														
 
															+        if (args[0].equals("qTest"))
														
 
															+            queueTest(args);
														
 
															+        else
														
 
															+            barrierTest(args);
														
 
															+
														
 
															+    }
														
 
															+
														
 
															+    public static void queueTest(String args[]) {
														
 
															+        Queue q = new Queue(args[1], "/app1");
														
 
															+
														
 
															+        System.out.println("Input: " + args[1]);
														
 
															+        int i;
														
 
															+        Integer max = new Integer(args[2]);
														
 
															+
														
 
															+        if (args[3].equals("p")) {
														
 
															+            System.out.println("Producer");
														
 
															+            for (i = 0; i &lt; max; i++)
														
 
															+                try{    
														
 
															+                    q.produce(10 + i);
														
 
															+                } catch (KeeperException e){
														
 
															+                    
														
 
															+                } catch (InterruptedException e){
														
 
															+                    
														
 
															+                }
														
 
															+        } else {
														
 
															+            System.out.println("Consumer");
														
 
															+   
														
 
															+            for (i = 0; i &lt; max; i++) {
														
 
															+                try{
														
 
															+                    int r = q.consume();
														
 
															+                    System.out.println("Item: " + r);
														
 
															+                } catch (KeeperException e){
														
 
															+                    i--;
														
 
															+                } catch (InterruptedException e){
														
 
															+                    
														
 
															+                }
														
 
															+            }
														
 
															+        }
														
 
															+    }
														
 
															+
														
 
															+    public static void barrierTest(String args[]) {
														
 
															+        Barrier b = new Barrier(args[1], "/b1", new Integer(args[2]));
														
 
															+        try{
														
 
															+            boolean flag = b.enter();
														
 
															+            System.out.println("Entered barrier: " + args[2]);
														
 
															+            if(!flag) System.out.println("Error when entering the barrier");
														
 
															+        } catch (KeeperException e){
														
 
															+            
														
 
															+        } catch (InterruptedException e){
														
 
															+            
														
 
															+        }
														
 
															+
														
 
															+        // Generate random integer
														
 
															+        Random rand = new Random();
														
 
															+        int r = rand.nextInt(100);
														
 
															+        // Loop for rand iterations
														
 
															+        for (int i = 0; i &lt; r; i++) {
														
 
															+            try {
														
 
															+                Thread.sleep(100);
														
 
															+            } catch (InterruptedException e) {
														
 
															+
														
 
															+            }
														
 
															+        }
														
 
															+        try{
														
 
															+            b.leave();
														
 
															+        } catch (KeeperException e){
														
 
															+            
														
 
															+        } catch (InterruptedException e){
														
 
															+            
														
 
															+        }
														
 
															+        System.out.println("Left barrier");
														
 
															+    }
														
 
															+}</pre>
														
 
															+</div>
														
 
															+</div>
														
 
															+</div>
														
 
															+
														
 
															+
														
 
															+<p align="right">
														
 
															+<font size="-2"></font>
														
 
															+</p>
														
 
															+</div>
														
 
															+<!--+
														
 
															+    |end content
														
 
															+    +-->
														
 
															+<div class="clearboth">&nbsp;</div>
														
 
															+</div>
														
 
															+<div id="footer">
														
 
															+<!--+
														
 
															+    |start bottomstrip
														
 
															+    +-->
														
 
															+<div class="lastmodified">
														
 
															+<script type="text/javascript"><!--
														
 
															+document.write("Last Published: " + document.lastModified);
														
 
															+//  --></script>
														
 
															+</div>
														
 
															+<div class="copyright">
														
 
															+        Copyright &copy;
														
 
															+         2008 <a href="http://www.apache.org/licenses/">The Apache Software Foundation.</a>
														
 
															+</div>
														
 
															+<!--+
														
 
															+    |end bottomstrip
														
 
															+    +-->
														
 
															+</div>
														
 
															+</body>
														
 
															+</html>
														
--- a/docs/zookeeperTutorial.pdf
+++ b/docs/zookeeperTutorial.pdf
--- a/src/docs/src/documentation/content/xdocs/index.xml
+++ b/src/docs/src/documentation/content/xdocs/index.xml
@@ -38,6 +38,9 @@ The following documents provide concepts and procedures to get you started using
 
															       <li><a href="zookeeperProgrammers.html">Programmer's Guide</a> - an application developer's guide to ZooKeeper</li>
														
 
															       <li><a href="recipes.html">ZooKeeper Recipes</a> - a set of common, higher level solutions using ZooKeeper</li>
														
 
															       <li><a href="zookeeperAdmin.html">Administrator's Guide</a> - a guide for system administrators and anyone else who might deploy Zookeeer</li>
														
 
															+      <li><a href="javaExample.html">ZooKeeper Java Example</a> - A simple Zookeeer Client, written in Java</li>
														
 
															+      <li><a href="zookeeperTutorial.html">Barrier and Queue Tutorial</a> - Simple Implementations of Barriers and Queues</li>
														
 
															+      <li><a href="zookeeperInternals.html">ZooKeeper Internals</a> - Assorted Topics on the Inner Workings of ZooKeeper</li>
														
 
															       <li><a href="ext:api/index">API Docs</a> - the technical reference to ZooKeeper APIs</li>
														
 
															       <li><a href="ext:wiki">Wiki</a> - miscellaneous, informal ZooKeeper documentation, in Wiki format</li>
														
 
															       <li><a href="ext:faq">FAQ</a> - frequently asked questions</li>    
														
--- a/src/docs/src/documentation/content/xdocs/javaExample.xml
+++ b/src/docs/src/documentation/content/xdocs/javaExample.xml
@@ -0,0 +1,672 @@
 
															+<?xml version="1.0" encoding="UTF-8"?>
														
 
															+<!--
														
 
															+  Copyright 2002-2004 The Apache Software Foundation
														
 
															+
														
 
															+  Licensed under the Apache License, Version 2.0 (the "License");
														
 
															+  you may not use this file except in compliance with the License.
														
 
															+  You may obtain a copy of the License at
														
 
															+
														
 
															+      http://www.apache.org/licenses/LICENSE-2.0
														
 
															+
														
 
															+  Unless required by applicable law or agreed to in writing, software
														
 
															+  distributed under the License is distributed on an "AS IS" BASIS,
														
 
															+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
														
 
															+  See the License for the specific language governing permissions and
														
 
															+  limitations under the License.
														
 
															+-->
														
 
															+
														
 
															+<!DOCTYPE article PUBLIC "-//OASIS//DTD Simplified DocBook XML V1.0//EN"
														
 
															+"http://www.oasis-open.org/docbook/xml/simple/1.0/sdocbook.dtd">
														
 
															+<article id="ar_JavaExample">
														
 
															+  <title>ZooKeeper Java Example</title>
														
 
															+
														
 
															+  <articleinfo>
														
 
															+    <legalnotice>
														
 
															+      <para>Licensed under the Apache License, Version 2.0 (the "License");
														
 
															+      you may not use this file except in compliance with the License. You may
														
 
															+      obtain a copy of the License at <ulink
														
 
															+      url="http://www.apache.org/licenses/LICENSE-2.0">http://www.apache.org/licenses/LICENSE-2.0</ulink>.</para>
														
 
															+
														
 
															+      <para>Unless required by applicable law or agreed to in writing,
														
 
															+      software distributed under the License is distributed on an "AS IS"
														
 
															+      BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
														
 
															+      implied. See the License for the specific language governing permissions
														
 
															+      and limitations under the License.</para>
														
 
															+    </legalnotice>
														
 
															+
														
 
															+    <abstract>
														
 
															+      <para>This article contains sample Java code for a simple watch client.</para>
														
 
															+
														
 
															+    </abstract>
														
 
															+  </articleinfo>
														
 
															+
														
 
															+  <section id="ch_Introduction">
														
 
															+    <title>A Simple Watch Client</title>
														
 
															+
														
 
															+    <para>To introduce you to the ZooKeeper Java API, we develop here a very simple 
														
 
															+    watch client. This ZooKeeper client watches a ZooKeeper node for changes 
														
 
															+    and responds to by starting or stopping a program.</para>
														
 
															+    
														
 
															+    <section id="sc_requirements"><title>Requirements</title>
														
 
															+    
														
 
															+    <para>The client has four requirements:</para>
														
 
															+    
														
 
															+    <itemizedlist><listitem><para>It takes as parameters:</para>
														
 
															+    	<itemizedlist>
														
 
															+		<listitem><para>the address of the ZooKeeper service</para></listitem>
														
 
															+		<listitem> <para> a znode, by name</para></listitem>
														
 
															+		<listitem><para> an executable with arguments.</para></listitem></itemizedlist></listitem>
														
 
															+	<listitem><para>It fetches the data associated with the znode and starts the executable.</para></listitem>
														
 
															+	<listitem><para>If the znode changes, the client refetches the contents and restarts the executable.</para></listitem>
														
 
															+	<listitem><para>If the znode disappears, the client kills the executable.</para></listitem></itemizedlist>
														
 
															+
														
 
															+   </section>
														
 
															+   
														
 
															+   <section id="sc_design">
														
 
															+   	<title>Program Design</title>
														
 
															+
														
 
															+   <para>Conventionally, ZooKeeper applications are broken into two units, one which maintains the connection, 
														
 
															+   and the other which monitors data.  In this application, the class called the <emphasis role="bold">Executor</emphasis> 
														
 
															+   maintains the ZooKeeper connection, and the class called the  <emphasis role="bold">DataMonitor</emphasis> monitors the data
														
 
															+   in the ZooKeeper tree. Also, Executor contains the main thread and contains the execution logic.
														
 
															+   It is responsible for what little user interaction there is, as well as interaction with the exectuable program you
														
 
															+   pass in as an argument and which the sample (per the requirements) shuts down and restarts, according to the 
														
 
															+   state of the znode.</para>
														
 
															+   
														
 
															+   </section>
														
 
															+  
														
 
															+   </section>
														
 
															+
														
 
															+   <section id="sc_executor"><title>The Executor Class</title>
														
 
															+    <para>The Executor object is the primary container of the sample application. It contains 
														
 
															+    both the <emphasis role="bold">ZooKeeper</emphasis> object, <emphasis role="bold">DataMonitor</emphasis>, as described above in 
														
 
															+    <xref linkend="sc_design"/>.  </para>
														
 
															+    
														
 
															+    <programlisting>
														
 
															+// from the Executor class...
														
 
															+    
														
 
															+public static void main(String[] args) {
														
 
															+    if (args.length &lt; 4) {
														
 
															+        System.err
														
 
															+                .println("USAGE: Executor hostPort znode filename program [args ...]");
														
 
															+        System.exit(2);
														
 
															+    }
														
 
															+    String hostPort = args[0];
														
 
															+    String znode = args[1];
														
 
															+    String filename = args[2];
														
 
															+    String exec[] = new String[args.length - 3];
														
 
															+    System.arraycopy(args, 3, exec, 0, exec.length);
														
 
															+    try {
														
 
															+        Executor theExectutor = new Executor(hostPort, znode, filename, exec);
														
 
															+	theExectutor.run();
														
 
															+    } catch (Exception e) {
														
 
															+        e.printStackTrace();
														
 
															+    }
														
 
															+}
														
 
															+    
														
 
															+public Executor(String hostPort, String znode, String filename,
														
 
															+    String exec[]) throws KeeperException, IOException {
														
 
															+    this.filename = filename;
														
 
															+    this.exec = exec;
														
 
															+    
														
 
															+    //create a new zookeeper object, passing a self-reference in a the Watcher
														
 
															+    zk = new ZooKeeper(hostPort, 3000, this);
														
 
															+    dm = new DataMonitor(zk, znode, null, this);
														
 
															+}
														
 
															+   
														
 
															+public void run() {
														
 
															+    try {
														
 
															+        synchronized (this) {
														
 
															+            while (!dm.dead) {
														
 
															+                wait();
														
 
															+            }
														
 
															+        }
														
 
															+    } catch (InterruptedException e) {    
														
 
															+    }
														
 
															+}
														
 
															+</programlisting>
														
 
															+
														
 
															+
														
 
															+    <para>
														
 
															+    Recall that the Executor's job is to starts and stop the executable whose name you pass in on the command line. 
														
 
															+    It does this in response to events fired by the ZooKeeper object. As you can see in the code above, the Executor passes
														
 
															+    a reference to itself as the Watcher argument in the ZooKeeper constructor. It also passes a reference to itself
														
 
															+    as DataMonitorListener argument to the DataMonitor constructor. Per the Executor's definition, it implements both these
														
 
															+    interfaces:
														
 
															+    </para>
														
 
															+    
														
 
															+    <programlisting>
														
 
															+public class Executor implements Watcher, Runnable, DataMonitorListener {
														
 
															+...
														
 
															+    </programlisting>
														
 
															+    
														
 
															+    <para>The <emphasis role="bold">Watcher</emphasis> interface is defined by the ZooKeeper Java API.
														
 
															+    ZooKeeper uses it to communicate back to its container. It supports only one method, <command>process()</command>, and ZooKeeper uses 
														
 
															+    it to communciates generic events that the main thread would be intersted in, such as the state of the ZooKeeper connection or the ZooKeeper session.The Executor 
														
 
															+    in this example simply forwards those events down to the DataMonitor to decide what to do with them. It does this simply to illustrate
														
 
															+    the point that, by convention, the Executor or some Executor-like object "owns" the ZooKeeper connection, but it is free to delegate the events to other
														
 
															+    events to other objects. It also uses this as the default channel on which to fire watch events. (More on this later.)</para>
														
 
															+    
														
 
															+<programlisting>
														
 
															+public void process(WatcherEvent event) {
														
 
															+    dm.process(event);
														
 
															+}
														
 
															+</programlisting>
														
 
															+    
														
 
															+    <para>The <emphasis role="bold">DataMonitorListener</emphasis> 
														
 
															+    interface, on the other hand, is not part of the the ZooKeeper API. It is a completely custom interface, 
														
 
															+    designed for this sample application. The DataMonitor object uses it to communicate back to its container, which
														
 
															+    is also the the Executor object.The DataMonitorListener interface looks like this:</para>
														
 
															+    <programlisting>
														
 
															+public interface DataMonitorListener {
														
 
															+    /**
														
 
															+    * The existence status of the node has changed.
														
 
															+    */
														
 
															+    void exists(byte data[]);
														
 
															+
														
 
															+    /**
														
 
															+    * The ZooKeeper session is no longer valid.
														
 
															+    * 
														
 
															+    * @param rc
														
 
															+    * the ZooKeeper reason code
														
 
															+    */
														
 
															+    void closing(int rc);
														
 
															+}
														
 
															+</programlisting>
														
 
															+    <para>This interface is defined in the DataMonitor class and implemented in the Executor class. 
														
 
															+    When <command>Executor.exists()</command> is invoked,
														
 
															+    the Executor decides whether to start up or shut down per the requirements. Recall that the requires say to kill the executable when the 
														
 
															+    znode ceases to <emphasis>exist</emphasis>. </para>
														
 
															+    
														
 
															+    <para>When <command>Executor.closing()</command>
														
 
															+    is invoked, the Executor decides whether or not to shut itself down in response to the ZooKeeper connection permanently disappearing.</para>
														
 
															+    
														
 
															+    <para>As you might have guessed, DataMonitor is the object that invokes 
														
 
															+    these methods, in response to changes in ZooKeeper's state.</para>
														
 
															+    
														
 
															+    <para>Here are Executor's implementation of 
														
 
															+    <command>DataMonitorListener.exists()</command> and <command>DataMonitorListener.closing</command>:
														
 
															+    </para>
														
 
															+    <programlisting>
														
 
															+public void exists( byte[] data ) {
														
 
															+    if (data == null) {
														
 
															+        if (child != null) {
														
 
															+            System.out.println("Killing process");
														
 
															+            child.destroy();
														
 
															+            try {
														
 
															+                child.waitFor();
														
 
															+            } catch (InterruptedException e) {
														
 
															+            }
														
 
															+        }
														
 
															+        child = null;
														
 
															+    } else {
														
 
															+        if (child != null) {
														
 
															+            System.out.println("Stopping child");
														
 
															+            child.destroy();
														
 
															+            try {
														
 
															+               child.waitFor();
														
 
															+            } catch (InterruptedException e) {
														
 
															+            e.printStackTrace();
														
 
															+            }
														
 
															+        }
														
 
															+        try {
														
 
															+            FileOutputStream fos = new FileOutputStream(filename);
														
 
															+            fos.write(data);
														
 
															+            fos.close();
														
 
															+        } catch (IOException e) {
														
 
															+            e.printStackTrace();
														
 
															+        }
														
 
															+        try {
														
 
															+            System.out.println("Starting child");
														
 
															+            child = Runtime.getRuntime().exec(exec);
														
 
															+            new StreamWriter(child.getInputStream(), System.out);
														
 
															+            new StreamWriter(child.getErrorStream(), System.err);
														
 
															+        } catch (IOException e) {
														
 
															+            e.printStackTrace();
														
 
															+        }
														
 
															+    }
														
 
															+}
														
 
															+
														
 
															+public void closing(int rc) {
														
 
															+    synchronized (this) {
														
 
															+        notifyAll();
														
 
															+    }
														
 
															+}
														
 
															+</programlisting>
														
 
															+    
														
 
															+</section>
														
 
															+<section id="sc_DataMonitor"><title>The DataMonitor Class</title>
														
 
															+<para>
														
 
															+The DataMonitor class has the meat of the ZooKeeper logic. It is mostly 
														
 
															+asynchronous and event driven. DataMonitor kicks things off in the constructor with:</para>
														
 
															+<programlisting>
														
 
															+public DataMonitor(ZooKeeper zk, String znode, Watcher chainedWatcher,
														
 
															+        DataMonitorListener listener) {
														
 
															+    this.zk = zk;
														
 
															+    this.znode = znode;
														
 
															+    this.chainedWatcher = chainedWatcher;
														
 
															+    this.listener = listener;
														
 
															+    
														
 
															+    // Get things started by checking if the node exists. We are going
														
 
															+    // to be completely event driven
														
 
															+    <emphasis role="bold">zk.exists(znode, true, this, null);</emphasis>
														
 
															+}
														
 
															+</programlisting>
														
 
															+
														
 
															+<para>The call to <command>ZooKeeper.exists()</command> checks for the existence of the znode, 
														
 
															+sets a watch, and passes a reference to itself (<command>this</command>)
														
 
															+as the completion callback object. In this sense, it kicks things off, since the
														
 
															+real processing happens when the watch is triggered.</para>
														
 
															+
														
 
															+<note>
														
 
															+<para>Don't confuse the completion callback with the watch callback. The <command>ZooKeeper.exists()</command> 
														
 
															+completion callback, which happens to be the method <command>StatCallback.processResult()</command> implemented 
														
 
															+in the DataMonitor object, is invoked when the asynchronous <emphasis>setting of the watch</emphasis> operation 
														
 
															+(by <command>ZooKeeper.exists()</command>) completes on the server. </para>
														
 
															+<para>
														
 
															+The triggering of the watch, on the other hand, sends an event to the <emphasis>Executor</emphasis> object, since
														
 
															+the Executor registered as the Watcher of the ZooKeeper object.</para>
														
 
															+
														
 
															+<para>As an aside, you might note that the DataMonitor could also register itself as the Watcher
														
 
															+for this particular watch event. This is new to ZooKeeper 3.0.0 (the support of multiple Watchers). In this
														
 
															+example, however, DataMonitor does not register as the Watcher.</para>
														
 
															+</note>
														
 
															+
														
 
															+<para>When the <command>ZooKeeper.exists()</command> operation completes on the server, the ZooKeeper API invokes this completion callback on 
														
 
															+the client:</para>
														
 
															+
														
 
															+<programlisting>
														
 
															+public void processResult(int rc, String path, Object ctx, Stat stat) {
														
 
															+    boolean exists;
														
 
															+    switch (rc) {
														
 
															+    case Code.Ok:
														
 
															+        exists = true;
														
 
															+        break;
														
 
															+    case Code.NoNode:
														
 
															+        exists = false;
														
 
															+        break;
														
 
															+    case Code.SessionExpired:
														
 
															+    case Code.NoAuth:
														
 
															+        dead = true;
														
 
															+        listener.closing(rc);
														
 
															+        return;
														
 
															+    default:
														
 
															+        // Retry errors
														
 
															+        zk.exists(znode, true, this, null);
														
 
															+        return;
														
 
															+    }
														
 
															+ 
														
 
															+    byte b[] = null;
														
 
															+    if (exists) {
														
 
															+        try {
														
 
															+            <emphasis role="bold">b = zk.getData(znode, false, null);</emphasis>
														
 
															+        } catch (KeeperException e) {
														
 
															+            // We don't need to worry about recovering now. The watch
														
 
															+            // callbacks will kick off any exception handling
														
 
															+            e.printStackTrace();
														
 
															+        } catch (InterruptedException e) {
														
 
															+            return;
														
 
															+        }
														
 
															+    }     
														
 
															+    if ((b == null &amp;&amp; b != prevData)
														
 
															+            || (b != null &amp;&amp; !Arrays.equals(prevData, b))) {
														
 
															+        <emphasis role="bold">listener.exists(b);</emphasis>
														
 
															+        prevData = b;
														
 
															+    }
														
 
															+}
														
 
															+</programlisting>
														
 
															+
														
 
															+<para>
														
 
															+The code first checks the error codes for znode existence, fatal errors, and 
														
 
															+recoverable errors. If the file (or znode) exists, it gets the data from the znode, and 
														
 
															+then invoke the exists() callback of Executor if the state has changed. Note, 
														
 
															+it doesn't have to do any Exception processing for the getData call because it 
														
 
															+has watches pending for anything that could cause an error: if the node is deleted 
														
 
															+before it calls <command>ZooKeeper.getData()</command>, the watch event set by 
														
 
															+the <command>ZooKeeper.exists()</command> triggers a callback; 
														
 
															+if there is a communication error, a connection watch event fires when 
														
 
															+the connection comes back up.
														
 
															+</para>
														
 
															+
														
 
															+<para>Finally, notice how DataMonitor processes watch events: </para>
														
 
															+<programlisting>
														
 
															+public void process(WatcherEvent event) {
														
 
															+    String path = event.getPath();
														
 
															+    if (event.getType() == Watcher.Event.EventNone) {
														
 
															+        // We are are being told that the state of the
														
 
															+        // connection has changed
														
 
															+        switch (event.getState()) {
														
 
															+        case Event.KeeperStateSyncConnected:
														
 
															+            // Everything is happy. Lets kick things off
														
 
															+            // again by checking the existence of the znode
														
 
															+            zk.exists(znode, true, this, null);
														
 
															+            break;
														
 
															+        case Event.KeeperStateExpired:
														
 
															+            // It's all over
														
 
															+            dead = true;
														
 
															+            listener.closing(KeeperException.Code.SessionExpired);
														
 
															+            break;
														
 
															+        }
														
 
															+    } else {
														
 
															+        if (path != null &amp;&amp; path.equals(znode)) {
														
 
															+            // Something has changed on the node, let's find out
														
 
															+            zk.exists(znode, true, this, null);
														
 
															+        }
														
 
															+   }
														
 
															+   if (chainedWatcher != null) {
														
 
															+        chainedWatcher.process(event);
														
 
															+   }
														
 
															+}
														
 
															+</programlisting>
														
 
															+<para>
														
 
															+If the client-side ZooKeeper libraries can reestablish the communication channel to ZooKeeper, DataMonitor simply kicks
														
 
															+everything off again with the call to <command>ZooKeeper.exists()</command>. 
														
 
															+If it gets an event for a znode, it calls <command>ZooKeeper.exists()</command> to find out what has changed.
														
 
															+</para>
														
 
															+</section>
														
 
															+
														
 
															+<section id="sc_completeSourceCode">
														
 
															+	<title>Complete Source Listings</title>
														
 
															+	<example id="eg_Executor_java"><title>Executor.java</title><programlisting>
														
 
															+/**
														
 
															+ * A simple example program to use DataMonitor to start and
														
 
															+ * stop executables based on a znode. The program watches the
														
 
															+ * specified znode and saves the data that corresponds to the
														
 
															+ * znode in the filesystem. It also starts the specified program
														
 
															+ * with the specified arguments when the znode exists and kills
														
 
															+ * the program if the znode goes away.
														
 
															+ */
														
 
															+package com.yahoo.zk.executor;
														
 
															+
														
 
															+import java.io.FileOutputStream;
														
 
															+import java.io.IOException;
														
 
															+import java.io.InputStream;
														
 
															+import java.io.OutputStream;
														
 
															+
														
 
															+import com.yahoo.zk.executor.DataMonitor.DataMonitorListener;
														
 
															+import com.yahoo.zookeeper.KeeperException;
														
 
															+import com.yahoo.zookeeper.Watcher;
														
 
															+import com.yahoo.zookeeper.ZooKeeper;
														
 
															+import com.yahoo.zookeeper.proto.WatcherEvent;
														
 
															+
														
 
															+public class Executor implements Watcher, Runnable, DataMonitorListener {
														
 
															+    String znode;
														
 
															+
														
 
															+    DataMonitor dm;
														
 
															+
														
 
															+    ZooKeeper zk;
														
 
															+
														
 
															+    String filename;
														
 
															+
														
 
															+    String exec[];
														
 
															+
														
 
															+    Process child;
														
 
															+
														
 
															+    public Executor(String hostPort, String znode, String filename,
														
 
															+            String exec[]) throws KeeperException, IOException {
														
 
															+        this.filename = filename;
														
 
															+        this.exec = exec;
														
 
															+        zk = new ZooKeeper(hostPort, 3000, this);
														
 
															+        dm = new DataMonitor(zk, znode, null, this);
														
 
															+    }
														
 
															+
														
 
															+    /**
														
 
															+     * @param args
														
 
															+     */
														
 
															+    public static void main(String[] args) {
														
 
															+        if (args.length &lt; 4) {
														
 
															+            System.err
														
 
															+                    .println("USAGE: Executor hostPort znode filename program [args ...]");
														
 
															+            System.exit(2);
														
 
															+        }
														
 
															+        String hostPort = args[0];
														
 
															+        String znode = args[1];
														
 
															+        String filename = args[2];
														
 
															+        String exec[] = new String[args.length - 3];
														
 
															+        System.arraycopy(args, 3, exec, 0, exec.length);
														
 
															+        try {
														
 
															+            new Executor(hostPort, znode, filename, exec).run();
														
 
															+        } catch (Exception e) {
														
 
															+            e.printStackTrace();
														
 
															+        }
														
 
															+    }
														
 
															+
														
 
															+    /***************************************************************************
														
 
															+     * We do process any events ourselves, we just need to forward them on.
														
 
															+     * 
														
 
															+     * @see com.yahoo.zookeeper.Watcher#process(com.yahoo.zookeeper.proto.WatcherEvent)
														
 
															+     */
														
 
															+    @Override
														
 
															+    public void process(WatcherEvent event) {
														
 
															+        dm.process(event);
														
 
															+    }
														
 
															+
														
 
															+    @Override
														
 
															+    public void run() {
														
 
															+        try {
														
 
															+            synchronized (this) {
														
 
															+                while (!dm.dead) {
														
 
															+                    wait();
														
 
															+                }
														
 
															+            }
														
 
															+        } catch (InterruptedException e) {
														
 
															+        }
														
 
															+    }
														
 
															+
														
 
															+    @Override
														
 
															+    public void closing(int rc) {
														
 
															+        synchronized (this) {
														
 
															+            notifyAll();
														
 
															+        }
														
 
															+    }
														
 
															+
														
 
															+    static class StreamWriter extends Thread {
														
 
															+        OutputStream os;
														
 
															+
														
 
															+        InputStream is;
														
 
															+
														
 
															+        StreamWriter(InputStream is, OutputStream os) {
														
 
															+            this.is = is;
														
 
															+            this.os = os;
														
 
															+            start();
														
 
															+        }
														
 
															+
														
 
															+        public void run() {
														
 
															+            byte b[] = new byte[80];
														
 
															+            int rc;
														
 
															+            try {
														
 
															+                while ((rc = is.read(b)) > 0) {
														
 
															+                    os.write(b, 0, rc);
														
 
															+                }
														
 
															+            } catch (IOException e) {
														
 
															+            }
														
 
															+
														
 
															+        }
														
 
															+    }
														
 
															+
														
 
															+    @Override
														
 
															+    public void exists(byte[] data) {
														
 
															+        if (data == null) {
														
 
															+            if (child != null) {
														
 
															+                System.out.println("Killing process");
														
 
															+                child.destroy();
														
 
															+                try {
														
 
															+                    child.waitFor();
														
 
															+                } catch (InterruptedException e) {
														
 
															+                }
														
 
															+            }
														
 
															+            child = null;
														
 
															+        } else {
														
 
															+            if (child != null) {
														
 
															+                System.out.println("Stopping child");
														
 
															+                child.destroy();
														
 
															+                try {
														
 
															+                    child.waitFor();
														
 
															+                } catch (InterruptedException e) {
														
 
															+                    e.printStackTrace();
														
 
															+                }
														
 
															+            }
														
 
															+            try {
														
 
															+                FileOutputStream fos = new FileOutputStream(filename);
														
 
															+                fos.write(data);
														
 
															+                fos.close();
														
 
															+            } catch (IOException e) {
														
 
															+                e.printStackTrace();
														
 
															+            }
														
 
															+            try {
														
 
															+                System.out.println("Starting child");
														
 
															+                child = Runtime.getRuntime().exec(exec);
														
 
															+                new StreamWriter(child.getInputStream(), System.out);
														
 
															+                new StreamWriter(child.getErrorStream(), System.err);
														
 
															+            } catch (IOException e) {
														
 
															+                e.printStackTrace();
														
 
															+            }
														
 
															+        }
														
 
															+    }
														
 
															+}
														
 
															+</programlisting>
														
 
															+	
														
 
															+</example>
														
 
															+
														
 
															+<example id="eg_DataMonitor_java">
														
 
															+	<title>DataMonitor.java</title>
														
 
															+	<programlisting>
														
 
															+/**
														
 
															+ * A simple class that monitors the data and existence of a ZooKeeper
														
 
															+ * node. It uses asynchronous ZooKeeper APIs.
														
 
															+ */
														
 
															+package com.yahoo.zk.executor;
														
 
															+
														
 
															+import java.util.Arrays;
														
 
															+
														
 
															+import com.yahoo.zookeeper.KeeperException;
														
 
															+import com.yahoo.zookeeper.Watcher;
														
 
															+import com.yahoo.zookeeper.ZooKeeper;
														
 
															+import com.yahoo.zookeeper.AsyncCallback.StatCallback;
														
 
															+import com.yahoo.zookeeper.KeeperException.Code;
														
 
															+import com.yahoo.zookeeper.data.Stat;
														
 
															+import com.yahoo.zookeeper.proto.WatcherEvent;
														
 
															+
														
 
															+public class DataMonitor implements Watcher, StatCallback {
														
 
															+
														
 
															+    ZooKeeper zk;
														
 
															+
														
 
															+    String znode;
														
 
															+
														
 
															+    Watcher chainedWatcher;
														
 
															+
														
 
															+    boolean dead;
														
 
															+
														
 
															+    DataMonitorListener listener;
														
 
															+
														
 
															+    byte prevData[];
														
 
															+
														
 
															+    public DataMonitor(ZooKeeper zk, String znode, Watcher chainedWatcher,
														
 
															+            DataMonitorListener listener) {
														
 
															+        this.zk = zk;
														
 
															+        this.znode = znode;
														
 
															+        this.chainedWatcher = chainedWatcher;
														
 
															+        this.listener = listener;
														
 
															+        // Get things started by checking if the node exists. We are going
														
 
															+        // to be completely event driven
														
 
															+        zk.exists(znode, true, this, null);
														
 
															+    }
														
 
															+
														
 
															+    /**
														
 
															+     * Other classes use the DataMonitor by implementing this method
														
 
															+     */
														
 
															+    public interface DataMonitorListener {
														
 
															+        /**
														
 
															+         * The existence status of the node has changed.
														
 
															+         */
														
 
															+        void exists(byte data[]);
														
 
															+
														
 
															+        /**
														
 
															+         * The ZooKeeper session is no longer valid.
														
 
															+         * 
														
 
															+         * @param rc
														
 
															+         *                the ZooKeeper reason code
														
 
															+         */
														
 
															+        void closing(int rc);
														
 
															+    }
														
 
															+
														
 
															+    @Override
														
 
															+    /**
														
 
															+     * This is a watch event callback. The node we were watching has changed or
														
 
															+     * something happened to our connection to ZooKeeper.
														
 
															+     */
														
 
															+    public void process(WatcherEvent event) {
														
 
															+        String path = event.getPath();
														
 
															+        if (event.getType() == Watcher.Event.EventNone) {
														
 
															+            // We are are being told that the state of the
														
 
															+            // connection has changed
														
 
															+            switch (event.getState()) {
														
 
															+            case Event.KeeperStateSyncConnected:
														
 
															+                // Everything is happy. Lets kick things off
														
 
															+                // again by checking the existence of the znode
														
 
															+                zk.exists(znode, true, this, null);
														
 
															+                break;
														
 
															+            case Event.KeeperStateExpired:
														
 
															+                // It's all over
														
 
															+                dead = true;
														
 
															+                listener.closing(KeeperException.Code.SessionExpired);
														
 
															+                break;
														
 
															+            }
														
 
															+        } else {
														
 
															+            if (path != null &amp;&amp; path.equals(znode)) {
														
 
															+                // Something has changed on the node, let's find out
														
 
															+                zk.exists(znode, true, this, null);
														
 
															+            }
														
 
															+        }
														
 
															+        if (chainedWatcher != null) {
														
 
															+            chainedWatcher.process(event);
														
 
															+        }
														
 
															+    }
														
 
															+
														
 
															+    @Override
														
 
															+    public void processResult(int rc, String path, Object ctx, Stat stat) {
														
 
															+        boolean exists;
														
 
															+        switch (rc) {
														
 
															+        case Code.Ok:
														
 
															+            exists = true;
														
 
															+            break;
														
 
															+        case Code.NoNode:
														
 
															+            exists = false;
														
 
															+            break;
														
 
															+        case Code.SessionExpired:
														
 
															+        case Code.NoAuth:
														
 
															+            dead = true;
														
 
															+            listener.closing(rc);
														
 
															+            return;
														
 
															+        default:
														
 
															+            // Retry errors
														
 
															+            zk.exists(znode, true, this, null);
														
 
															+            return;
														
 
															+        }
														
 
															+
														
 
															+        byte b[] = null;
														
 
															+        if (exists) {
														
 
															+            try {
														
 
															+                b = zk.getData(znode, false, null);
														
 
															+            } catch (KeeperException e) {
														
 
															+                // We don't need to worry about recovering now. The watch
														
 
															+                // callbacks will kick off any exception handling
														
 
															+                e.printStackTrace();
														
 
															+            } catch (InterruptedException e) {
														
 
															+                return;
														
 
															+            }
														
 
															+        }
														
 
															+        if ((b == null &amp;&amp; b != prevData)
														
 
															+                || (b != null &amp;&amp; !Arrays.equals(prevData, b))) {
														
 
															+            listener.exists(b);
														
 
															+            prevData = b;
														
 
															+        }
														
 
															+    }
														
 
															+}
														
 
															+</programlisting>
														
 
															+</example>
														
 
															+</section>
														
 
															+
														
 
															+
														
 
															+
														
 
															+</article>
														
--- a/src/docs/src/documentation/content/xdocs/site.xml
+++ b/src/docs/src/documentation/content/xdocs/site.xml
@@ -38,6 +38,9 @@ See http://forrest.apache.org/docs/linking.html for more info.
 
															     <program   label="Programmer's Guide"     href="zookeeperProgrammers.html" />
														
 
															     <recipes   label="Recipes"		      href="recipes.html" />
														
 
															     <admin     label="Administrator's Guide"  href="zookeeperAdmin.html" />
														
 
															+    <javaEx    label="Java Example"     href="javaExample.html" />
														
 
															+    <barTutor  label="Barrie and Queue Tutorial" href="zookeeperTutorial.html" />
														
 
															+    <internals label="ZooKeeper Internals"	href="zookeeperInternals.html" />
														
 
															     <api       label="API Docs"               href="ext:api/index" />
														
 
															     <wiki      label="Wiki"                   href="ext:wiki" />
														
 
															     <faq       label="FAQ"                    href="ext:faq" />
														
--- a/src/docs/src/documentation/content/xdocs/zookeeperAdmin.xml
+++ b/src/docs/src/documentation/content/xdocs/zookeeperAdmin.xml
@@ -116,7 +116,6 @@
 
															           <para>Install the ZooKeeper Server Package. It can be downloaded
														
 
															             from:
														
 
															           </para>
														
 
															-
														
 
															           <para>
														
 
															             <ulink url="http://hadoop.apache.org/zookeeper/releases.html">
														
 
															               http://hadoop.apache.org/zookeeper/releases.html
														
@@ -312,7 +311,16 @@ server.3=zoo3:2888</computeroutput></para>
 
															     <section id="sc_logging">
														
 
															       <title>Logging</title>
														
 
															-      <para></para>
														
 
															+      <para>ZooKeeper uses <emphasis role="bold">log4j</emphasis> version 1.2 as 
														
 
															+      its logging infrastructure. The  ZooKeeper default <filename>log4j.properties</filename> 
														
 
															+      file resides in the <filename>conf</filename> directory. Log4j requires that 
														
 
															+      <filename>log4j.properties</filename> either be in the working directory 
														
 
															+      (the directory from which ZooKeeper is run) or be accessible from the classpath.</para>
														
 
															+
														
 
															+      <para>For more information, see 
														
 
															+      <ulink url="http://logging.apache.org/log4j/1.2/manual.html#defaultInit">Log4j Default Initialization Procedure</ulink> 
														
 
															+      of the log4j manual.</para>
														
 
															+      
														
 
															     </section>
														
 
															     <section id="sc_troubleshooting">
														
--- a/src/docs/src/documentation/content/xdocs/zookeeperInternals.xml
+++ b/src/docs/src/documentation/content/xdocs/zookeeperInternals.xml
@@ -0,0 +1,447 @@
 
															+<?xml version="1.0" encoding="UTF-8"?>
														
 
															+<!--
														
 
															+  Copyright 2002-2004 The Apache Software Foundation
														
 
															+
														
 
															+  Licensed under the Apache License, Version 2.0 (the "License");
														
 
															+  you may not use this file except in compliance with the License.
														
 
															+  You may obtain a copy of the License at
														
 
															+
														
 
															+      http://www.apache.org/licenses/LICENSE-2.0
														
 
															+
														
 
															+  Unless required by applicable law or agreed to in writing, software
														
 
															+  distributed under the License is distributed on an "AS IS" BASIS,
														
 
															+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
														
 
															+  See the License for the specific language governing permissions and
														
 
															+  limitations under the License.
														
 
															+-->
														
 
															+
														
 
															+<!DOCTYPE article PUBLIC "-//OASIS//DTD Simplified DocBook XML V1.0//EN"
														
 
															+"http://www.oasis-open.org/docbook/xml/simple/1.0/sdocbook.dtd">
														
 
															+<article id="ar_ZooKeeperInternals">
														
 
															+  <title>ZooKeeper Internals</title>
														
 
															+
														
 
															+  <articleinfo>
														
 
															+    <legalnotice>
														
 
															+      <para>Licensed under the Apache License, Version 2.0 (the "License");
														
 
															+      you may not use this file except in compliance with the License. You may
														
 
															+      obtain a copy of the License at <ulink
														
 
															+      url="http://www.apache.org/licenses/LICENSE-2.0">http://www.apache.org/licenses/LICENSE-2.0</ulink>.</para>
														
 
															+
														
 
															+      <para>Unless required by applicable law or agreed to in writing,
														
 
															+      software distributed under the License is distributed on an "AS IS"
														
 
															+      BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
														
 
															+      implied. See the License for the specific language governing permissions
														
 
															+      and limitations under the License.</para>
														
 
															+    </legalnotice>
														
 
															+
														
 
															+    <abstract>
														
 
															+      <para>This article contains topics which discuss the inner workings of
														
 
															+       ZooKeeper. So far, that's logging and atomic broadcast. </para>
														
 
															+
														
 
															+    </abstract>
														
 
															+  </articleinfo>
														
 
															+
														
 
															+  <section id="ch_Introduction">
														
 
															+    <title>Introduction</title>
														
 
															+
														
 
															+    <para>This document contains information on the inner workings of ZooKeeper. 
														
 
															+    So far, it discusses these topics:
														
 
															+    </para>
														
 
															+
														
 
															+<itemizedlist>    
														
 
															+<listitem><para><xref linkend="sc_atomicBroadcast"/></para></listitem>
														
 
															+<listitem><para><xref linkend="sc_logging"/></para></listitem>
														
 
															+</itemizedlist>
														
 
															+
														
 
															+</section>
														
 
															+
														
 
															+<section id="sc_atomicBroadcast">
														
 
															+<title>Atomic Broadcast</title>
														
 
															+
														
 
															+<para>
														
 
															+At the heart of ZooKeeper is an atomic messaging system that keeps all of the servers in sync.</para>
														
 
															+
														
 
															+<section id="sc_guaranteesPropertiesDefinitions"><title>Guarantees, Properties, and Definitions</title>
														
 
															+<para>
														
 
															+The specific guarantees provided by the messaging system used by ZooKeeper are the following:</para>
														
 
															+
														
 
															+<variablelist>
														
 
															+
														
 
															+<varlistentry><term><emphasis >Reliable delivery</emphasis></term>
														
 
															+<listitem><para>If a message, m, is delivered 
														
 
															+by one server, it will be eventually delivered by all servers.</para></listitem></varlistentry>
														
 
															+
														
 
															+<varlistentry><term><emphasis >Total order</emphasis></term>
														
 
															+<listitem><para> If a message is 
														
 
															+delivered before message b by one server, a will be delivered before b by all 
														
 
															+servers. If a and b are delivered messages, either a will be delivered before b 
														
 
															+or b will be delivered before a.</para></listitem></varlistentry>
														
 
															+
														
 
															+<varlistentry><term><emphasis >Causal order</emphasis> </term>
														
 
															+
														
 
															+<listitem><para>
														
 
															+If a message b is sent after a message a has been delivered by the sender of b, 
														
 
															+a must be ordered before b. If a sender sends c after sending b, c must be ordered after b.
														
 
															+</para></listitem></varlistentry>
														
 
															+
														
 
															+</variablelist>
														
 
															+
														
 
															+
														
 
															+<para>
														
 
															+The ZooKeeper messaging system also needs to be efficient, reliable, and easy to 
														
 
															+implement and maintain. We make heavy use of messaging, so we need the system to 
														
 
															+be able to handle thousands of requests per second. Although we can require at 
														
 
															+least k+1 correct servers to send new messages, we must be able to recover from 
														
 
															+correlated failures such as power outages. When we implemented the system we had 
														
 
															+little time and few engineering resources, so we needed a protocol that is 
														
 
															+accessible to engineers and is easy to implement. We found that our protocol 
														
 
															+satisfied all of these goals.
														
 
															+
														
 
															+</para>
														
 
															+
														
 
															+<para>
														
 
															+Our protocol assumes that we can construct point-to-point FIFO channels between 
														
 
															+the servers. While similar services usually assume message delivery that can 
														
 
															+lose or reorder messages, our assumption of FIFO channels is very practical 
														
 
															+given that we use TCP for communication. Specifically we rely on the following property of TCP:</para>
														
 
															+
														
 
															+<variablelist>
														
 
															+
														
 
															+<varlistentry>
														
 
															+<term><emphasis >Ordered delivery</emphasis></term>
														
 
															+<listitem><para>Data is delivered in the same order it is sent and a message m is 
														
 
															+delivered only after all messages sent before m have been delivered. 
														
 
															+(The corollary to this is that if message m is lost all messages after m will be lost.)</para></listitem></varlistentry>
														
 
															+
														
 
															+<varlistentry><term><emphasis >No message after close</emphasis></term>
														
 
															+<listitem><para>Once a FIFO channel is closed, no messages will be received from it.</para></listitem></varlistentry>
														
 
															+
														
 
															+</variablelist>
														
 
															+
														
 
															+<para>
														
 
															+FLP proved that consensus cannot be achieved in asynchronous distributed systems 
														
 
															+if failures are possible. To ensure we achieve consensus in the presence of failures 
														
 
															+we use timeouts. However, we rely on times for liveness not for correctness. So, 
														
 
															+if timeouts stop working (clocks malfunction for example) the messaging system may 
														
 
															+hang, but it will not violate its guarantees.</para>
														
 
															+
														
 
															+<para>When describing the ZooKeeper messaging protocol we will talk of packets, 
														
 
															+proposals, and messages:</para>
														
 
															+<variablelist>
														
 
															+<varlistentry><term><emphasis >Packet</emphasis></term>
														
 
															+<listitem><para>a sequence of bytes sent through a FIFO channel</para></listitem></varlistentry><varlistentry>
														
 
															+
														
 
															+<term><emphasis >Proposal</emphasis></term>
														
 
															+<listitem><para>a unit of agreement. Proposals are agreed upon by exchanging packets 
														
 
															+with a quorum of ZooKeeper servers. Most proposals contain messages, however the 
														
 
															+NEW_LEADER proposal is an example of a proposal that does not correspond to a message.</para></listitem>
														
 
															+</varlistentry><varlistentry>
														
 
															+
														
 
															+<term><emphasis >Message</emphasis></term>
														
 
															+<listitem><para>a sequence of bytes to be atomically broadcast to all ZooKeeper 
														
 
															+servers. A message put into a proposal and agreed upon before it is delivered.</para></listitem>
														
 
															+</varlistentry>
														
 
															+
														
 
															+</variablelist>
														
 
															+
														
 
															+<para>
														
 
															+As stated above, ZooKeeper guarantees a total order of messages, and it also 
														
 
															+guarantees a total order of proposals. ZooKeeper exposes the total ordering using
														
 
															+a ZooKeeper transaction id (<emphasis>zxid</emphasis>). All proposals will be stamped with a zxid when 
														
 
															+it is proposed and exactly reflects the total ordering. Proposals are sent to all 
														
 
															+ZooKeeper servers and committed when a quorum of them acknowledge the proposal. 
														
 
															+If a proposal contains a message, the message will be delivered when the proposal 
														
 
															+is committed. Acknowledgement means the server has recorded the proposal to persistent storage. 
														
 
															+Our quorums have the requirement that any pair of quorum must have at least one server 
														
 
															+in common. We ensure this by requiring that all quorums have size (<emphasis>n/2+1</emphasis>) where 
														
 
															+n is the number of servers that make up a ZooKeeper service.
														
 
															+</para>
														
 
															+
														
 
															+<para>
														
 
															+The zxid has two parts: the epoch and a counter. In our implementation the zxid 
														
 
															+is a 64-bit number. We use the high order 32-bits for the epoch and the low order 
														
 
															+32-bits for the counter. Because it has two parts represent the zxid both as a 
														
 
															+number and as a pair of integers, (<emphasis>epoch, count</emphasis>). The epoch number represents a 
														
 
															+change in leadership. Each time a new leader comes into power it will have its 
														
 
															+own epoch number. We have a simple algorithm to assign a unique zxid to a proposal: 
														
 
															+the leader simply increments the zxid to obtain a unique zxid for each proposal. 
														
 
															+<emphasis>Leadership activation will ensure that only one leader uses a given epoch, so our 
														
 
															+simple algorithm guarantees that every proposal will have a unique id.</emphasis>
														
 
															+</para>
														
 
															+
														
 
															+<para>
														
 
															+ZooKeeper messaging consists of two phases:</para>
														
 
															+
														
 
															+<variablelist>
														
 
															+<varlistentry><term><emphasis >Leader activation</emphasis></term>
														
 
															+<listitem><para>In this phase a leader establishes the correct state of the system 
														
 
															+and gets ready to start making proposals.</para></listitem>
														
 
															+</varlistentry>
														
 
															+
														
 
															+<varlistentry><term><emphasis >Active messaging</emphasis></term>
														
 
															+<listitem><para>In this phase a leader accepts messages to propose and coordinates message delivery.</para></listitem>
														
 
															+</varlistentry>
														
 
															+</variablelist>
														
 
															+
														
 
															+<para>
														
 
															+ZooKeeper is a holistic protocol. We do not focus on individual proposals, rather 
														
 
															+look at the stream of proposals as a whole. Our strict ordering allows us to do this 
														
 
															+efficiently and greatly simplifies our protocol. Leadership activation embodies 
														
 
															+this holistic concept. A leader becomes active only when a quorum of followers 
														
 
															+(The leader counts as a follower as well. You can always vote for yourself ) has synced 
														
 
															+up with the leader, they have the same state. This state consists of all of the 
														
 
															+proposals that the leader believes have been committed and the proposal to follow 
														
 
															+the leader, the NEW_LEADER proposal. (Hopefully you are thinking to 
														
 
															+yourself, <emphasis>Does the set of proposals that the leader believes has been committed 
														
 
															+included all the proposals that really have been committed?</emphasis> The answer is <emphasis>yes</emphasis>. 
														
 
															+Below, we make clear why.)
														
 
															+</para>
														
 
															+
														
 
															+</section>
														
 
															+
														
 
															+<section id="sc_leaderElection">
														
 
															+
														
 
															+<title>Leader Activation</title>
														
 
															+<para>
														
 
															+Leader activation includes leader election. We currently have two leader election 
														
 
															+algorithms in ZooKeeper: LeaderElection and FastLeaderElection (AuthFastLeaderElection 
														
 
															+is a variant of FastLeaderElection). ZooKeeper messaging doesn't care about the 
														
 
															+exact method of electing a leader has long as the following holds:
														
 
															+</para>
														
 
															+
														
 
															+<itemizedlist>
														
 
															+
														
 
															+<listitem><para>The leader has seen the highest zxid of all the followers.</para></listitem>
														
 
															+<listitem><para>A quorum of servers have committed to following the leader.</para></listitem>
														
 
															+
														
 
															+</itemizedlist>
														
 
															+
														
 
															+<para>
														
 
															+Of these two requirements only the first, the highest zxid amoung the followers 
														
 
															+needs to hold for correct operation. The second requirement, a quorum of followers, 
														
 
															+just needs to hold with high probability. We are going to recheck the second requirement, 
														
 
															+so if a failure happens during or after the leader election and quorum is lost, 
														
 
															+we will recover by abandoning leader activation and running another election.
														
 
															+</para>
														
 
															+
														
 
															+<para>
														
 
															+After leader election a single server will be designated as a leader and start 
														
 
															+waiting for followers to connect. The rest of the servers will try to connect to 
														
 
															+the leader. The leader will sync up with followers by sending any proposals they 
														
 
															+are missing, or if a follower is missing too many proposals, it will send a full 
														
 
															+snapshot of the state to the follower.
														
 
															+</para>
														
 
															+
														
 
															+<para>
														
 
															+There is a corner cases that a follower arrives that has proposals, U, not seen 
														
 
															+by a leader. Proposals are seen in order, so the proposals of U will have a zxids 
														
 
															+higher than zxids seen by the leader. The follower must have arrived after the 
														
 
															+leader election, otherwise the follower would have been elected leader given that 
														
 
															+it has seen a higher zxid. Since committed proposals must be seen by a quorum of 
														
 
															+servers, and a quorum of servers that elected the leader did not see U, the proposals 
														
 
															+of you have not been committed, so they can be discarded. When the follower connects 
														
 
															+to the leader, the leader will tell the follower to discard U.
														
 
															+</para>
														
 
															+
														
 
															+<para>
														
 
															+A new leader establishes a zxid to start using for new proposals by getting the 
														
 
															+epoch, e, of the highest zxid it has seen and setting the next zxid to use to be 
														
 
															+(e+1, 0), fter the leader syncs with a follower, it will propose a NEW_LEADER 
														
 
															+proposal. Once the NEW_LEADER proposal has been committed, the leader will activate 
														
 
															+and start receiving and issuing proposals.
														
 
															+</para>
														
 
															+
														
 
															+<para>
														
 
															+It all sounds complicated but here are the basic rules of operation during leader 
														
 
															+activation:
														
 
															+</para>
														
 
															+
														
 
															+<itemizedlist>
														
 
															+<listitem><para>A follower will ACK the NEW_LEADER proposal after it has synced with the leader.</para></listitem>
														
 
															+<listitem><para>A follower will only ACK a NEW_LEADER proposal with a given zxid from a single server.</para></listitem>
														
 
															+<listitem><para>A new leader will COMMIT the NEW_LEADER proposal when a quorum of followers have ACKed it.</para></listitem>
														
 
															+<listitem><para>A follower will commit any state it received from the leader when the NEW_LEADER proposal is COMMIT.</para></listitem>
														
 
															+<listitem><para>A new leader will not accept new proposals until the NEW_LEADER proposal has been COMMITED.</para></listitem>
														
 
															+</itemizedlist>
														
 
															+
														
 
															+<para>
														
 
															+If leader election terminates erroneously, we don't have a problem since the 
														
 
															+NEW_LEADER proposal will not be committed since the leader will not have quorum. 
														
 
															+When this happens, the leader and any remaining followers will timeout and go back 
														
 
															+to leader election.
														
 
															+</para>
														
 
															+
														
 
															+</section>
														
 
															+
														
 
															+<section id="sc_activeMessaging">
														
 
															+<title>Active Messaging</title>
														
 
															+<para>
														
 
															+Leader Activation does all the heavy lifting. Once the leader is coronated he can 
														
 
															+start blasting out proposals. As long as he remains the leader no other leader can 
														
 
															+emerge since no other leader will be able to get a quorum of followers. If a new 
														
 
															+leader does emerge, 
														
 
															+it means that the leader has lost quorum, and the new leader will clean up any 
														
 
															+mess left over during her leadership activation.
														
 
															+</para>
														
 
															+
														
 
															+<para>ZooKeeper messaging operates similar to a classic two-phase commit.</para>
														
 
															+
														
 
															+<mediaobject id="fg_2phaseCommit" >
														
 
															+  <imageobject>
														
 
															+    <imagedata fileref="images/2pc.png"/>
														
 
															+  </imageobject>
														
 
															+</mediaobject>
														
 
															+
														
 
															+<para>
														
 
															+All communication channels are FIFO, so everything is done in order. Specifically 
														
 
															+the following operating constraints are observed:</para>
														
 
															+
														
 
															+<itemizedlist>
														
 
															+
														
 
															+<listitem><para>The leader sends proposals to all followers using 
														
 
															+the same order. Moreover, this order follows the order in which requests have been 
														
 
															+received. Because we use FIFO channels this means that followers also receive proposals in order.
														
 
															+</para></listitem>
														
 
															+
														
 
															+<listitem><para>Followers process messages in the order they are received. This 
														
 
															+means that messages will be ACKed in order and the leader will receive ACKs from 
														
 
															+followers in order, due to the FIFO channels. It also means that if message $m$ 
														
 
															+has been written to non-volatile storage, all messages that were proposed before 
														
 
															+$m$ have been written to non-volatile storage.</para></listitem>
														
 
															+
														
 
															+<listitem><para>The leader will issue a COMMIT to all followers as soon as a 
														
 
															+quorum of followers have ACKed a message. Since messages are ACKed in order, 
														
 
															+COMMITs will be sent by the leader as received by the followers in order.</para></listitem>
														
 
															+
														
 
															+<listitem><para>COMMITs are processed in order. Followers deliver a proposals 
														
 
															+message when that proposal is committed.</para></listitem>
														
 
															+
														
 
															+</itemizedlist>
														
 
															+
														
 
															+</section>
														
 
															+
														
 
															+<section id="sc_summary">
														
 
															+<title>Summary</title>
														
 
															+<para>So there you go. Why does it work? Specifically, why does is set of proposals 
														
 
															+believed by a new leader always contain any proposal that has actually been committed? 
														
 
															+First, all proposals have a unique zxid, so unlike other protocols, we never have 
														
 
															+to worry about two different values being proposed for the same zxid; followers 
														
 
															+(a leader is also a follower) see and record proposals in order; proposals are 
														
 
															+committed in order; there is only one active leader at a time since followers only 
														
 
															+follow a single leader at a time; a new leader has seen all committed proposals 
														
 
															+from the previous epoch since it has seen the highest zxid from a quorum of servers; 
														
 
															+any uncommited proposals from a previous epoch seen by a new leader will be committed 
														
 
															+by that leader before it becomes active.</para></section>
														
 
															+
														
 
															+<section id="sc_comparisons"><title>Comparisons</title>
														
 
															+<para>
														
 
															+Isn't this just Multi-Paxos? No, Multi-Paxos requires some way of assuring that 
														
 
															+there is only a single coordinator. We do not count on such assurances. Instead 
														
 
															+we use the leader activation to recover from leadership change or old leaders 
														
 
															+believing they are still active.
														
 
															+</para>
														
 
															+
														
 
															+<para>
														
 
															+Isn't this just Paxos? Your active messaging phase looks just like phase 2 of Paxos? 
														
 
															+Actually, to us active messaging looks just like 2 phase commit without the need to 
														
 
															+handle aborts. Active messaging is different from both in the sense that it has 
														
 
															+cross proposal ordering requirements. If we do not maintain strict FIFO ordering of 
														
 
															+all packets, it all falls apart. Also, our leader activation phase is different from 
														
 
															+both of them. In particular, our use of epochs allows us to skip blocks of uncommitted
														
 
															+proposals and to not worry about duplicate proposals for a given zxid.
														
 
															+</para>
														
 
															+
														
 
															+</section>
														
 
															+
														
 
															+</section>
														
 
															+
														
 
															+
														
 
															+<section id="sc_logging">
														
 
															+
														
 
															+<title>Logging</title>
														
 
															+
														
 
															+<para>
														
 
															+ZooKeeper uses 
														
 
															+<ulink url="http://logging.apache.org/log4j">log4j</ulink>
														
 
															+version 1.2 as its logging infrastructure. For information on configuring log4j for
														
 
															+ZooKeeper, see the <ulink url="zookeeperAdmin.html#sc_logging">Logging</ulink> section 
														
 
															+of the <ulink url="zookeeperAdmin.html">ZooKeeper Administrator's Guide.</ulink>
														
 
															+</para>
														
 
															+
														
 
															+<section id="sc_developerGuidelines"><title>Developer Guidelines</title>
														
 
															+
														
 
															+<para>Please follow these guidelines when submitting code. Patch reviewers will look for the following:</para>
														
 
															+<section id="sc_rightLevel"><title>Logging at the Right Level</title>
														
 
															+<para>
														
 
															+There are <ulink url="http://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/Level.html#FATAL">6 levels of logging in log4j</ulink>. 
														
 
															+It's important to pick the right one. In order of higher to lower severity:</para>
														
 
															+<orderedlist>
														
 
															+   <listitem><para> FATAL level designates very severe error events that will presumably lead the application to abort</para></listitem>
														
 
															+   <listitem><para>ERROR level designates error events that might still allow the application to continue running.</para></listitem>
														
 
															+   <listitem><para>WARN level designates potentially harmful situations.</para></listitem>
														
 
															+   <listitem><para>INFO level designates informational messages that highlight the progress of the application at coarse-grained level.</para></listitem>
														
 
															+   <listitem><para>EBUG Level designates fine-grained informational events that are most useful to debug an application.</para></listitem>
														
 
															+   <listitem><para>TRACE Level designates finer-grained informational events than the DEBUG.</para></listitem>
														
 
															+</orderedlist>
														
 
															+
														
 
															+<para>
														
 
															+ZooKeeper is typically run in production such that log messages of INFO level 
														
 
															+severity and higher (more severe) are output to the log.</para>
														
 
															+
														
 
															+
														
 
															+</section>
														
 
															+
														
 
															+<section id="sc_log4jIdioms"><title>Use of Standard log4j Idioms</title>
														
 
															+
														
 
															+<para><emphasis>Static Message Logging</emphasis></para>
														
 
															+<programlisting>
														
 
															+LOG.debug("process completed successfully!");
														
 
															+</programlisting>
														
 
															+
														
 
															+<para>However when creating a message from a number of components (string 
														
 
															+concatenation), the log call should be wrapped with a "isXEnabled()" call. this 
														
 
															+eliminates the string concatenation overhead when debug level logging is not enabled.
														
 
															+</para>
														
 
															+
														
 
															+<programlisting>
														
 
															+if (LOG.isDebugEnabled()) {
														
 
															+    LOG.debug("got " + count + " messages in " + time + " minutes");
														
 
															+}
														
 
															+</programlisting>
														
 
															+
														
 
															+<para><emphasis>Naming</emphasis></para>
														
 
															+
														
 
															+<para>
														
 
															+Loggers should be named after the class in which they are used. (See the 
														
 
															+<ulink url="http://logging.apache.org/log4j/1.2/faq.html#2.4">log4j faq</ulink> 
														
 
															+for reasons why this is a good idea.)
														
 
															+</para>
														
 
															+
														
 
															+<programlisting>
														
 
															+public class Foo {
														
 
															+    private static final Logger LOG = Logger.getLogger(Foo.class);
														
 
															+    ....
														
 
															+    public Foo() {
														
 
															+       LOG.info("constructing Foo");
														
 
															+</programlisting>
														
 
															+
														
 
															+<para><emphasis>Exception handling</emphasis></para>
														
 
															+<programlisting>
														
 
															+try {
														
 
															+  // code
														
 
															+} catch (XYZException e) {
														
 
															+  // do this
														
 
															+  LOG.error("Something bad happened", e);
														
 
															+  // don't do this (generally)
														
 
															+  // LOG.error(e);
														
 
															+  // why? because "don't do" case hides the stack trace
														
 
															+ 
														
 
															+  // continue process here as you need... recover or (re)throw
														
 
															+}
														
 
															+</programlisting>
														
 
															+</section>
														
 
															+</section>
														
 
															+
														
 
															+</section>
														
 
															+
														
 
															+</article>
														
--- a/src/docs/src/documentation/content/xdocs/zookeeperStarted.xml
+++ b/src/docs/src/documentation/content/xdocs/zookeeperStarted.xml
@@ -71,19 +71,20 @@
 
															       </note>
														
 
															       <para>Once you have downloaded the ZooKeeper source, cd to the root of
														
 
															-      your ZooKeeper source, and run "ant jar". For example:<computeroutput>$ cd ~/dev/zookeeper
														
 
															-
														
 
															-$ ~/dev/zookeeper/: ant jar</computeroutput></para>
														
 
															+      your ZooKeeper source, and run "ant jar". For example:</para>
														
 
															+<programlisting>
														
 
															+$ cd ~/dev/zookeeper
														
 
															+$ ~/dev/zookeeper/: ant jar
														
 
															+</programlisting>
														
 
															       <para>This should generate a JAR file called zookeeper.jar. To start
														
 
															-      Zookeeper, compile and run zookeeper.jar. <emphasis>[tbd, some more
														
 
															-      instruction here. Perhaps a command line? Are these two steps or
														
 
															-      one?]</emphasis></para>
														
 
															+      Zookeeper, compile and run zookeeper.jar.</para>
														
 
															       <para>To start ZooKeeper you need a configuration file. Here is a sample
														
 
															       file:</para>
														
 
															-      <programlisting>tickTime=2000
														
 
															+<programlisting>
														
 
															+tickTime=2000
														
 
															 dataDir=/var/zookeeper/ 
														
 
															 clientPort=2181
														
 
															 </programlisting>
														
@@ -127,7 +128,7 @@ clientPort=2181
 
															       <para>Now that you created the configuration file, you can start
														
 
															       ZooKeeper:</para>
														
 
															-      <para><computeroutput>java -cp zookeeper-dev.jar:java/lib/log4j-1.2.15.jar:conf org.apache.zookeeper.server.quorum.QuorumPeerMain zoo.cfg</computeroutput></para>
														
 
															+      <programlisting>java -cp zookeeper-dev.jar:java/lib/log4j-1.2.15.jar:conf org.apache.zookeeper.server.quorum.QuorumPeerMain zoo.cfg</programlisting>
														
 
															       <para>ZooKeeper logs messages using log4j -- more detail available in
														
 
															       the <ulink url="zookeeperProgrammers.html#Logging">Logging</ulink>
														
@@ -163,7 +164,7 @@ clientPort=2181
 
															         <listitem>
														
 
															           <para><emphasis role="bold">C</emphasis>: compile cli_mt
														
 
															           (multi-threaded) or cli_st (single-threaded) by running
														
 
															-          <command>_make cli_mt_</command> or <command>_make cli_st_</command>
														
 
															+          <command>make cli_mt</command> or <command>make cli_st</command>
														
 
															           in the c subdirectory in the ZooKeeper sources.</para>
														
 
															           <para>You can run the program using <emphasis>LD_LIBRARY_PATH=.
														
@@ -197,13 +198,16 @@ clientPort=2181
 
															       file. The file is similar to the one used in standalone mode, but with a
														
 
															       few differences. Here is an example:</para>
														
 
															-      <para><computeroutput>tickTime=2000 
														
 
															+<programlisting>
														
 
															+tickTime=2000 
														
 
															 dataDir=/var/zookeeper/ 
														
 
															 clientPort=2181 
														
 
															 initLimit=5 
														
 
															 syncLimit=2 
														
 
															-server.1=zoo1:2888 server.2=zoo2:2888 
														
 
															-server.3=zoo3:2888 </computeroutput></para>
														
 
															+server.1=zoo1:2888
														
 
															+server.2=zoo2:2888 
														
 
															+server.3=zoo3:2888 
														
 
															+</programlisting>
														
 
															       <para>The new entry, <emphasis role="bold">initLimit</emphasis> is
														
 
															       timeouts ZooKeeper uses to limit the length of time the Zookeeper
														
--- a/src/docs/src/documentation/content/xdocs/zookeeperTutorial.xml
+++ b/src/docs/src/documentation/content/xdocs/zookeeperTutorial.xml
@@ -0,0 +1,674 @@
 
															+<?xml version="1.0" encoding="UTF-8"?>
														
 
															+<!--
														
 
															+  Copyright 2002-2004 The Apache Software Foundation
														
 
															+
														
 
															+  Licensed under the Apache License, Version 2.0 (the "License");
														
 
															+  you may not use this file except in compliance with the License.
														
 
															+  You may obtain a copy of the License at
														
 
															+
														
 
															+      http://www.apache.org/licenses/LICENSE-2.0
														
 
															+
														
 
															+  Unless required by applicable law or agreed to in writing, software
														
 
															+  distributed under the License is distributed on an "AS IS" BASIS,
														
 
															+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
														
 
															+  See the License for the specific language governing permissions and
														
 
															+  limitations under the License.
														
 
															+-->
														
 
															+
														
 
															+<!DOCTYPE article PUBLIC "-//OASIS//DTD Simplified DocBook XML V1.0//EN"
														
 
															+"http://www.oasis-open.org/docbook/xml/simple/1.0/sdocbook.dtd">
														
 
															+<article id="ar_Tutorial">
														
 
															+  <title>Programming with ZooKeeper - A basic tutorial</title>
														
 
															+
														
 
															+  <articleinfo>
														
 
															+    <legalnotice>
														
 
															+      <para>Licensed under the Apache License, Version 2.0 (the "License");
														
 
															+      you may not use this file except in compliance with the License. You may
														
 
															+      obtain a copy of the License at <ulink
														
 
															+      url="http://www.apache.org/licenses/LICENSE-2.0">http://www.apache.org/licenses/LICENSE-2.0</ulink>.</para>
														
 
															+
														
 
															+      <para>Unless required by applicable law or agreed to in writing,
														
 
															+      software distributed under the License is distributed on an "AS IS"
														
 
															+      BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
														
 
															+      implied. See the License for the specific language governing permissions
														
 
															+      and limitations under the License.</para>
														
 
															+    </legalnotice>
														
 
															+
														
 
															+    <abstract>
														
 
															+      <para>This article contains sample Java code for simple implementations of barrier
														
 
															+      and consumers queues..</para>
														
 
															+
														
 
															+    </abstract>
														
 
															+  </articleinfo>
														
 
															+
														
 
															+  <section id="ch_Introduction">
														
 
															+    <title>Introduction</title>
														
 
															+
														
 
															+    <para>In this tutorial, we show simple implementations of barriers and 
														
 
															+    producer-consumer queues using ZooKeeper. We call the respective classes Barrier and Queue. 
														
 
															+    These examples assume that you have at least one ZooKeeper server running.</para>
														
 
															+    
														
 
															+    <para>Both primitives use the following common excerpt of code:</para>
														
 
															+    
														
 
															+    <programlisting>
														
 
															+static ZooKeeper zk = null;
														
 
															+static Integer mutex;
														
 
															+
														
 
															+String root;
														
 
															+
														
 
															+SyncPrimitive(String address) {
														
 
															+	if(zk == null){
														
 
															+	    try {
														
 
															+		System.out.println("Starting ZK:");
														
 
															+		zk = new ZooKeeper(address, 3000, this);
														
 
															+		mutex = new Integer(-1);
														
 
															+		System.out.println("Finished starting ZK: " + zk);
														
 
															+	    } catch (KeeperException e) {
														
 
															+		System.out.println("Keeper exception when starting new session: "
														
 
															+			+ e.toString());
														
 
															+		zk = null;
														
 
															+	    } catch (IOException e) {
														
 
															+		System.out.println(e.toString());
														
 
															+		zk = null;
														
 
															+	    }
														
 
															+	}
														
 
															+}
														
 
															+
														
 
															+synchronized public void process(WatcherEvent event) {
														
 
															+	synchronized (mutex) {
														
 
															+	    mutex.notify();
														
 
															+	}
														
 
															+}
														
 
															+
														
 
															+</programlisting>
														
 
															+
														
 
															+<para>Both classes extend SyncPrimitive. In this way, we execute steps that are 
														
 
															+common to all primitives in the constructor of SyncPrimitive. To keep the examples 
														
 
															+simple, we create a ZooKeeper object the first time we instantiate either a barrier 
														
 
															+object or a queue object, and we declare a static variable that is a reference 
														
 
															+to this object. The subsequent instances of Barrier and Queue check whether a 
														
 
															+ZooKeeper object exists. Alternatively, we could have the application creating a
														
 
															+ZooKeeper object and passing it to the constructor of Barrier and Queue.</para>
														
 
															+<para>
														
 
															+We use the process() method to process notifications triggered due to watches. 
														
 
															+In the following discussion, we present code that sets watches. A watch is internal 
														
 
															+structure that enables ZooKeeper to notify a client of a change to a node. For example, 
														
 
															+if a client is waiting for other clients to leave a barrier, then it can set a watch and 
														
 
															+wait for modifications to a particular node, which can indicate that it is the end of the wait. 
														
 
															+This point becomes clear once we go over the examples.
														
 
															+</para>
														
 
															+</section>
														
 
															+   
														
 
															+ <section id="sc_barriers"><title>Barriers</title>
														
 
															+ 
														
 
															+ <para>
														
 
															+ A barrier is a primitive that enables a group of processes to synchronize the 
														
 
															+ beginning and the end of a computation. The general idea of this implementation 
														
 
															+ is to have a barrier node that serves the purpose of being a parent for individual 
														
 
															+ process nodes. Suppose that we call the barrier node "/b1". Each process "p" then 
														
 
															+ creates a node "/b1/p". Once enough processes have created their corresponding 
														
 
															+ nodes, joined processes can start the computation.
														
 
															+ </para>
														
 
															+ 
														
 
															+ <para>In this example, each process instantiates a Barrier object, and its constructor takes as parameters:</para>
														
 
															+
														
 
															+ <itemizedlist><listitem><para>the address of a ZooKeeper server (e.g., "zoo1.foo.com:2181")</para></listitem>
														
 
															+<listitem><para>the path of the barrier node on ZooKeeper (e.g., "/b1")</para></listitem>
														
 
															+<listitem><para>the size of the group of processes</para></listitem>
														
 
															+</itemizedlist>
														
 
															+
														
 
															+<para>The constructor of Barrier passes the address of the Zookeeper server to the 
														
 
															+constructor of the parent class. The parent class creates a ZooKeeper instance if 
														
 
															+one does not exist. The constructor of Barrier then creates a 
														
 
															+barrier node on ZooKeeper, which is the parent node of all process nodes, and 
														
 
															+we call root (<emphasis role="bold">Note:</emphasis> This is not the ZooKeeper root "/").</para>
														
 
															+
														
 
															+<programlisting>
														
 
															+ /**
														
 
															+ * Barrier constructor
														
 
															+ *
														
 
															+ * @param address
														
 
															+ * @param name
														
 
															+ * @param size
														
 
															+ */
														
 
															+Barrier(String address, String name, int size) {
														
 
															+    super(address);
														
 
															+    this.root = name;
														
 
															+    this.size = size;
														
 
															+
														
 
															+    // Create barrier node
														
 
															+    if (zk != null) {
														
 
															+	try {
														
 
															+	    Stat s = zk.exists(root, false);
														
 
															+	    if (s == null) {
														
 
															+		zk.create(root, new byte[0], Ids.OPEN_ACL_UNSAFE, 0);
														
 
															+	    }
														
 
															+	} catch (KeeperException e) {
														
 
															+	    System.out.println("Keeper exception when instantiating queue: " + e.toString());
														
 
															+	} catch (InterruptedException e) {
														
 
															+	    System.out.println("Interrupted exception");
														
 
															+	}
														
 
															+    }
														
 
															+
														
 
															+    // My node name
														
 
															+    try {
														
 
															+	name = new String(InetAddress.getLocalHost().getCanonicalHostName().toString());
														
 
															+    } catch (UnknownHostException e) {
														
 
															+	System.out.println(e.toString());
														
 
															+    }
														
 
															+
														
 
															+}
														
 
															+</programlisting>
														
 
															+<para>
														
 
															+To enter the barrier, a process calls enter(). The process creates a node under 
														
 
															+the root to represent it, using its host name to form the node name. It then wait 
														
 
															+until enough processes have entered the barrier. A process does it by checking 
														
 
															+the number of children the root node has with "getChildren()", and waiting for 
														
 
															+notifications in the case it does not have enough. To receive a notification when 
														
 
															+there is a change to the root node, a process has to set a watch, and does it 
														
 
															+through the call to "getChildren()". In the code, we have that "getChildren()" 
														
 
															+has two parameters. The first one states the node to read from, and the second is
														
 
															+a boolean flag that enables the process to set a watch. In the code the flag is true.
														
 
															+</para>
														
 
															+
														
 
															+<programlisting>
														
 
															+ /**
														
 
															+ * Join barrier
														
 
															+ *
														
 
															+ * @return
														
 
															+ * @throws KeeperException
														
 
															+ * @throws InterruptedException
														
 
															+ */
														
 
															+
														
 
															+boolean enter() throws KeeperException, InterruptedException{
														
 
															+    zk.create(root + "/" + name, new byte[0], Ids.OPEN_ACL_UNSAFE,
														
 
															+	    CreateFlags.EPHEMERAL);
														
 
															+    while (true) {
														
 
															+	synchronized (mutex) {
														
 
															+	    ArrayList&lt;String&gt; list = zk.getChildren(root, true);
														
 
															+
														
 
															+	    if (list.size() &lt; size) {
														
 
															+		mutex.wait();
														
 
															+	    } else {
														
 
															+		return true;
														
 
															+	    }
														
 
															+	}
														
 
															+    }
														
 
															+}
														
 
															+</programlisting>
														
 
															+<para>
														
 
															+Note that enter() throws both KeeperException and InterruptedException, so it is 
														
 
															+the reponsability of the application to catch and handle such exceptions.</para>
														
 
															+
														
 
															+<para>
														
 
															+Once the computation is finished, a process calls leave() to leave the barrier. 
														
 
															+First it deletes its corresponding node, and then it gets the children of the root 
														
 
															+node. If there is at least one child, then it waits for a notification (obs: note 
														
 
															+that the second parameter of the call to getChildren() is true, meaning that 
														
 
															+ZooKeeper has to set a watch on the the root node). Upon reception of a notification, 
														
 
															+it checks once more whether the root node has any child.</para>
														
 
															+
														
 
															+<programlisting>
														
 
															+ /**
														
 
															+ * Wait until all reach barrier
														
 
															+ *
														
 
															+ * @return
														
 
															+ * @throws KeeperException
														
 
															+ * @throws InterruptedException
														
 
															+ */
														
 
															+
														
 
															+boolean leave() throws KeeperException, InterruptedException{
														
 
															+    zk.delete(root + "/" + name, 0);
														
 
															+    while (true) {
														
 
															+	synchronized (mutex) {
														
 
															+	    ArrayList&lt;String&gt; list = zk.getChildren(root, true);
														
 
															+		if (list.size() &gt; 0) {
														
 
															+		    mutex.wait();
														
 
															+		} else {
														
 
															+		    return true;
														
 
															+		}
														
 
															+	    }
														
 
															+	}
														
 
															+}
														
 
															+</programlisting>
														
 
															+</section>
														
 
															+<section id="sc_producerConsumerQueues"><title>Producer-Consumer Queues</title>
														
 
															+<para>
														
 
															+A producer-consumer queue is a distributed data estructure thata group of processes 
														
 
															+use to generate and consume items. Producer processes create new elements and add 
														
 
															+them to the queue. Consumer processes remove elements from the list, and process them. 
														
 
															+In this implementation, the elements are simple integers. The queue is represented 
														
 
															+by a root node, and to add an element to the queue, a producer process creates a new node, 
														
 
															+a child of the root node.
														
 
															+</para>
														
 
															+
														
 
															+<para>
														
 
															+The following excerpt of code corresponds to the constructor of the object. As 
														
 
															+with Barrier objects, it first calls the constructor of the parent class, SyncPrimitive, 
														
 
															+that creates a ZooKeeper object if one doesn't exist. It then verifies if the root 
														
 
															+node of the queue exists, and creates if it doesn't.
														
 
															+</para>
														
 
															+<programlisting>
														
 
															+/**
														
 
															+ * Constructor of producer-consumer queue
														
 
															+ *
														
 
															+ * @param address
														
 
															+ * @param name
														
 
															+ */
														
 
															+Queue(String address, String name) {
														
 
															+    super(address);
														
 
															+    this.root = name;
														
 
															+    // Create ZK node name
														
 
															+    if (zk != null) {
														
 
															+	try {
														
 
															+	    Stat s = zk.exists(root, false);
														
 
															+	    if (s == null) {
														
 
															+		zk.create(root, new byte[0], Ids.OPEN_ACL_UNSAFE, 0);
														
 
															+	    }
														
 
															+	} catch (KeeperException e) {
														
 
															+	    System.out
														
 
															+		    .println("Keeper exception when instantiating queue: "
														
 
															+			    + e.toString());
														
 
															+	} catch (InterruptedException e) {
														
 
															+	    System.out.println("Interrupted exception");
														
 
															+	}
														
 
															+    }
														
 
															+}
														
 
															+ </programlisting>
														
 
															+ 
														
 
															+<para>
														
 
															+A producer process calls "produce()" to add an element to the queue, and passes 
														
 
															+an integer as an argument. To add an element to the queue, the method creates a 
														
 
															+new node using "create()", and uses the SEQUENCE flag to instruct ZooKeeper to 
														
 
															+append the value of the sequencer counter associated to the root node. In this way, 
														
 
															+we impose a total order on the elements of the queue, thus guaranteeing that the 
														
 
															+oldest element of the queue is the next one consumed.
														
 
															+</para>
														
 
															+
														
 
															+<programlisting>
														
 
															+/**
														
 
															+ * Add element to the queue.
														
 
															+ *
														
 
															+ * @param i
														
 
															+ * @return
														
 
															+ */
														
 
															+
														
 
															+boolean produce(int i) throws KeeperException, InterruptedException{
														
 
															+    ByteBuffer b = ByteBuffer.allocate(4);
														
 
															+    byte[] value;
														
 
															+
														
 
															+    // Add child with value i
														
 
															+    b.putInt(i);
														
 
															+    value = b.array();
														
 
															+    zk.create(root + "/element", value, Ids.OPEN_ACL_UNSAFE,
														
 
															+		CreateFlags.SEQUENCE);
														
 
															+
														
 
															+    return true;
														
 
															+}
														
 
															+</programlisting>
														
 
															+<para>
														
 
															+To consume an element, a consumer process obtains the children of the root node, 
														
 
															+reads the node with smallest counter value, and returns the element. Note that 
														
 
															+if there is a conflict, then one of the two contending processes won't be able to 
														
 
															+delete the node and the delete operation will throw an exception.</para>
														
 
															+
														
 
															+<para>
														
 
															+A call to getChildren() returns the list of children in lexicographic order. 
														
 
															+As lexicographic order does not necessary follow the numerical order of the counter 
														
 
															+values, we need to decide which element is the smallest. To decide which one has 
														
 
															+the smallest counter value, we traverse the list, and remove the prefix "element" 
														
 
															+from each one.</para>
														
 
															+
														
 
															+<programlisting>
														
 
															+/**
														
 
															+ * Remove first element from the queue.
														
 
															+ *
														
 
															+ * @return
														
 
															+ * @throws KeeperException
														
 
															+ * @throws InterruptedException
														
 
															+ */
														
 
															+int consume() throws KeeperException, InterruptedException{
														
 
															+    int retvalue = -1;
														
 
															+    Stat stat = null;
														
 
															+
														
 
															+    // Get the first element available
														
 
															+    while (true) {
														
 
															+	synchronized (mutex) {
														
 
															+	    ArrayList&lt;String&gt; list = zk.getChildren(root, true);
														
 
															+	    if (list.size() == 0) {
														
 
															+		System.out.println("Going to wait");
														
 
															+		mutex.wait();
														
 
															+	    } else {
														
 
															+		Integer min = new Integer(list.get(0).substring(7));
														
 
															+		for(String s : list){
														
 
															+		    Integer tempValue = new Integer(s.substring(7));
														
 
															+		    if(tempValue &gt; min) min = tempValue;
														
 
															+		}
														
 
															+		System.out.println("Temporary value: " + root + "/element" + min);
														
 
															+		byte[] b = zk.getData(root + "/element" + min, false, stat);
														
 
															+		zk.delete(root + "/element" + min, 0);
														
 
															+		ByteBuffer buffer = ByteBuffer.wrap(b);
														
 
															+		retvalue = buffer.getInt();
														
 
															+
														
 
															+		return retvalue;
														
 
															+	    }
														
 
															+	}
														
 
															+    }
														
 
															+}
														
 
															+</programlisting>
														
 
															+ 
														
 
															+</section>
														
 
															+<section id="sc_sourceListing"><title>Complete Source Listing</title>
														
 
															+<example id="eg_SyncPrimitive_java">
														
 
															+<title>SyncPrimitive.Java</title>
														
 
															+<programlisting>
														
 
															+package com.yahoo.SyncPrimitive;
														
 
															+
														
 
															+import java.io.IOException;
														
 
															+import java.net.InetAddress;
														
 
															+import java.net.UnknownHostException;
														
 
															+import java.nio.ByteBuffer;
														
 
															+import java.lang.InterruptedException;
														
 
															+import java.util.ArrayList;
														
 
															+import java.util.Random;
														
 
															+
														
 
															+import com.yahoo.zookeeper.Watcher;
														
 
															+import com.yahoo.zookeeper.data.Stat;
														
 
															+import com.yahoo.zookeeper.proto.WatcherEvent;
														
 
															+import com.yahoo.zookeeper.KeeperException;
														
 
															+import com.yahoo.zookeeper.ZooDefs.Ids;
														
 
															+import com.yahoo.zookeeper.ZooDefs.CreateFlags;
														
 
															+import com.yahoo.zookeeper.ZooKeeper;
														
 
															+
														
 
															+public class SyncPrimitive implements Watcher {
														
 
															+
														
 
															+    static ZooKeeper zk = null;
														
 
															+    static Integer mutex;
														
 
															+    
														
 
															+    String root;
														
 
															+
														
 
															+    SyncPrimitive(String address) {
														
 
															+        if(zk == null){
														
 
															+            try {
														
 
															+                System.out.println("Starting ZK:");
														
 
															+                zk = new ZooKeeper(address, 3000, this);
														
 
															+                mutex = new Integer(-1);
														
 
															+                System.out.println("Finished starting ZK: " + zk);
														
 
															+            } catch (KeeperException e) {
														
 
															+                System.out.println("Keeper exception when starting new session: "
														
 
															+                        + e.toString());
														
 
															+                zk = null;
														
 
															+            } catch (IOException e) {
														
 
															+                System.out.println(e.toString());
														
 
															+                zk = null;
														
 
															+            }
														
 
															+        }
														
 
															+        //else mutex = new Integer(-1);
														
 
															+    }
														
 
															+
														
 
															+    synchronized public void process(WatcherEvent event) {
														
 
															+        synchronized (mutex) {
														
 
															+            //System.out.println("Process: " + event.getType());
														
 
															+            mutex.notify();
														
 
															+        }
														
 
															+    }
														
 
															+
														
 
															+    /**
														
 
															+     * Barrier
														
 
															+     */
														
 
															+    static public class Barrier extends SyncPrimitive {
														
 
															+        int size;
														
 
															+        String name;
														
 
															+
														
 
															+        /**
														
 
															+         * Barrier constructor
														
 
															+         * 
														
 
															+         * @param address
														
 
															+         * @param name
														
 
															+         * @param size
														
 
															+         */
														
 
															+        Barrier(String address, String name, int size) {
														
 
															+            super(address);
														
 
															+            this.root = name;
														
 
															+            this.size = size;
														
 
															+
														
 
															+            // Create barrier node
														
 
															+            if (zk != null) {
														
 
															+                try {
														
 
															+                    Stat s = zk.exists(root, false);
														
 
															+                    if (s == null) {
														
 
															+                        zk.create(root, new byte[0], Ids.OPEN_ACL_UNSAFE, 0);
														
 
															+                    }
														
 
															+                } catch (KeeperException e) {
														
 
															+                    System.out
														
 
															+                            .println("Keeper exception when instantiating queue: "
														
 
															+                                    + e.toString());
														
 
															+                } catch (InterruptedException e) {
														
 
															+                    System.out.println("Interrupted exception");
														
 
															+                }
														
 
															+            }
														
 
															+            
														
 
															+            // My node name
														
 
															+            try {
														
 
															+                name = new String(InetAddress.getLocalHost().getCanonicalHostName().toString());
														
 
															+            } catch (UnknownHostException e) {
														
 
															+                System.out.println(e.toString());
														
 
															+            }
														
 
															+            
														
 
															+        }
														
 
															+
														
 
															+        /**
														
 
															+         * Join barrier
														
 
															+         * 
														
 
															+         * @return
														
 
															+         * @throws KeeperException
														
 
															+         * @throws InterruptedException
														
 
															+         */
														
 
															+        
														
 
															+        boolean enter() throws KeeperException, InterruptedException{
														
 
															+            zk.create(root + "/" + name, new byte[0], Ids.OPEN_ACL_UNSAFE,
														
 
															+                    CreateFlags.EPHEMERAL);
														
 
															+            while (true) {
														
 
															+                synchronized (mutex) {                    
														
 
															+                    ArrayList&lt;String&gt; list = zk.getChildren(root, true);
														
 
															+       
														
 
															+                    if (list.size() &lt; size) {
														
 
															+                        mutex.wait();
														
 
															+                    } else {
														
 
															+                        return true;
														
 
															+                    }
														
 
															+                }
														
 
															+            }
														
 
															+        }
														
 
															+
														
 
															+        /**
														
 
															+         * Wait until all reach barrier
														
 
															+         * 
														
 
															+         * @return
														
 
															+         * @throws KeeperException
														
 
															+         * @throws InterruptedException
														
 
															+         */
														
 
															+        
														
 
															+        boolean leave() throws KeeperException, InterruptedException{
														
 
															+            zk.delete(root + "/" + name, 0);
														
 
															+            while (true) {
														
 
															+                synchronized (mutex) {
														
 
															+                    ArrayList&lt;String&gt; list = zk.getChildren(root, true);
														
 
															+                        if (list.size() > 0) {
														
 
															+                            mutex.wait();
														
 
															+                        } else {
														
 
															+                            return true;
														
 
															+                        }
														
 
															+                    }
														
 
															+                }
														
 
															+        }
														
 
															+    }
														
 
															+
														
 
															+    /**
														
 
															+     * Producer-Consumer queue
														
 
															+     */
														
 
															+    static public class Queue extends SyncPrimitive {
														
 
															+
														
 
															+        /**
														
 
															+         * Constructor of producer-consumer queue
														
 
															+         * 
														
 
															+         * @param address
														
 
															+         * @param name
														
 
															+         */
														
 
															+        Queue(String address, String name) {
														
 
															+            super(address);
														
 
															+            this.root = name;
														
 
															+            // Create ZK node name
														
 
															+            if (zk != null) {
														
 
															+                try {
														
 
															+                    Stat s = zk.exists(root, false);
														
 
															+                    if (s == null) {
														
 
															+                        zk.create(root, new byte[0], Ids.OPEN_ACL_UNSAFE, 0);
														
 
															+                    }
														
 
															+                } catch (KeeperException e) {
														
 
															+                    System.out
														
 
															+                            .println("Keeper exception when instantiating queue: "
														
 
															+                                    + e.toString());
														
 
															+                } catch (InterruptedException e) {
														
 
															+                    System.out.println("Interrupted exception");
														
 
															+                }
														
 
															+            }
														
 
															+        }
														
 
															+
														
 
															+        /**
														
 
															+         * Add element to the queue.
														
 
															+         * 
														
 
															+         * @param i
														
 
															+         * @return
														
 
															+         */
														
 
															+        
														
 
															+        boolean produce(int i) throws KeeperException, InterruptedException{
														
 
															+            ByteBuffer b = ByteBuffer.allocate(4);
														
 
															+            byte[] value;
														
 
															+
														
 
															+            // Add child with value i
														
 
															+            b.putInt(i);
														
 
															+            value = b.array();
														
 
															+            zk.create(root + "/element", value, Ids.OPEN_ACL_UNSAFE,
														
 
															+                        CreateFlags.SEQUENCE);
														
 
															+            
														
 
															+            return true;
														
 
															+        }
														
 
															+
														
 
															+
														
 
															+        /**
														
 
															+         * Remove first element from the queue.
														
 
															+         * 
														
 
															+         * @return
														
 
															+         * @throws KeeperException
														
 
															+         * @throws InterruptedException
														
 
															+         */
														
 
															+        int consume() throws KeeperException, InterruptedException{
														
 
															+            int retvalue = -1;
														
 
															+            Stat stat = null;
														
 
															+            
														
 
															+            // Get the first element available
														
 
															+            while (true) {
														
 
															+                synchronized (mutex) {
														
 
															+                    ArrayList&lt;String&gt; list = zk.getChildren(root, true);
														
 
															+                    if (list.size() == 0) {
														
 
															+                        System.out.println("Going to wait");
														
 
															+                        mutex.wait();
														
 
															+                    } else {
														
 
															+                        Integer min = new Integer(list.get(0).substring(7));
														
 
															+                        for(String s : list){
														
 
															+                            Integer tempValue = new Integer(s.substring(7));
														
 
															+                            //System.out.println("Temporary value: " + tempValue);
														
 
															+                            if(tempValue &lt; min) min = tempValue;
														
 
															+                        }
														
 
															+                        System.out.println("Temporary value: " + root + "/element" + min);
														
 
															+                        byte[] b = zk.getData(root + "/element" + min,
														
 
															+                                    false, stat);
														
 
															+                        zk.delete(root + "/element" + min, 0);
														
 
															+                        ByteBuffer buffer = ByteBuffer.wrap(b);
														
 
															+                        retvalue = buffer.getInt();
														
 
															+                        
														
 
															+                        return retvalue;
														
 
															+                    }
														
 
															+                }
														
 
															+            }           
														
 
															+        }
														
 
															+    }
														
 
															+
														
 
															+    public static void main(String args[]) {
														
 
															+        if (args[0].equals("qTest"))
														
 
															+            queueTest(args);
														
 
															+        else
														
 
															+            barrierTest(args);
														
 
															+
														
 
															+    }
														
 
															+
														
 
															+    public static void queueTest(String args[]) {
														
 
															+        Queue q = new Queue(args[1], "/app1");
														
 
															+
														
 
															+        System.out.println("Input: " + args[1]);
														
 
															+        int i;
														
 
															+        Integer max = new Integer(args[2]);
														
 
															+
														
 
															+        if (args[3].equals("p")) {
														
 
															+            System.out.println("Producer");
														
 
															+            for (i = 0; i &lt; max; i++)
														
 
															+                try{    
														
 
															+                    q.produce(10 + i);
														
 
															+                } catch (KeeperException e){
														
 
															+                    
														
 
															+                } catch (InterruptedException e){
														
 
															+                    
														
 
															+                }
														
 
															+        } else {
														
 
															+            System.out.println("Consumer");
														
 
															+   
														
 
															+            for (i = 0; i &lt; max; i++) {
														
 
															+                try{
														
 
															+                    int r = q.consume();
														
 
															+                    System.out.println("Item: " + r);
														
 
															+                } catch (KeeperException e){
														
 
															+                    i--;
														
 
															+                } catch (InterruptedException e){
														
 
															+                    
														
 
															+                }
														
 
															+            }
														
 
															+        }
														
 
															+    }
														
 
															+
														
 
															+    public static void barrierTest(String args[]) {
														
 
															+        Barrier b = new Barrier(args[1], "/b1", new Integer(args[2]));
														
 
															+        try{
														
 
															+            boolean flag = b.enter();
														
 
															+            System.out.println("Entered barrier: " + args[2]);
														
 
															+            if(!flag) System.out.println("Error when entering the barrier");
														
 
															+        } catch (KeeperException e){
														
 
															+            
														
 
															+        } catch (InterruptedException e){
														
 
															+            
														
 
															+        }
														
 
															+
														
 
															+        // Generate random integer
														
 
															+        Random rand = new Random();
														
 
															+        int r = rand.nextInt(100);
														
 
															+        // Loop for rand iterations
														
 
															+        for (int i = 0; i &lt; r; i++) {
														
 
															+            try {
														
 
															+                Thread.sleep(100);
														
 
															+            } catch (InterruptedException e) {
														
 
															+
														
 
															+            }
														
 
															+        }
														
 
															+        try{
														
 
															+            b.leave();
														
 
															+        } catch (KeeperException e){
														
 
															+            
														
 
															+        } catch (InterruptedException e){
														
 
															+            
														
 
															+        }
														
 
															+        System.out.println("Left barrier");
														
 
															+    }
														
 
															+}</programlisting></example>
														
 
															+</section>
														
 
															+
														
 
															+</article>
														
--- a/src/docs/src/documentation/resources/images/2pc.png
+++ b/src/docs/src/documentation/resources/images/2pc.png
--- a/src/docs/src/documentation/skinconf.xml
+++ b/src/docs/src/documentation/skinconf.xml
@@ -142,10 +142,20 @@ which will be used to configure the chosen Forrest skin.
 
															       font-family: monospace;
														
 
															     }
														
 
															+    pre.code {
														
 
															+      margin-left: 0em;
														
 
															+      padding: 0.5em;
														
 
															+      background-color: #f0f0f0;
														
 
															+      font-family: monospace;
														
 
															+    }
														
 
															+
														
 
															+<!-- patricks
														
 
															     .code {
														
 
															       font-family: "Courier New", Courier, monospace;
														
 
															       font-size: 110%;
														
 
															     }
														
 
															+-->
														
 
															+
														
 
															   </extra-css>
														
 
															   <colors>