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<h1>ZooKeeper Administrator's Guide</h1>
<h3>A Guide to Deployment and Administration</h3>
<div id="front-matter">
<div id="minitoc-area">
<ul class="minitoc">
<li>
<a href="#ch_deployment">Deployment</a>
<ul class="minitoc">
<li>
<a href="#sc_systemReq">System Requirements</a>
<ul class="minitoc">
<li>
<a href="#sc_supportedPlatforms">Supported Platforms</a>
</li>
<li>
<a href="#sc_requiredSoftware">Required Software </a>
</li>
</ul>
</li>
<li>
<a href="#sc_zkMulitServerSetup">Clustered (Multi-Server) Setup</a>
</li>
<li>
<a href="#sc_singleAndDevSetup">Single Server and Developer Setup</a>
</li>
</ul>
</li>
<li>
<a href="#ch_administration">Administration</a>
<ul class="minitoc">
<li>
<a href="#sc_designing">Designing a ZooKeeper Deployment</a>
<ul class="minitoc">
<li>
<a href="#sc_CrossMachineRequirements">Cross Machine Requirements</a>
</li>
<li>
<a href="#Single+Machine+Requirements">Single Machine Requirements</a>
</li>
</ul>
</li>
<li>
<a href="#sc_provisioning">Provisioning</a>
</li>
<li>
<a href="#sc_strengthsAndLimitations">Things to Consider: ZooKeeper Strengths and Limitations</a>
</li>
<li>
<a href="#sc_administering">Administering</a>
</li>
<li>
<a href="#sc_maintenance">Maintenance</a>
<ul class="minitoc">
<li>
<a href="#Ongoing+Data+Directory+Cleanup">Ongoing Data Directory Cleanup</a>
</li>
<li>
<a href="#Debug+Log+Cleanup+%28log4j%29">Debug Log Cleanup (log4j)</a>
</li>
</ul>
</li>
<li>
<a href="#sc_supervision">Supervision</a>
</li>
<li>
<a href="#sc_monitoring">Monitoring</a>
</li>
<li>
<a href="#sc_logging">Logging</a>
</li>
<li>
<a href="#sc_troubleshooting">Troubleshooting</a>
</li>
<li>
<a href="#sc_configuration">Configuration Parameters</a>
<ul class="minitoc">
<li>
<a href="#sc_minimumConfiguration">Minimum Configuration</a>
</li>
<li>
<a href="#sc_advancedConfiguration">Advanced Configuration</a>
</li>
<li>
<a href="#sc_clusterOptions">Cluster Options</a>
</li>
<li>
<a href="#sc_authOptions">Authentication &amp; Authorization Options</a>
</li>
<li>
<a href="#Experimental+Options%2FFeatures">Experimental Options/Features</a>
</li>
<li>
<a href="#Unsafe+Options">Unsafe Options</a>
</li>
<li>
<a href="#Disabling+data+directory+autocreation">Disabling data directory autocreation</a>
</li>
<li>
<a href="#sc_performance_options">Performance Tuning Options</a>
</li>
<li>
<a href="#Communication+using+the+Netty+framework">Communication using the Netty framework</a>
</li>
</ul>
</li>
<li>
<a href="#sc_zkCommands">ZooKeeper Commands: The Four Letter Words</a>
</li>
<li>
<a href="#sc_dataFileManagement">Data File Management</a>
<ul class="minitoc">
<li>
<a href="#The+Data+Directory">The Data Directory</a>
</li>
<li>
<a href="#The+Log+Directory">The Log Directory</a>
</li>
<li>
<a href="#sc_filemanagement">File Management</a>
</li>
</ul>
</li>
<li>
<a href="#sc_commonProblems">Things to Avoid</a>
</li>
<li>
<a href="#sc_bestPractices">Best Practices</a>
</li>
</ul>
</li>
</ul>
</div>
</div>
  

<a name="ch_deployment"></a>
<h2 class="h3">Deployment</h2>
<div class="section">
<p>This section contains information about deploying Zookeeper and
    covers these topics:</p>
<ul>
      
<li>
        
<p>
<a href="#sc_systemReq">System Requirements</a>
</p>
      
</li>

      
<li>
        
<p>
<a href="#sc_zkMulitServerSetup">Clustered (Multi-Server) Setup</a>
</p>
      
</li>

      
<li>
        
<p>
<a href="#sc_singleAndDevSetup">Single Server and Developer Setup</a>
</p>
      
</li>
    
</ul>
<p>The first two sections assume you are interested in installing
    ZooKeeper in a production environment such as a datacenter. The final
    section covers situations in which you are setting up ZooKeeper on a
    limited basis - for evaluation, testing, or development - but not in a
    production environment.</p>
<a name="sc_systemReq"></a>
<h3 class="h4">System Requirements</h3>
<a name="sc_supportedPlatforms"></a>
<h4>Supported Platforms</h4>
<ul>
          
<li>
            
<p>GNU/Linux is supported as a development and production
              platform for both server and client.</p>
          
</li>
          
<li>
            
<p>Sun Solaris is supported as a development and production
              platform for both server and client.</p>
          
</li>
          
<li>
            
<p>FreeBSD is supported as a development and production
              platform for both server and client.</p>
          
</li>
          
<li>
            
<p>Win32 is supported as a <em>development
            platform</em> only for both server and client.</p>
          
</li>
          
<li>
            
<p>Win64 is supported as a <em>development
            platform</em> only for both server and client.</p>
          
</li>
          
<li>
            
<p>MacOSX is supported as a <em>development
            platform</em> only for both server and client.</p>
          
</li>
        
</ul>
<a name="sc_requiredSoftware"></a>
<h4>Required Software </h4>
<p>ZooKeeper runs in Java, release 1.6 or greater (JDK 6 or
        greater, FreeBSD support requires openjdk7).  It runs as an
        <em>ensemble</em> of ZooKeeper servers. Three
        ZooKeeper servers is the minimum recommended size for an
        ensemble, and we also recommend that they run on separate
        machines. At Yahoo!, ZooKeeper is usually deployed on
        dedicated RHEL boxes, with dual-core processors, 2GB of RAM,
        and 80GB IDE hard drives.</p>
<a name="sc_zkMulitServerSetup"></a>
<h3 class="h4">Clustered (Multi-Server) Setup</h3>
<p>For reliable ZooKeeper service, you should deploy ZooKeeper in a
      cluster known as an <em>ensemble</em>. As long as a majority
      of the ensemble are up, the service will be available. Because Zookeeper
      requires a majority, it is best to use an
      odd number of machines. For example, with four machines ZooKeeper can
      only handle the failure of a single machine; if two machines fail, the
      remaining two machines do not constitute a majority. However, with five
      machines ZooKeeper can handle the failure of two machines. </p>
<p>Here are the steps to setting a server that will be part of an
      ensemble. These steps should be performed on every host in the
      ensemble:</p>
<ol>
        
<li>
          
<p>Install the Java JDK. You can use the native packaging system
          for your system, or download the JDK from:</p>

          
<p>
<a href="http://java.sun.com/javase/downloads/index.jsp">http://java.sun.com/javase/downloads/index.jsp</a>
</p>
        
</li>

        
<li>
          
<p>Set the Java heap size. This is very important to avoid
          swapping, which will seriously degrade ZooKeeper performance. To
          determine the correct value, use load tests, and make sure you are
          well below the usage limit that would cause you to swap. Be
          conservative - use a maximum heap size of 3GB for a 4GB
          machine.</p>
        
</li>

        
<li>
          
<p>Install the ZooKeeper Server Package. It can be downloaded
            from:
          </p>
          
<p>
            
<a href="http://zookeeper.apache.org/releases.html">
              http://zookeeper.apache.org/releases.html
            </a>
          
</p>
        
</li>

        
<li>
          
<p>Create a configuration file. This file can be called anything.
          Use the following settings as a starting point:</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>You can find the meanings of these and other configuration
          settings in the section <a href="#sc_configuration">Configuration Parameters</a>. A word
          though about a few here:</p>

          
<p>Every machine that is part of the ZooKeeper ensemble should know
          about every other machine in the ensemble. You accomplish this with
          the series of lines of the form <strong>server.id=host:port:port</strong>. The parameters <strong>host</strong> and <strong>port</strong> are straightforward. You attribute the
          server id to each machine by creating a file named
          <span class="codefrag filename">myid</span>, one for each server, which resides in
          that server's data directory, as specified by the configuration file
          parameter <strong>dataDir</strong>.</p>
</li>

          
<li>
<p>The myid file
          consists of a single line containing only the text of that machine's
          id. So <span class="codefrag filename">myid</span> of server 1 would contain the text
          "1" and nothing else. The id must be unique within the
          ensemble and should have a value between 1 and 255.</p>
        
</li>

        
<li>
          
<p>If your configuration file is set up, you can start a
          ZooKeeper server:</p>

          
<p>
<span class="codefrag computeroutput">$ java -cp zookeeper.jar:lib/slf4j-api-1.7.5.jar:lib/slf4j-log4j12-1.7.5.jar:lib/log4j-1.2.16.jar:conf \
              org.apache.zookeeper.server.quorum.QuorumPeerMain zoo.cfg
          </span>
</p>
          
          
<p>QuorumPeerMain starts a ZooKeeper server,
            <a href="http://java.sun.com/javase/technologies/core/mntr-mgmt/javamanagement/">JMX</a>
            management beans are also registered which allows
            management through a JMX management console. 
            The <a href="zookeeperJMX.html">ZooKeeper JMX
            document</a> contains details on managing ZooKeeper with JMX.
          </p>

          
<p>See the script <em>bin/zkServer.sh</em>,
            which is included in the release, for an example
            of starting server instances.</p>

        
</li>

        
<li>
          
<p>Test your deployment by connecting to the hosts:</p>

          
<ul>
            
<li>
              
<p>In Java, you can run the following command to execute
              simple operations:</p>

              
<p>
<span class="codefrag computeroutput">$ java -cp zookeeper.jar:lib/slf4j-api-1.7.5.jar:lib/slf4j-log4j12-1.7.5.jar:lib/log4j-1.2.16.jar:conf:src/java/lib/jline-2.11.jar \
      org.apache.zookeeper.ZooKeeperMain -server 127.0.0.1:2181</span>
</p>
            
</li>

            
<li>
              
<p>In C, you can compile either the single threaded client or
              the multithreaded client: or n the c subdirectory in the
              ZooKeeper sources. This compiles the single threaded
              client:</p>

              
<p>
<span class="codefrag computeroutput">$ make cli_st</span>
</p>

              
<p>And this compiles the mulithreaded client:</p>

              
<p>
<span class="codefrag computeroutput">$ make cli_mt</span>
</p>
            
</li>
          
</ul>

          
<p>Running either program gives you a shell in which to execute
          simple file-system-like operations. To connect to ZooKeeper with the
          multithreaded client, for example, you would run:</p>

          
<p>
<span class="codefrag computeroutput">$ cli_mt 127.0.0.1:2181</span>
</p>
        
</li>
      
</ol>
<a name="sc_singleAndDevSetup"></a>
<h3 class="h4">Single Server and Developer Setup</h3>
<p>If you want to setup ZooKeeper for development purposes, you will
      probably want to setup a single server instance of ZooKeeper, and then
      install either the Java or C client-side libraries and bindings on your
      development machine.</p>
<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
      Guide</a>.</p>
<p>For information on installing the client side libraries, refer to
      the <a href="zookeeperProgrammers.html#Bindings">Bindings</a>
      section of the <a href="zookeeperProgrammers.html">ZooKeeper
      Programmer's Guide</a>.</p>
</div>

  
<a name="ch_administration"></a>
<h2 class="h3">Administration</h2>
<div class="section">
<p>This section contains information about running and maintaining
    ZooKeeper and covers these topics: </p>
<ul>
        
<li>
          
<p>
<a href="#sc_designing">Designing a ZooKeeper Deployment</a>
</p>
        
</li>

        
<li>
          
<p>
<a href="#sc_provisioning">Provisioning</a>
</p>
        
</li>

        
<li>
          
<p>
<a href="#sc_strengthsAndLimitations">Things to Consider: ZooKeeper Strengths and Limitations</a>
</p>
        
</li>

        
<li>
          
<p>
<a href="#sc_administering">Administering</a>
</p>
        
</li>

        
<li>
          
<p>
<a href="#sc_maintenance">Maintenance</a>
</p>
        
</li>

        
<li>
          
<p>
<a href="#sc_supervision">Supervision</a>
</p>
        
</li>

        
<li>
          
<p>
<a href="#sc_monitoring">Monitoring</a>
</p>
        
</li>

        
<li>
          
<p>
<a href="#sc_logging">Logging</a>
</p>
        
</li>

        
<li>
          
<p>
<a href="#sc_troubleshooting">Troubleshooting</a>
</p>
        
</li>

        
<li>
          
<p>
<a href="#sc_configuration">Configuration Parameters</a>
</p>
        
</li>

        
<li>
          
<p>
<a href="#sc_zkCommands">ZooKeeper Commands: The Four Letter Words</a>
</p>
        
</li>

        
<li>
          
<p>
<a href="#sc_dataFileManagement">Data File Management</a>
</p>
        
</li>

        
<li>
          
<p>
<a href="#sc_commonProblems">Things to Avoid</a>
</p>
        
</li>

        
<li>
          
<p>
<a href="#sc_bestPractices">Best Practices</a>
</p>
        
</li>
      
</ul>
<a name="sc_designing"></a>
<h3 class="h4">Designing a ZooKeeper Deployment</h3>
<p>The reliablity of ZooKeeper rests on two basic assumptions.</p>
<ol>
        
<li>
<p> Only a minority of servers in a deployment
            will fail. <em>Failure</em> in this context
            means a machine crash, or some error in the network that
            partitions a server off from the majority.</p>
        
</li>
        
<li>
<p> Deployed machines operate correctly. To
            operate correctly means to execute code correctly, to have
            clocks that work properly, and to have storage and network
            components that perform consistently.</p>
        
</li>
      
</ol>
<p>The sections below contain considerations for ZooKeeper
      administrators to maximize the probability for these assumptions
      to hold true. Some of these are cross-machines considerations,
      and others are things you should consider for each and every
      machine in your deployment.</p>
<a name="sc_CrossMachineRequirements"></a>
<h4>Cross Machine Requirements</h4>
<p>For the ZooKeeper service to be active, there must be a
        majority of non-failing machines that can communicate with
        each other. To create a deployment that can tolerate the
        failure of F machines, you should count on deploying 2xF+1
        machines.  Thus, a deployment that consists of three machines
        can handle one failure, and a deployment of five machines can
        handle two failures. Note that a deployment of six machines
        can only handle two failures since three machines is not a
        majority.  For this reason, ZooKeeper deployments are usually
        made up of an odd number of machines.</p>
<p>To achieve the highest probability of tolerating a failure
        you should try to make machine failures independent. For
        example, if most of the machines share the same switch,
        failure of that switch could cause a correlated failure and
        bring down the service. The same holds true of shared power
        circuits, cooling systems, etc.</p>
<a name="Single+Machine+Requirements"></a>
<h4>Single Machine Requirements</h4>
<p>If ZooKeeper has to contend with other applications for
        access to resourses like storage media, CPU, network, or
        memory, its performance will suffer markedly.  ZooKeeper has
        strong durability guarantees, which means it uses storage
        media to log changes before the operation responsible for the
        change is allowed to complete. You should be aware of this
        dependency then, and take great care if you want to ensure
        that ZooKeeper operations aren&rsquo;t held up by your media. Here
        are some things you can do to minimize that sort of
        degradation:
      </p>
<ul>
        
<li>
          
<p>ZooKeeper's transaction log must be on a dedicated
            device. (A dedicated partition is not enough.) ZooKeeper
            writes the log sequentially, without seeking Sharing your
            log device with other processes can cause seeks and
            contention, which in turn can cause multi-second
            delays.</p>
        
</li>

        
<li>
          
<p>Do not put ZooKeeper in a situation that can cause a
            swap. In order for ZooKeeper to function with any sort of
            timeliness, it simply cannot be allowed to swap.
            Therefore, make certain that the maximum heap size given
            to ZooKeeper is not bigger than the amount of real memory
            available to ZooKeeper.  For more on this, see
            <a href="#sc_commonProblems">Things to Avoid</a>
            below. </p>
        
</li>
      
</ul>
<a name="sc_provisioning"></a>
<h3 class="h4">Provisioning</h3>
<p></p>
<a name="sc_strengthsAndLimitations"></a>
<h3 class="h4">Things to Consider: ZooKeeper Strengths and Limitations</h3>
<p></p>
<a name="sc_administering"></a>
<h3 class="h4">Administering</h3>
<p></p>
<a name="sc_maintenance"></a>
<h3 class="h4">Maintenance</h3>
<p>Little long term maintenance is required for a ZooKeeper
        cluster however you must be aware of the following:</p>
<a name="Ongoing+Data+Directory+Cleanup"></a>
<h4>Ongoing Data Directory Cleanup</h4>
<p>The ZooKeeper <a href="#var_datadir">Data
          Directory</a> contains files which are a persistent copy
          of the znodes stored by a particular serving ensemble. These
          are the snapshot and transactional log files. As changes are
          made to the znodes these changes are appended to a
          transaction log, occasionally, when a log grows large, a
          snapshot of the current state of all znodes will be written
          to the filesystem. This snapshot supercedes all previous
          logs.
        </p>
<p>A ZooKeeper server <strong>will not remove
        old snapshots and log files</strong> when using the default
        configuration (see autopurge below), this is the
        responsibility of the operator. Every serving environment is
        different and therefore the requirements of managing these
        files may differ from install to install (backup for example).
        </p>
<p>The PurgeTxnLog utility implements a simple retention
        policy that administrators can use. The <a href="api/index.html">API docs</a> contains details on
        calling conventions (arguments, etc...).
        </p>
<p>In the following example the last count snapshots and
        their corresponding logs are retained and the others are
        deleted.  The value of &lt;count&gt; should typically be
        greater than 3 (although not required, this provides 3 backups
        in the unlikely event a recent log has become corrupted). This
        can be run as a cron job on the ZooKeeper server machines to
        clean up the logs daily.</p>
<pre class="code"> java -cp zookeeper.jar:lib/slf4j-api-1.7.5.jar:lib/slf4j-log4j12-1.7.5.jar:lib/log4j-1.2.16.jar:conf org.apache.zookeeper.server.PurgeTxnLog &lt;dataDir&gt; &lt;snapDir&gt; -n &lt;count&gt;</pre>
<p>Automatic purging of the snapshots and corresponding
        transaction logs was introduced in version 3.4.0 and can be
        enabled via the following configuration parameters <strong>autopurge.snapRetainCount</strong> and <strong>autopurge.purgeInterval</strong>. For more on
        this, see <a href="#sc_advancedConfiguration">Advanced Configuration</a>
        below.</p>
<a name="Debug+Log+Cleanup+%28log4j%29"></a>
<h4>Debug Log Cleanup (log4j)</h4>
<p>See the section on <a href="#sc_logging">logging</a> in this document. It is
        expected that you will setup a rolling file appender using the
        in-built log4j feature. The sample configuration file in the
        release tar's conf/log4j.properties provides an example of
        this.
        </p>
<a name="sc_supervision"></a>
<h3 class="h4">Supervision</h3>
<p>You will want to have a supervisory process that manages
      each of your ZooKeeper server processes (JVM). The ZK server is
      designed to be "fail fast" meaning that it will shutdown
      (process exit) if an error occurs that it cannot recover
      from. As a ZooKeeper serving cluster is highly reliable, this
      means that while the server may go down the cluster as a whole
      is still active and serving requests. Additionally, as the
      cluster is "self healing" the failed server once restarted will
      automatically rejoin the ensemble w/o any manual
      interaction.</p>
<p>Having a supervisory process such as <a href="http://cr.yp.to/daemontools.html">daemontools</a> or
      <a href="http://en.wikipedia.org/wiki/Service_Management_Facility">SMF</a>
      (other options for supervisory process are also available, it's
      up to you which one you would like to use, these are just two
      examples) managing your ZooKeeper server ensures that if the
      process does exit abnormally it will automatically be restarted
      and will quickly rejoin the cluster.</p>
<a name="sc_monitoring"></a>
<h3 class="h4">Monitoring</h3>
<p>The ZooKeeper service can be monitored in one of two
      primary ways; 1) the command port through the use of <a href="#sc_zkCommands">4 letter words</a> and 2) <a href="zookeeperJMX.html">JMX</a>. See the appropriate section for
      your environment/requirements.</p>
<a name="sc_logging"></a>
<h3 class="h4">Logging</h3>
<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="sc_troubleshooting"></a>
<h3 class="h4">Troubleshooting</h3>
<dl>
		
<dt>
<term> Server not coming up because of file corruption</term>
</dt>
<dd>
<p>A server might not be able to read its database and fail to come up because of 
		some file corruption in the transaction logs of the ZooKeeper server. You will
		see some IOException on loading ZooKeeper database. In such a case,
		make sure all the other servers in your ensemble are up and  working. Use "stat" 
		command on the command port to see if they are in good health. After you have verified that
		all the other servers of the ensemble are up, you can go ahead and clean the database
		of the corrupt server. Delete all the files in datadir/version-2 and datalogdir/version-2/.
		Restart the server.
		</p>
</dd>
		
</dl>
<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
      all the servers that make up a ZooKeeper server assuming the disk
      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="sc_minimumConfiguration"></a>
<h4>Minimum Configuration</h4>
<p>Here are the minimum configuration keywords that must be defined
        in the configuration file:</p>
<dl>
          
<dt>
<term>clientPort</term>
</dt>
<dd>
<p>the port to listen for client connections; that is, the
              port that clients attempt to connect to.</p>
</dd>

          
<dt>
<term>dataDir</term>
</dt>
<dd>
<p>the location where ZooKeeper will store the in-memory
              database snapshots and, unless specified otherwise, the
              transaction log of updates to the database.</p>
<div class="note">
<div class="label">Note</div>
<div class="content">
                
<p>Be careful where you put the transaction log. A
                dedicated transaction log device is key to consistent good
                performance. Putting the log on a busy device will adversely
                effect performance.</p>
              
</div>
</div>
</dd>

          
<dt>
<term>tickTime</term>
</dt>
<dd>
<p>the length of a single tick, which is the basic time unit
              used by ZooKeeper, as measured in milliseconds. It is used to
              regulate heartbeats, and timeouts. For example, the minimum
              session timeout will be two ticks.</p>
</dd>
        
</dl>
<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.
        Some can also be set using Java system properties, generally of the
        form <em>zookeeper.keyword</em>. The exact system
        property, when available, is noted below.</p>
<dl>
          
<dt>
<term>dataLogDir</term>
</dt>
<dd>
<p>(No Java system property)</p>
<p>This option will direct the machine to write the
              transaction log to the <strong>dataLogDir</strong> rather than the <strong>dataDir</strong>. This allows a dedicated log
              device to be used, and helps avoid competition between logging
              and snaphots.</p>
<div class="note">
<div class="label">Note</div>
<div class="content">
                
<p>Having a dedicated log device has a large impact on
                throughput and stable latencies. It is highly recommened to
                dedicate a log device and set <strong>dataLogDir</strong> to point to a directory on
                that device, and then make sure to point <strong>dataDir</strong> to a directory
                <em>not</em> residing on that device.</p>
              
</div>
</div>
</dd>

          
<dt>
<term>globalOutstandingLimit</term>
</dt>
<dd>
<p>(Java system property: <strong>zookeeper.globalOutstandingLimit.</strong>)</p>
<p>Clients can submit requests faster than ZooKeeper can
              process them, especially if there are a lot of clients. To
              prevent ZooKeeper from running out of memory due to queued
              requests, ZooKeeper will throttle clients so that there is no
              more than globalOutstandingLimit outstanding requests in the
              system. The default limit is 1,000.</p>
</dd>

          
<dt>
<term>preAllocSize</term>
</dt>
<dd>
<p>(Java system property: <strong>zookeeper.preAllocSize</strong>)</p>
<p>To avoid seeks ZooKeeper allocates space in the
              transaction log file in blocks of preAllocSize kilobytes. The
              default block size is 64M. One reason for changing the size of
              the blocks is to reduce the block size if snapshots are taken
              more often. (Also, see <strong>snapCount</strong>).</p>
</dd>

          
<dt>
<term>snapCount</term>
</dt>
<dd>
<p>(Java system property: <strong>zookeeper.snapCount</strong>)</p>
<p>ZooKeeper logs transactions to a transaction
              log. After snapCount transactions are written to a log
              file a snapshot is started and a new transaction log
              file is created. The default snapCount is
              100,000.</p>
</dd>

          
<dt>
<term>traceFile</term>
</dt>
<dd>
<p>(Java system property: <strong>requestTraceFile</strong>)</p>
<p>If this option is defined, requests will be will logged to
              a trace file named traceFile.year.month.day. Use of this option
              provides useful debugging information, but will impact
              performance. (Note: The system property has no zookeeper prefix,
              and the configuration variable name is different from the system
              property. Yes - it's not consistent, and it's annoying.)</p>
</dd>

          
<dt>
<term>maxClientCnxns</term>
</dt>
<dd>
<p>(No Java system property)</p>
<p>Limits the number of concurrent connections (at the socket 
              level) that a single client, identified by IP address, may make
              to a single member of the ZooKeeper ensemble. This is used to 
              prevent certain classes of DoS attacks, including file 
              descriptor exhaustion. The default is 60. Setting this to 0
              entirely removes the limit on concurrent connections.</p>
</dd>

           
<dt>
<term>clientPortAddress</term>
</dt>
<dd>
<p>
<strong>New in 3.3.0:</strong> the
               address (ipv4, ipv6 or hostname) to listen for client
               connections; that is, the address that clients attempt
               to connect to. This is optional, by default we bind in
               such a way that any connection to the <strong>clientPort</strong> for any
               address/interface/nic on the server will be
               accepted.</p>
</dd>

          
<dt>
<term>minSessionTimeout</term>
</dt>
<dd>
<p>(No Java system property)</p>
<p>
<strong>New in 3.3.0:</strong> the
              minimum session timeout in milliseconds that the server
              will allow the client to negotiate. Defaults to 2 times
              the <strong>tickTime</strong>.</p>
</dd>

          
<dt>
<term>maxSessionTimeout</term>
</dt>
<dd>
<p>(No Java system property)</p>
<p>
<strong>New in 3.3.0:</strong> the
              maximum session timeout in milliseconds that the server
              will allow the client to negotiate. Defaults to 20 times
              the <strong>tickTime</strong>.</p>
</dd>
           
           
<dt>
<term>fsync.warningthresholdms</term>
</dt>
<dd>
<p>(Java system property: <strong>fsync.warningthresholdms</strong>)</p>
<p>
<strong>New in 3.3.4:</strong> A
               warning message will be output to the log whenever an
               fsync in the Transactional Log (WAL) takes longer than
               this value. The values is specified in milliseconds and
               defaults to 1000. This value can only be set as a
               system property.</p>
</dd>

          
<dt>
<term>autopurge.snapRetainCount</term>
</dt>
<dd>
<p>(No Java system property)</p>
<p>
<strong>New in 3.4.0:</strong> 
              When enabled, ZooKeeper auto purge feature retains
              the <strong>autopurge.snapRetainCount</strong> most
              recent snapshots and the corresponding transaction logs in the 
              <strong>dataDir</strong> and <strong>dataLogDir</strong> respectively and deletes the rest.
              Defaults to 3. Minimum value is 3.</p>
</dd>
          
          
<dt>
<term>autopurge.purgeInterval</term>
</dt>
<dd>
<p>(No Java system property)</p>
<p>
<strong>New in 3.4.0:</strong> The
              time interval in hours for which the purge task has to
              be triggered. Set to a positive integer (1 and above)
              to enable the auto purging. Defaults to 0.</p>
</dd>

          
<dt>
<term>syncEnabled</term>
</dt>
<dd>
<p>(Java system property: <strong>zookeeper.observer.syncEnabled</strong>)</p>
<p>
<strong>New in 3.4.6, 3.5.0:</strong>
              The observers now log transaction and write snapshot to disk
              by default like the participants. This reduces the recovery time
              of the observers on restart. Set to "false" to disable this
              feature. Default is "true"</p>
</dd>
        
</dl>
<a name="sc_clusterOptions"></a>
<h4>Cluster Options</h4>
<p>The options in this section are designed for use with an ensemble
        of servers -- that is, when deploying clusters of servers.</p>
<dl>
          
<dt>
<term>electionAlg</term>
</dt>
<dd>
<p>(No Java system property)</p>
<p>Election implementation to use. A value of "0" corresponds
              to the original UDP-based version, "1" corresponds to the
              non-authenticated UDP-based version of fast leader election, "2"
              corresponds to the authenticated UDP-based version of fast
              leader election, and "3" corresponds to TCP-based version of
              fast leader election. Currently, algorithm 3 is the default</p>
<div class="note">
<div class="label">Note</div>
<div class="content">
              
<p> The implementations of leader election 0, 1, and 2 are now 
              <strong> deprecated </strong>. We have the intention
              of removing them in the next release, at which point only the 
              FastLeaderElection will be available. 
              </p>
              
</div>
</div>
</dd>

          
<dt>
<term>initLimit</term>
</dt>
<dd>
<p>(No Java system property)</p>
<p>Amount of time, in ticks (see <a href="#id_tickTime">tickTime</a>), to allow followers to
              connect and sync to a leader. Increased this value as needed, if
              the amount of data managed by ZooKeeper is large.</p>
</dd>

          
<dt>
<term>leaderServes</term>
</dt>
<dd>
<p>(Java system property: zookeeper.<strong>leaderServes</strong>)</p>
<p>Leader accepts client connections. Default value is "yes".
              The leader machine coordinates updates. For higher update
              throughput at thes slight expense of read throughput the leader
              can be configured to not accept clients and focus on
              coordination. The default to this option is yes, which means
              that a leader will accept client connections.</p>
<div class="note">
<div class="label">Note</div>
<div class="content">
                
<p>Turning on leader selection is highly recommended when
                you have more than three ZooKeeper servers in an ensemble.</p>
              
</div>
</div>
</dd>

          
<dt>
<term>server.x=[hostname]:nnnnn[:nnnnn], etc</term>
</dt>
<dd>
<p>(No Java system property)</p>
<p>servers making up the ZooKeeper ensemble. When the server
              starts up, it determines which server it is by looking for the
              file <span class="codefrag filename">myid</span> in the data directory. That file
              contains the server number, in ASCII, and it should match
              <strong>x</strong> in <strong>server.x</strong> in the left hand side of this
              setting.</p>
<p>The list of servers that make up ZooKeeper servers that is
              used by the clients must match the list of ZooKeeper servers
              that each ZooKeeper server has.</p>
<p>There are two port numbers <strong>nnnnn</strong>. 
              The first followers use to connect to the leader, and the second is for 
              leader election. The leader election port is only necessary if electionAlg 
              is 1, 2, or 3 (default). If electionAlg is 0, then the second port is not 
              necessary. If you want to test multiple servers on a single machine, then 
              different ports can be used for each server.</p>
</dd>

          
<dt>
<term>syncLimit</term>
</dt>
<dd>
<p>(No Java system property)</p>
<p>Amount of time, in ticks (see <a href="#id_tickTime">tickTime</a>), to allow followers to sync
              with ZooKeeper. If followers fall too far behind a leader, they
              will be dropped.</p>
</dd>

          
<dt>
<term>group.x=nnnnn[:nnnnn]</term>
</dt>
<dd>
<p>(No Java system property)</p>
<p>Enables a hierarchical quorum construction."x" is a group identifier
              and the numbers following the "=" sign correspond to server identifiers. 
              The left-hand side of the assignment is a colon-separated list of server
              identifiers. Note that groups must be disjoint and the union of all groups
              must be the ZooKeeper ensemble. </p>
<p> You will find an example <a href="zookeeperHierarchicalQuorums.html">here</a>
              
</p>
</dd>

          
<dt>
<term>weight.x=nnnnn</term>
</dt>
<dd>
<p>(No Java system property)</p>
<p>Used along with "group", it assigns a weight to a server when
              forming quorums. Such a value corresponds to the weight of a server
              when voting. There are a few parts of ZooKeeper that require voting
              such as leader election and the atomic broadcast protocol. By default
              the weight of server is 1. If the configuration defines groups, but not
              weights, then a value of 1 will be assigned to all servers.  
              </p>
<p> You will find an example <a href="zookeeperHierarchicalQuorums.html">here</a>
              
</p>
</dd>
          
          
<dt>
<term>cnxTimeout</term>
</dt>
<dd>
<p>(Java system property: zookeeper.<strong>cnxTimeout</strong>)</p>
<p>Sets the timeout value for opening connections for leader election notifications. 
              Only applicable if you are using electionAlg 3. 
              </p>
<div class="note">
<div class="label">Note</div>
<div class="content">
                
<p>Default value is 5 seconds.</p>
              
</div>
</div>
</dd>

          
<dt>
<term>standaloneEnabled</term>
</dt>
<dd>
<p>(No Java system property)</p>
<p>
<strong>New in 3.5.0:</strong>
              When set to false, a single server can be started in replicated
              mode, a lone participant can run with observers, and a cluster
              can reconfigure down to one node, and up from one node. The
              default is true for backwards compatibility. It can be set
              using QuorumPeerConfig's setStandaloneEnabled method or by
              adding "standaloneEnabled=false" or "standaloneEnabled=true"
              to a server's config file.
              </p>
</dd>
        
</dl>
<p></p>
<a name="sc_authOptions"></a>
<h4>Authentication &amp; Authorization Options</h4>
<p>The options in this section allow control over
        authentication/authorization performed by the service.</p>
<dl>
          
<dt>
<term>zookeeper.DigestAuthenticationProvider.superDigest</term>
</dt>
<dd>
<p>(Java system property only: <strong>zookeeper.DigestAuthenticationProvider.superDigest</strong>)</p>
<p>By default this feature is <strong>disabled</strong>
</p>
<p>
<strong>New in 3.2:</strong>
              Enables a ZooKeeper ensemble administrator to access the
              znode hierarchy as a "super" user. In particular no ACL
              checking occurs for a user authenticated as
              super.</p>
<p>org.apache.zookeeper.server.auth.DigestAuthenticationProvider
              can be used to generate the superDigest, call it with
              one parameter of "super:&lt;password&gt;". Provide the
              generated "super:&lt;data&gt;" as the system property value
              when starting each server of the ensemble.</p>
<p>When authenticating to a ZooKeeper server (from a
              ZooKeeper client) pass a scheme of "digest" and authdata
              of "super:&lt;password&gt;". Note that digest auth passes
              the authdata in plaintext to the server, it would be
              prudent to use this authentication method only on
              localhost (not over the network) or over an encrypted
              connection.</p>
</dd>
        
</dl>
<a name="Experimental+Options%2FFeatures"></a>
<h4>Experimental Options/Features</h4>
<p>New features that are currently considered experimental.</p>
<dl>
          
<dt>
<term>Read Only Mode Server</term>
</dt>
<dd>
<p>(Java system property: <strong>readonlymode.enabled</strong>)</p>
<p>
<strong>New in 3.4.0:</strong>
              Setting this value to true enables Read Only Mode server
              support (disabled by default). ROM allows clients
              sessions which requested ROM support to connect to the
              server even when the server might be partitioned from
              the quorum. In this mode ROM clients can still read
              values from the ZK service, but will be unable to write
              values and see changes from other clients. See
              ZOOKEEPER-784 for more details.
              </p>
</dd>

        
</dl>
<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
        the variable does.</p>
<dl>
          
<dt>
<term>forceSync</term>
</dt>
<dd>
<p>(Java system property: <strong>zookeeper.forceSync</strong>)</p>
<p>Requires updates to be synced to media of the transaction
              log before finishing processing the update. If this option is
              set to no, ZooKeeper will not require updates to be synced to
              the media.</p>
</dd>

          
<dt>
<term>jute.maxbuffer:</term>
</dt>
<dd>
<p>(Java system property:<strong>
              jute.maxbuffer</strong>)</p>
<p>This option can only be set as a Java system property.
              There is no zookeeper prefix on it. It specifies the maximum
              size of the data that can be stored in a znode. The default is
              0xfffff, or just under 1M. If this option is changed, the system
              property must be set on all servers and clients otherwise
              problems will arise. This is really a sanity check. ZooKeeper is
              designed to store data on the order of kilobytes in size.</p>
</dd>

          
<dt>
<term>skipACL</term>
</dt>
<dd>
<p>(Java system property: <strong>zookeeper.skipACL</strong>)</p>
<p>Skips ACL checks. This results in a boost in throughput,
              but opens up full access to the data tree to everyone.</p>
</dd>

          
<dt>
<term>quorumListenOnAllIPs</term>
</dt>
<dd>
<p>When set to true the ZooKeeper server will listen  
              for connections from its peers on all available IP addresses,
              and not only the address configured in the server list of the
              configuration file. It affects the connections handling the 
              ZAB protocol and the Fast Leader Election protocol. Default
              value is <strong>false</strong>.</p>
</dd>

        
</dl>
<a name="Disabling+data+directory+autocreation"></a>
<h4>Disabling data directory autocreation</h4>
<p>
<strong>New in 3.5:</strong> The default
        behavior of a ZooKeeper server is to automatically create the
        data directory (specified in the configuration file) when
        started if that directory does not already exist. This can be
        inconvenient and even dangerous in some cases. Take the case
        where a configuration change is made to a running server,
        wherein the <strong>dataDir</strong> parameter
        is accidentally changed. When the ZooKeeper server is
        restarted it will create this non-existent directory and begin
        serving - with an empty znode namespace. This scenario can
        result in an effective "split brain" situation (i.e. data in
        both the new invalid directory and the original valid data
        store). As such is would be good to have an option to turn off
        this autocreate behavior. In general for production
        environments this should be done, unfortunately however the
        default legacy behavior cannot be changed at this point and
        therefore this must be done on a case by case basis. This is
        left to users and to packagers of ZooKeeper distributions.
        </p>
<p>When running <strong>zkServer.sh</strong> autocreate can be disabled
        by setting the environment variable <strong>ZOO_DATADIR_AUTOCREATE_DISABLE</strong> to 1.
        When running ZooKeeper servers directly from class files this
        can be accomplished by setting <strong>zookeeper.datadir.autocreate=false</strong> on
        the java command line, i.e. <strong>-Dzookeeper.datadir.autocreate=false</strong>
        
</p>
<p>When this feature is disabled, and the ZooKeeper server
        determines that the required directories do not exist it will
        generate an error and refuse to start.
        </p>
<p>A new script <strong>zkServer-initialize.sh</strong> is provided to
        support this new feature. If autocreate is disabled it is
        necessary for the user to first install ZooKeeper, then create
        the data directory (and potentially txnlog directory), and
        then start the server. Otherwise as mentioned in the previous
        paragraph the server will not start. Running <strong>zkServer-initialize.sh</strong> will create the
        required directories, and optionally setup the myid file
        (optional command line parameter). This script can be used
        even if the autocreate feature itself is not used, and will
        likely be of use to users as this (setup, including creation
        of the myid file) has been an issue for users in the past.
        Note that this script ensures the data directories exist only,
        it does not create a config file, but rather requires a config
        file to be available in order to execute.
        </p>
<a name="sc_performance_options"></a>
<h4>Performance Tuning Options</h4>
<p>
<strong>New in 3.5.0:</strong> Several subsystems have been reworked
        to improve read throughput. This includes multi-threading of the NIO communication subsystem and
        request processing pipeline (Commit Processor). NIO is the default client/server communication
        subsystem. Its threading model comprises 1 acceptor thread, 1-N selector threads and 0-M
        socket I/O worker threads. In the request processing pipeline the system can be configured
        to process multiple read request at once while maintaining the same consistency guarantee
        (same-session read-after-write). The Commit Processor threading model comprises 1 main
        thread and 0-N worker threads.
        </p>
<p>
        The default values are aimed at maximizing read throughput on a dedicated ZooKeeper machine.
        Both subsystems need to have sufficient amount of threads to achieve peak read throughput.
        </p>
<dl>

          
<dt>
<term>zookeeper.nio.numSelectorThreads</term>
</dt>
<dd>
<p>(Java system property only: <strong>zookeeper.nio.numSelectorThreads</strong>)
              </p>
<p>
<strong>New in 3.5.0:</strong>
              Number of NIO selector threads. At least 1 selector thread required.
              It is recommended to use more than one selector for large numbers
              of client connections. The default value is sqrt( number of cpu cores / 2 ).
              </p>
</dd>

          
<dt>
<term>zookeeper.nio.numWorkerThreads</term>
</dt>
<dd>
<p>(Java system property only: <strong>zookeeper.nio.numWorkerThreads</strong>)
              </p>
<p>
<strong>New in 3.5.0:</strong>
              Number of NIO worker threads. If configured with 0 worker threads, the selector threads
              do the socket I/O directly. The default value is 2 times the number of cpu cores.
              </p>
</dd>

          
<dt>
<term>zookeeper.commitProcessor.numWorkerThreads</term>
</dt>
<dd>
<p>(Java system property only: <strong>zookeeper.commitProcessor.numWorkerThreads</strong>)
              </p>
<p>
<strong>New in 3.5.0:</strong>
              Number of Commit Processor worker threads. If configured with 0 worker threads, the main thread
              will process the request directly. The default value is the number of cpu cores.
              </p>
</dd>

        
</dl>
<a name="Communication+using+the+Netty+framework"></a>
<h4>Communication using the Netty framework</h4>
<p>
<strong>New in
            3.4:</strong> <a href="http://jboss.org/netty">Netty</a>
            is an NIO based client/server communication framework, it
            simplifies (over NIO being used directly) many of the
            complexities of network level communication for java
            applications. Additionally the Netty framework has built
            in support for encryption (SSL) and authentication
            (certificates). These are optional features and can be
            turned on or off individually.
        </p>
<p>Prior to version 3.4 ZooKeeper has always used NIO
            directly, however in versions 3.4 and later Netty is
            supported as an option to NIO (replaces). NIO continues to
            be the default, however Netty based communication can be
            used in place of NIO by setting the environment variable
            "zookeeper.serverCnxnFactory" to
            "org.apache.zookeeper.server.NettyServerCnxnFactory". You
            have the option of setting this on either the client(s) or
            server(s), typically you would want to set this on both,
            however that is at your discretion.
        </p>
<p>
          TBD - tuning options for netty - currently there are none that are netty specific but we should add some. Esp around max bound on the number of reader worker threads netty creates.
        </p>
<p>
          TBD - how to manage encryption
        </p>
<p>
          TBD - how to manage certificates
        </p>
<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
      or nc, at the client port.</p>
<p>Three of the more interesting commands: "stat" gives some
      general information about the server and connected clients,
      while "srvr" and "cons" give extended details on server and
      connections respectively.</p>
<dl>
        
<dt>
<term>conf</term>
</dt>
<dd>
<p>
<strong>New in 3.3.0:</strong> Print
            details about serving configuration.</p>
</dd>

        
<dt>
<term>cons</term>
</dt>
<dd>
<p>
<strong>New in 3.3.0:</strong> List
            full connection/session details for all clients connected
            to this server. Includes information on numbers of packets
            received/sent, session id, operation latencies, last
            operation performed, etc...</p>
</dd>

        
<dt>
<term>crst</term>
</dt>
<dd>
<p>
<strong>New in 3.3.0:</strong> Reset
            connection/session statistics for all connections.</p>
</dd>

        
<dt>
<term>dump</term>
</dt>
<dd>
<p>Lists the outstanding sessions and ephemeral nodes. This
            only works on the leader.</p>
</dd>

        
<dt>
<term>envi</term>
</dt>
<dd>
<p>Print details about serving environment</p>
</dd>

        
<dt>
<term>ruok</term>
</dt>
<dd>
<p>Tests if server is running in a non-error state. The server
            will respond with imok if it is running. Otherwise it will not
            respond at all.</p>
<p>A response of "imok" does not necessarily indicate that the
            server has joined the quorum, just that the server process is active
            and bound to the specified client port. Use "stat" for details on
            state wrt quorum and client connection information.</p>
</dd>

        
<dt>
<term>srst</term>
</dt>
<dd>
<p>Reset server statistics.</p>
</dd>

        
<dt>
<term>srvr</term>
</dt>
<dd>
<p>
<strong>New in 3.3.0:</strong> Lists
            full details for the server.</p>
</dd>

        
<dt>
<term>stat</term>
</dt>
<dd>
<p>Lists brief details for the server and connected
            clients.</p>
</dd>

        
<dt>
<term>wchs</term>
</dt>
<dd>
<p>
<strong>New in 3.3.0:</strong> Lists
            brief information on watches for the server.</p>
</dd>

        
<dt>
<term>wchc</term>
</dt>
<dd>
<p>
<strong>New in 3.3.0:</strong> Lists
            detailed information on watches for the server, by
            session.  This outputs a list of sessions(connections)
            with associated watches (paths). Note, depending on the
            number of watches this operation may be expensive (ie
            impact server performance), use it carefully.</p>
</dd>

        
<dt>
<term>wchp</term>
</dt>
<dd>
<p>
<strong>New in 3.3.0:</strong> Lists
            detailed information on watches for the server, by path.
            This outputs a list of paths (znodes) with associated
            sessions. Note, depending on the number of watches this
            operation may be expensive (ie impact server performance),
            use it carefully.</p>
</dd>


<dt>
<term>mntr</term>
</dt>
<dd>
<p>
<strong>New in 3.4.0:</strong> Outputs a list 
            of variables that could be used for monitoring the health of the cluster.</p>
<pre class="code">$ echo mntr | nc localhost 2185

zk_version  3.4.0
zk_avg_latency  0
zk_max_latency  0
zk_min_latency  0
zk_packets_received 70
zk_packets_sent 69
zk_outstanding_requests 0
zk_server_state leader
zk_znode_count   4
zk_watch_count  0
zk_ephemerals_count 0
zk_approximate_data_size    27
zk_followers    4                   - only exposed by the Leader
zk_synced_followers 4               - only exposed by the Leader
zk_pending_syncs    0               - only exposed by the Leader
zk_open_file_descriptor_count 23    - only available on Unix platforms
zk_max_file_descriptor_count 1024   - only available on Unix platforms
</pre>
<p>The output is compatible with java properties format and the content 
        may change over time (new keys added). Your scripts should expect changes.</p>
<p>ATTENTION: Some of the keys are platform specific and some of the keys are only exported by the Leader. </p>
<p>The output contains multiple lines with the following format:</p>
<pre class="code">key \t value</pre>
</dd>
      
</dl>
<p>Here's an example of the <strong>ruok</strong>
      command:</p>
<pre class="code">$ echo ruok | nc 127.0.0.1 5111
imok
</pre>
<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
      the same. The server can (and should) be configured to store the
      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="The+Data+Directory"></a>
<h4>The Data Directory</h4>
<p>This directory has two files in it:</p>
<ul>
          
<li>
            
<p>
<span class="codefrag filename">myid</span> - contains a single integer in
            human readable ASCII text that represents the server id.</p>
          
</li>

          
<li>
            
<p>
<span class="codefrag filename">snapshot.&lt;zxid&gt;</span> - holds the fuzzy
            snapshot of a data tree.</p>
          
</li>
        
</ul>
<p>Each ZooKeeper server has a unique id. This id is used in two
        places: the <span class="codefrag filename">myid</span> file and the configuration file.
        The <span class="codefrag filename">myid</span> file identifies the server that
        corresponds to the given data directory. The configuration file lists
        the contact information for each server identified by its server id.
        When a ZooKeeper server instance starts, it reads its id from the
        <span class="codefrag filename">myid</span> file and then, using that id, reads from the
        configuration file, looking up the port on which it should
        listen.</p>
<p>The <span class="codefrag filename">snapshot</span> files stored in the data
        directory are fuzzy snapshots in the sense that during the time the
        ZooKeeper server is taking the snapshot, updates are occurring to the
        data tree. The suffix of the <span class="codefrag filename">snapshot</span> file names
        is the <em>zxid</em>, the ZooKeeper transaction id, of the
        last committed transaction at the start of the snapshot. Thus, the
        snapshot includes a subset of the updates to the data tree that
        occurred while the snapshot was in process. The snapshot, then, may
        not correspond to any data tree that actually existed, and for this
        reason we refer to it as a fuzzy snapshot. Still, ZooKeeper can
        recover using this snapshot because it takes advantage of the
        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="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="sc_filemanagement"></a>
<h4>File Management</h4>
<p>The format of snapshot and log files does not change between
        standalone ZooKeeper servers and different configurations of
        replicated ZooKeeper servers. Therefore, you can pull these files from
        a running replicated ZooKeeper server to a development machine with a
        stand-alone ZooKeeper server for trouble shooting.</p>
<p>Using older log and snapshot files, you can look at the previous
        state of ZooKeeper servers and even restore that state. The
        LogFormatter class allows an administrator to look at the transactions
        in a log.</p>
<p>The ZooKeeper server creates snapshot and log files, but
        never deletes them. The retention policy of the data and log
        files is implemented outside of the ZooKeeper server. The
        server itself only needs the latest complete fuzzy snapshot
        and the log files from the start of that snapshot. See the
        <a href="#sc_maintenance">maintenance</a> section in
        this document for more details on setting a retention policy
        and maintenance of ZooKeeper storage.
        </p>
<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>
<dl>
        
<dt>
<term>inconsistent lists of servers</term>
</dt>
<dd>
<p>The list of ZooKeeper servers used by the clients must match
            the list of ZooKeeper servers that each ZooKeeper server has.
            Things work okay if the client list is a subset of the real list,
            but things will really act strange if clients have a list of
            ZooKeeper servers that are in different ZooKeeper clusters. Also,
            the server lists in each Zookeeper server configuration file
            should be consistent with one another.</p>
</dd>

        
<dt>
<term>incorrect placement of transasction log</term>
</dt>
<dd>
<p>The most performance critical part of ZooKeeper is the
            transaction log. ZooKeeper syncs transactions to media before it
            returns a response. A dedicated transaction log device is key to
            consistent good performance. Putting the log on a busy device will
            adversely effect performance. If you only have one storage device,
            put trace files on NFS and increase the snapshotCount; it doesn't
            eliminate the problem, but it should mitigate it.</p>
</dd>

        
<dt>
<term>incorrect Java heap size</term>
</dt>
<dd>
<p>You should take special care to set your Java max heap size
            correctly. In particular, you should not create a situation in
            which ZooKeeper swaps to disk. The disk is death to ZooKeeper.
            Everything is ordered, so if processing one request swaps the
            disk, all other queued requests will probably do the same. the
            disk. DON'T SWAP.</p>
<p>Be conservative in your estimates: if you have 4G of RAM, do
            not set the Java max heap size to 6G or even 4G. For example, it
            is more likely you would use a 3G heap for a 4G machine, as the
            operating system and the cache also need memory. The best and only
            recommend practice for estimating the heap size your system needs
            is to run load tests, and then make sure you are well below the
            usage limit that would cause the system to swap.</p>
</dd>
      
</dl>
<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:</p>
<p>For multi-tennant installations see the <a href="zookeeperProgrammers.html#ch_zkSessions">section</a>
      detailing ZooKeeper "chroot" support, this can be very useful
      when deploying many applications/services interfacing to a
      single ZooKeeper cluster.</p>
</div>

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