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<h1>ZooKeeper Administrator's Guide</h1>
<h3>A Guide to Deployment and Administration</h3>
<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>
</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>
</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_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="#Unsafe+Options">Unsafe Options</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="#File+Management">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>
  

  

  

  
<a name="N1000B"></a><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="N10032"></a><a name="sc_systemReq"></a>
<h3 class="h4">System Requirements</h3>
<p>ZooKeeper runs in Java, release 1.5 or greater (JDK 5 or greater).
      It runs as group of hosts called a quorum. Three ZooKeeper hosts per
      quorum is the minimum recommended quorum size. At Yahoo!, ZooKeeper is
      usually deployed on dedicated RHEL boxes, with dual-core processors, 2GB
      of RAM, and 80GB IDE harddrives.</p>
<a name="N1003C"></a><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 a <em>quorum</em>. As long as a majority
      of the quorum 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 a
      quorum. These steps should be performed on every host in the
      quorum:</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://hadoop.apache.org/zookeeper/releases.html">
              http://hadoop.apache.org/zookeeper/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>

          
<p>
<span class="codefrag computeroutput">
tickTime=2000
dataDir=/var/zookeeper/
clientPort=2181
initLimit=5
syncLimit=2
server.1=zoo1:2888
server.2=zoo2:2888
server.3=zoo3:2888</span>
</p>

          
<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 quorum should know
          about every other machine in the quorum. You accomplish this with
          the series of lines of the form <strong>server.id=host:port</strong>. The integers <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>. 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
          quorum.</p>
        
</li>

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

          
<p>
<span class="codefrag computeroutput">$ java -cp zookeeper-dev.jar:java/lib/log4j-1.2.15.jar:conf \
        org.apache.zookeeper.server.quorum.QuorumPeerMain zoo.cfg</span>
</p>
        
</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:java/lib/log4j-1.2.15.jar:conf \
      org.apache.zookeeper.ZooKeeperMain 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="N100D0"></a><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="N100F1"></a><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_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="N1015D"></a><a name="sc_designing"></a>
<h3 class="h4">Designing a ZooKeeper Deployment</h3>
<p></p>
<a name="N10166"></a><a name="sc_provisioning"></a>
<h3 class="h4">Provisioning</h3>
<p></p>
<a name="N1016F"></a><a name="sc_strengthsAndLimitations"></a>
<h3 class="h4">Things to Consider: ZooKeeper Strengths and Limitations</h3>
<p></p>
<a name="N10178"></a><a name="sc_administering"></a>
<h3 class="h4">Administering</h3>
<p></p>
<a name="N10181"></a><a name="sc_monitoring"></a>
<h3 class="h4">Monitoring</h3>
<p></p>
<a name="N1018A"></a><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="N101AA"></a><a name="sc_troubleshooting"></a>
<h3 class="h4">Troubleshooting</h3>
<p></p>
<a name="N101B3"></a><a name="sc_configuration"></a>
<h3 class="h4">Configuration Parameters</h3>
<p>ZooKeeper's behavior is governed by the ZooKeeper configuration
      file. This file is designed so that the exact same file can be used by
      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="N101BC"></a><a name="sc_minimumConfiguration"></a>
<h4>Minimum Configuration</h4>
<p>Here are the minimum configuration keywords that must be defined
        in the configuration file:</p>
<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="N101E3"></a><a name="sc_advancedConfiguration"></a>
<h4>Advanced Configuration</h4>
<p>The configuration settings in the section are optional. You can
        use them to further fine tune the behaviour of your ZooKeeper servers.
        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>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.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 started. The default snapCount is 10,000.</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>
        
</dl>
<a name="N10243"></a><a name="sc_clusterOptions"></a>
<h4>Cluster Options</h4>
<p>The options in this section are designed for use in quorums --
        that is, when deploying clusters of servers.</p>
<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</p>
</dd>

          
<dt>
<term>electionPort</term>
</dt>
<dd>
<p>(No Java system property)</p>
<p>Port used for leader election. It is only used when the
              election algorithm is not "0". When the election algorithm is
              "0" a UDP port with the same port number as the port listed in
              the <strong>server.num</strong> option will be
              used.</p>
</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 a quorum.</p>
              
</div>
</div>
</dd>

          
<dt>
<term>server.x=[hostname]:nnnn, etc</term>
</dt>
<dd>
<p>(No Java system property)</p>
<p>servers making up the ZooKeeper quorum. 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>The port numbers <strong>nnnn</strong> in
              this setting are the <em>electionPort</em> numbers
              of the servers (as opposed to clientPorts). If you want to test
              multiple servers on a single machine, the individual choices of
              electionPort for each server can be defined in each server's
              config files using the line electionPort=xxxx to avoid
              clashes.</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>
        
</dl>
<p></p>
<a name="N102AF"></a><a name="Unsafe+Options"></a>
<h4>Unsafe Options</h4>
<p>The following options can be useful, but be careful when you use
        them. The risk of each is explained along with the explanation of what
        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>
        
</dl>
<a name="N102E1"></a><a name="sc_zkCommands"></a>
<h3 class="h4">ZooKeeper Commands: The Four Letter Words</h3>
<p>ZooKeeper responds to a small set of commands. Each command is
      composed of four letters. You issue the commands to ZooKeeper via telnet
      or nc, at the client port.</p>
<dl>
        
<dt>
<term>dump</term>
</dt>
<dd>
<p>Lists the outstanding sessions and ephemeral nodes. This
            only works on the leader.</p>
</dd>

        
<dt>
<term>kill</term>
</dt>
<dd>
<p>Shuts down the server. This must be issued from the machine
            the ZooKeeper server is running on.</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>
</dd>

        
<dt>
<term>stat</term>
</dt>
<dd>
<p>Lists statistics about performance and connected
            clients.</p>
</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="N10313"></a><a name="sc_dataFileManagement"></a>
<h3 class="h4">Data File Management</h3>
<p>ZooKeeper stores its data in a data directory and its transaction
      log in a transaction log directory. By default these two directories are
      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="N1031C"></a><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="N10358"></a><a name="The+Log+Directory"></a>
<h4>The Log Directory</h4>
<p>The Log Directory contains the ZooKeeper transaction logs.
        Before any update takes place, ZooKeeper ensures that the transaction
        that represents the update is written to non-volatile storage. A new
        log file is started each time a snapshot is begun. The log file's
        suffix is the first zxid written to that log.</p>
<a name="N10362"></a><a name="File+Management"></a>
<h4>File Management</h4>
<p>The format of snapshot and log files does not change between
        standalone ZooKeeper servers and different configurations of
        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. The PurgeTxnLog utility implements a simple
        retention policy that administrators can use.</p>
<a name="N10373"></a><a name="sc_commonProblems"></a>
<h3 class="h4">Things to Avoid</h3>
<p>Here are some common problems you can avoid by configuring
      ZooKeeper correctly:</p>
<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="N10397"></a><a name="sc_bestPractices"></a>
<h3 class="h4">Best Practices</h3>
<p>For best results, take note of the following list of good
      Zookeeper practices. <em>[tbd...]</em>
</p>
</div>

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