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<h1>ZooKeeper Getting Started Guide</h1>
<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_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>
  

  

  
<a name="N10009"></a><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="N10016"></a><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="N10024"></a><a name="sc_Download"></a>
<h3 class="h4">Download</h3>
<p>To get a ZooKeeper distribution, download a recent
        <a href="http://hadoop.apache.org/zookeeper/releases.html">
          stable</a> release from one of the Apache Download
        Mirrors.</p>
<a name="N10032"></a><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/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="N10083"></a><a name="sc_ConnectingToZooKeeper"></a>
<h3 class="h4">Connecting to ZooKeeper</h3>
<p>Once ZooKeeper is running, you have several options for connection
      to it:</p>
<ul>
        
<li>
          
<p>
<strong>Java</strong>: Use</p>

          
<pre class="code">bin/zkCli.sh 127.0.0.1:2181</pre>

          
<p>This lets you perform simple, file-like operations.</p>
        
</li>

        
<li>
          
<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> in
          the <strong>src/c</strong> subdirectory in
          the ZooKeeper sources. See the README contained within
          <strong>src/c</strong> for full details.</p>

          
<p>You can run the program
          from <strong>src/c</strong> using:</p>

          
<pre class="code">LD_LIBRARY_PATH=. cli_mt 127.0.0.1:2181</pre>

          
<p>or</p>

          
<pre class="code">LD_LIBRARY_PATH=. cli_st 127.0.0.1:2181</pre>
          
<p>This will give you a simple shell to execute file
          system like operations on ZooKeeper.</p>
        
</li>
      
</ul>
<a name="N100C6"></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
      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.html">Programming
      Examples in the ZooKeeper Programmer's Guide</a> for
      sample code using of the different APIs.</p>
<a name="N100D4"></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
      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. The file 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/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>
      
</div>
</div>
<a name="N10111"></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>
<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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