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<h1>ZooKeeper Getting Started Guide</h1>
<div id="front-matter">
<div id="minitoc-area">
<ul class="minitoc">
<li>
<a href="#ch_GettingStarted">Getting Started: Coordinating Distributed Applications with
      ZooKeeper</a>
<ul class="minitoc">
<li>
<a href="#sc_Prerequisites">Pre-requisites</a>
</li>
<li>
<a href="#sc_Download">Download</a>
</li>
<li>
<a href="#sc_InstallingSingleMode">Standalone Operation</a>
</li>
<li>
<a href="#sc_FileManagement">Managing ZooKeeper Storage</a>
</li>
<li>
<a href="#sc_ConnectingToZooKeeper">Connecting to ZooKeeper</a>
</li>
<li>
<a href="#sc_ProgrammingToZooKeeper">Programming to ZooKeeper</a>
</li>
<li>
<a href="#sc_RunningReplicatedZooKeeper">Running Replicated ZooKeeper</a>
</li>
<li>
<a href="#Other+Optimizations">Other Optimizations</a>
</li>
</ul>
</li>
</ul>
</div>
</div>
  

  

  
<a name="ch_GettingStarted"></a>
<h2 class="h3">Getting Started: Coordinating Distributed Applications with
      ZooKeeper</h2>
<div class="section">
<p>This document contains information to get you started quickly with
    ZooKeeper. It is aimed primarily at developers hoping to try it out, and
    contains simple installation instructions for a single ZooKeeper server, a
    few commands to verify that it is running, and a simple programming
    example. Finally, as a convenience, there are a few sections regarding
    more complicated installations, for example running replicated
    deployments, and optimizing the transaction log. However for the complete
    instructions for commercial deployments, please refer to the <a href="zookeeperAdmin.html">ZooKeeper
    Administrator's Guide</a>.</p>
<a name="sc_Prerequisites"></a>
<h3 class="h4">Pre-requisites</h3>
<p>See <a href="zookeeperAdmin.html#sc_systemReq">
          System Requirements</a> in the Admin guide.</p>
<a name="sc_Download"></a>
<h3 class="h4">Download</h3>
<p>To get a ZooKeeper distribution, download a recent
        <a href="http://zookeeper.apache.org/releases.html">
          stable</a> release from one of the Apache Download
        Mirrors.</p>
<a name="sc_InstallingSingleMode"></a>
<h3 class="h4">Standalone Operation</h3>
<p>Setting up a ZooKeeper server in standalone mode is
      straightforward. The server is contained in a single JAR file,
      so installation consists of creating a configuration.</p>
<p>Once you've downloaded a stable ZooKeeper release unpack
      it and cd to the root</p>
<p>To start ZooKeeper you need a configuration file. Here is a sample,
      create it in <strong>conf/zoo.cfg</strong>:</p>
<pre class="code">
tickTime=2000
dataDir=/var/lib/zookeeper
clientPort=2181
</pre>
<p>This file can be called anything, but for the sake of this
      discussion call
      it <strong>conf/zoo.cfg</strong>. Change the
      value of <strong>dataDir</strong> to specify an
      existing (empty to start with) directory.  Here are the meanings
      for each of the fields:</p>
<dl>
        
<dt>
<term>
<strong>tickTime</strong>
</term>
</dt>
<dd>
<p>the basic time unit in milliseconds used by ZooKeeper. It is
            used to do heartbeats and the minimum session timeout will be
            twice the tickTime.</p>
</dd>
      
</dl>
<dl>
        
<dt>
<term>
<strong>dataDir</strong>
</term>
</dt>
<dd>
<p>the location to store the in-memory database snapshots and,
            unless specified otherwise, the transaction log of updates to the
            database.</p>
</dd>

        
<dt>
<term>
<strong>clientPort</strong>
</term>
</dt>
<dd>
<p>the port to listen for client connections</p>
</dd>
      
</dl>
<p>Now that you created the configuration file, you can start
      ZooKeeper:</p>
<pre class="code">bin/zkServer.sh start</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. You will see log messages
      coming to the console (default) and/or a log file depending on
      the log4j configuration.</p>
<p>The steps outlined here run ZooKeeper in standalone mode. There is
      no replication, so if ZooKeeper process fails, the service will go down.
      This is fine for most development situations, but to run ZooKeeper in
      replicated mode, please see <a href="#sc_RunningReplicatedZooKeeper">Running Replicated
      ZooKeeper</a>.</p>
<a name="sc_FileManagement"></a>
<h3 class="h4">Managing ZooKeeper Storage</h3>
<p>For long running production systems ZooKeeper storage must
      be managed externally (dataDir and logs). See the section on
      <a href="zookeeperAdmin.html#sc_maintenance">maintenance</a> for
      more details.</p>
<a name="sc_ConnectingToZooKeeper"></a>
<h3 class="h4">Connecting to ZooKeeper</h3>
<pre class="code">$ bin/zkCli.sh -server 127.0.0.1:2181</pre>
<p>This lets you perform simple, file-like operations.</p>
<p>Once you have connected, you should see something like:
        </p>
<pre class="code">

Connecting to localhost:2181
log4j:WARN No appenders could be found for logger (org.apache.zookeeper.ZooKeeper).
log4j:WARN Please initialize the log4j system properly.
Welcome to ZooKeeper!
JLine support is enabled
[zkshell: 0]
        </pre>
<p>
        From the shell, type <span class="codefrag command">help</span> to get a listing of commands that can be executed from the client, as in:
      </p>
<pre class="code">

[zkshell: 0] help
ZooKeeper host:port cmd args
        get path [watch]
        ls path [watch]
        set path data [version]
        delquota [-n|-b] path
        quit
        printwatches on|off
        createpath data acl
        stat path [watch]
        listquota path
        history
        setAcl path acl
        getAcl path
        sync path
        redo cmdno
        addauth scheme auth
        delete path [version]
        setquota -n|-b val path

        </pre>
<p>From here, you can try a few simple commands to get a feel for this simple command line interface.  First, start by issuing the list command, as
      in <span class="codefrag command">ls</span>, yielding:
      </p>
<pre class="code">

[zkshell: 8] ls /
[zookeeper]
        </pre>
<p>Next, create a new znode by running <span class="codefrag command">create /zk_test my_data</span>. This creates a new znode and associates the string "my_data" with the node.
      You should see:</p>
<pre class="code">

[zkshell: 9] create /zk_test my_data
Created /zk_test
      </pre>
<p>  Issue another <span class="codefrag command">ls /</span> command to see what the directory looks like:
        </p>
<pre class="code">

[zkshell: 11] ls /
[zookeeper, zk_test]

        </pre>
<p>
      Notice that the zk_test directory has now been created.
      </p>
<p>Next, verify that the data was associated with the znode by running the <span class="codefrag command">get</span> command, as in:
      </p>
<pre class="code">

[zkshell: 12] get /zk_test
my_data
cZxid = 5
ctime = Fri Jun 05 13:57:06 PDT 2009
mZxid = 5
mtime = Fri Jun 05 13:57:06 PDT 2009
pZxid = 5
cversion = 0
dataVersion = 0
aclVersion = 0
ephemeralOwner = 0
dataLength = 7
numChildren = 0
        </pre>
<p>We can change the data associated with zk_test by issuing the <span class="codefrag command">set</span> command, as in:
        </p>
<pre class="code">

[zkshell: 14] set /zk_test junk
cZxid = 5
ctime = Fri Jun 05 13:57:06 PDT 2009
mZxid = 6
mtime = Fri Jun 05 14:01:52 PDT 2009
pZxid = 5
cversion = 0
dataVersion = 1
aclVersion = 0
ephemeralOwner = 0
dataLength = 4
numChildren = 0
[zkshell: 15] get /zk_test
junk
cZxid = 5
ctime = Fri Jun 05 13:57:06 PDT 2009
mZxid = 6
mtime = Fri Jun 05 14:01:52 PDT 2009
pZxid = 5
cversion = 0
dataVersion = 1
aclVersion = 0
ephemeralOwner = 0
dataLength = 4
numChildren = 0
      </pre>
<p>
       (Notice we did a <span class="codefrag command">get</span> after setting the data and it did, indeed, change.</p>
<p>Finally, let's <span class="codefrag command">delete</span> the node by issuing:
      </p>
<pre class="code">

[zkshell: 16] delete /zk_test
[zkshell: 17] ls /
[zookeeper]
[zkshell: 18]
</pre>
<p>That's it for now.  To explore more, continue with the rest of this document and see the <a href="zookeeperProgrammers.html">Programmer's Guide</a>. </p>
<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
      threaded and multi-threaded. These differ only in how the messaging loop
      is done. For more information, see the <a href="zookeeperProgrammers.html#ch_programStructureWithExample">Programming
      Examples in the ZooKeeper Programmer's Guide</a> for
      sample code using of the different APIs.</p>
<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
      ZooKeeper in replicated mode. A replicated group of servers in the same
      application is called a <em>quorum</em>, and in replicated
      mode, all servers in the quorum have copies of the same configuration
      file.</p>
<div class="note">
<div class="label">Note</div>
<div class="content">
      
<p>
         For replicated mode, a minimum of three servers are required,
         and it is strongly recommended that you have an odd number of
         servers. If you only have two servers, then you are in a
         situation where if one of them fails, there are not enough
         machines to form a majority quorum. Two servers is inherently
         <strong>less</strong>
         stable than a single server, because there are two single
         points of failure.
      </p>
   
</div>
</div>
<p>
      The required
      <strong>conf/zoo.cfg</strong>
      file for replicated mode is similar to the one used in standalone
      mode, but with a few differences. Here is an example:
   </p>
<pre class="code">
tickTime=2000
dataDir=/var/lib/zookeeper
clientPort=2181
initLimit=5
syncLimit=2
server.1=zoo1:2888:3888
server.2=zoo2:2888:3888
server.3=zoo3:2888:3888
</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
      be from a leader.</p>
<p>With both of these timeouts, you specify the unit of time using
      <strong>tickTime</strong>. In this example, the timeout
      for initLimit is 5 ticks at 2000 milleseconds a tick, or 10
      seconds.</p>
<p>The entries of the form <em>server.X</em> list the
      servers that make up the ZooKeeper service. When the server starts up,
      it knows which server it is by looking for the file
      <em>myid</em> in the data directory. That file has the 
      contains the server number, in ASCII.</p>
<p>Finally, note the two port numbers after each server
       name: " 2888" and "3888". Peers use the former port to connect
       to other peers. Such a connection is necessary so that peers
       can communicate, for example, to agree upon the order of
       updates. More specifically, a ZooKeeper server uses this port
       to connect followers to the leader. When a new leader arises, a
       follower opens a TCP connection to the leader using this
       port. Because the default leader election also uses TCP, we
       currently require another port for leader election. This is the
       second port in the server entry.
       </p>
<div class="note">
<div class="label">Note</div>
<div class="content">
        
<p>If you want to test multiple servers on a single
        machine, specify the servername
        as <em>localhost</em> with unique quorum &amp;
        leader election ports (i.e. 2888:3888, 2889:3889, 2890:3890 in
        the example above) for each server.X in that server's config
        file. Of course separate <em>dataDir</em>s and
        distinct <em>clientPort</em>s are also necessary
        (in the above replicated example, running on a
        single <em>localhost</em>, you would still have
        three config files).</p>
        
<p>Please be aware that setting up multiple servers on a single
            machine will not create any redundancy. If something were to
            happen which caused the machine to die, all of the zookeeper
            servers would be offline. Full redundancy requires that each
            server have its own machine. It must be a completely separate
            physical server. Multiple virtual machines on the same physical
            host are still vulnerable to the complete failure of that host.</p>
      
</div>
</div>
<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>
<ul>
        
<li>
          
<p>To get low latencies on updates it is important to
          have a dedicated transaction log directory. By default
          transaction logs are put in the same directory as the data
          snapshots and <em>myid</em> file. The dataLogDir
          parameters indicates a different directory to use for the
          transaction logs.</p>
        
</li>

        
<li>
          
<p>
<em>[tbd: what is the other config param?]</em>
</p>
        
</li>
      
</ul>
</div>

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