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<h1>Programming with ZooKeeper - A basic tutorial</h1>
<div id="front-matter">
<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>
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</div>
  

<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 (IOException e) {
                System.out.println(e.toString());
                zk = null;
            }
        }
    }

    synchronized public void process(WatchedEvent 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="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 root
         * @param size
         */
        Barrier(String address, String root, int size) {
            super(address);
            this.root = root;
            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,
                                CreateMode.PERSISTENT);
                    }
                } 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,
                    CreateMode.EPHEMERAL_SEQUENTIAL);
            while (true) {
                synchronized (mutex) {
                    List&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) {
                    List&lt;String&gt; list = zk.getChildren(root, true);
                        if (list.size() &gt; 0) {
                            mutex.wait();
                        } else {
                            return true;
                        }
                    }
                }
        }
    }
</pre>
</div>

<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,
                                CreateMode.PERSISTENT);
                    }
                } 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,
                        CreateMode.PERSISTENT_SEQUENTIAL);

            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) {
                    List&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;
                    }
                }
            }
        }
    }
</pre>
</div>

<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">
import java.io.IOException;
import java.net.InetAddress;
import java.net.UnknownHostException;
import java.nio.ByteBuffer;
import java.util.List;
import java.util.Random;

import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.ZooDefs.Ids;
import org.apache.zookeeper.data.Stat;

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 (IOException e) {
                System.out.println(e.toString());
                zk = null;
            }
        }
        //else mutex = new Integer(-1);
    }

    synchronized public void process(WatchedEvent 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 root
         * @param size
         */
        Barrier(String address, String root, int size) {
            super(address);
            this.root = root;
            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,
                                CreateMode.PERSISTENT);
                    }
                } 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,
                    CreateMode.EPHEMERAL_SEQUENTIAL);
            while (true) {
                synchronized (mutex) {
                    List&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) {
                    List&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,
                                CreateMode.PERSISTENT);
                    }
                } 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,
                        CreateMode.PERSISTENT_SEQUENTIAL);

            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) {
                    List&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>
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