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|alternative credits - +--> -<div id="credit2"></div> -</div> -<!--+ - |end Menu - +--> -<!--+ - |start content - +--> -<div id="content"> -<div title="Portable Document Format" class="pdflink"> -<a class="dida" href="zookeeperAdmin.pdf"><img alt="PDF -icon" src="skin/images/pdfdoc.gif" class="skin"><br> - PDF</a> -</div> -<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 & 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="#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> -<li> -<a href="#Recovery+-+TxnLogToolkit">Recovery - TxnLogToolkit</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> -<p>ZooKeeper consists of multiple components. Some components are - supported broadly, and other components are supported only on a smaller - set of platforms.</p> -<ul> - -<li> - -<p> -<strong>Client</strong> is the Java client - library, used by applications to connect to a ZooKeeper ensemble. - </p> - -</li> - -<li> - -<p> -<strong>Server</strong> is the Java server - that runs on the ZooKeeper ensemble nodes.</p> - -</li> - -<li> - -<p> -<strong>Native Client</strong> is a client - implemented in C, similar to the Java client, used by applications - to connect to a ZooKeeper ensemble.</p> - -</li> - -<li> - -<p> -<strong>Contrib</strong> refers to multiple - optional add-on components.</p> - -</li> - -</ul> -<p>The following matrix describes the level of support committed for - running each component on different operating system platforms.</p> -<table class="ForrestTable" cellspacing="1" cellpadding="4"> -<caption>Support Matrix</caption> - -<title>Support Matrix</title> - - -<tr> - -<th>Operating System</th> - <th>Client</th> - <th>Server</th> - <th>Native Client</th> - <th>Contrib</th> - -</tr> - - -<tr> - -<td>GNU/Linux</td> - <td>Development and Production</td> - <td>Development and Production</td> - <td>Development and Production</td> - <td>Development and Production</td> - -</tr> - -<tr> - -<td>Solaris</td> - <td>Development and Production</td> - <td>Development and Production</td> - <td>Not Supported</td> - <td>Not Supported</td> - -</tr> - -<tr> - -<td>FreeBSD</td> - <td>Development and Production</td> - <td>Development and Production</td> - <td>Not Supported</td> - <td>Not Supported</td> - -</tr> - -<tr> - -<td>Windows</td> - <td>Development and Production</td> - <td>Development and Production</td> - <td>Not Supported</td> - <td>Not Supported</td> - -</tr> - -<tr> - -<td>Mac OS X</td> - <td>Development Only</td> - <td>Development Only</td> - <td>Not Supported</td> - <td>Not Supported</td> - -</tr> - - -</table> -<p>For any operating system not explicitly mentioned as supported in - the matrix, components may or may not work. The ZooKeeper community - will fix obvious bugs that are reported for other platforms, but there - is no full support.</p> -<a name="sc_requiredSoftware"></a> -<h4>Required Software </h4> -<p>ZooKeeper runs in Java, release 1.6 or greater (JDK 6 or - greater). 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> -<div class="note"> -<div class="label">Note</div> -<div class="content"> - -<p> - As mentioned in the - <a href="zookeeperStarted.html">ZooKeeper Getting Started Guide</a> - , a minimum of three servers are required for a fault tolerant - clustered setup, and it is strongly recommended that you have an - odd number of servers. - </p> - -<p>Usually three servers is more than enough for a production - install, but for maximum reliability during maintenance, you may - wish to install five servers. With three servers, if you perform - maintenance on one of them, you are vulnerable to a failure on one - of the other two servers during that maintenance. If you have five - of them running, you can take one down for maintenance, and know - that you're still OK if one of the other four suddenly fails. - </p> - -<p>Your redundancy considerations should include all aspects of - your environment. If you have three ZooKeeper servers, but their - network cables are all plugged into the same network switch, then - the failure of that switch will take down your entire ensemble. - </p> - -</div> -</div> -<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-1.6.1.jar:lib/slf4j-log4j12-1.6.1.jar:lib/log4j-1.2.15.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> - - -<p>In Java, you can run the following command to execute - simple operations:</p> - - -<p> -<span class="codefrag computeroutput">$ bin/zkCli.sh -server 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’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 and a new transaction log file is created - for future transactions. During snapshotting, ZooKeeper may - continue appending incoming transactions to the old log file. - Therefore, some transactions which are newer than a snapshot - may be found in the last transaction log preceding the - snapshot. - </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 <count> 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-1.6.1.jar:lib/slf4j-log4j12-1.6.1.jar:lib/log4j-1.2.15.jar:conf org.apache.zookeeper.server.PurgeTxnLog <dataDir> <snapDir> -n <count></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 records its transactions using snapshots and - a transaction log (think write-ahead log).The number of - transactions recorded in the transaction log before a snapshot - can be taken (and the transaction log rolled) is determined - by snapCount. In order to prevent all of the machines in the quorum - from taking a snapshot at the same time, each ZooKeeper server - will take a snapshot when the number of transactions in the transaction log - reaches a runtime generated random value in the [snapCount/2+1, snapCount] - range.The default snapCount is 100,000.</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>zookeeper.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>4lw.commands.whitelist</term> -</dt> -<dd> -<p>(Java system property: <strong>zookeeper.4lw.commands.whitelist</strong>)</p> -<p> -<strong>New in 3.4.10:</strong> - This property contains a list of comma separated - <a href="#sc_zkCommands">Four Letter Words</a> commands. It is introduced - to provide fine grained control over the set of commands ZooKeeper can execute, - so users can turn off certain commands if necessary. - By default it contains all supported four letter word commands except "wchp" and "wchc", - if the property is not specified. If the property is specified, then only commands listed - in the whitelist are enabled. - </p> -<p>Here's an example of the configuration that enables stat, ruok, conf, and isro - command while disabling the rest of Four Letter Words command:</p> -<pre class="code"> - 4lw.commands.whitelist=stat, ruok, conf, isro - </pre> -<p>Users can also use asterisk option so they don't have to include every command one by one in the list. - As an example, this will enable all four letter word commands: - </p> -<pre class="code"> - 4lw.commands.whitelist=* - </pre> -</dd> - - -<dt> -<term>ipReachableTimeout</term> -</dt> -<dd> -<p>(Java system property: <strong>zookeeper.ipReachableTimeout</strong>)</p> -<p> -<strong>New in 3.4.11:</strong> - Set this timeout value for IP addresses reachable checking when hostname is resolved, as mesured in - milliseconds. - By default, ZooKeeper will use the first IP address of the hostname(without any reachable checking). - When zookeeper.ipReachableTimeout is set(larger than 0), ZooKeeper will will try to pick up the first - IP address which is reachable. This is done by calling Java API InetAddress.isReachable(long timeout) - function, in which this timeout value is used. If none of such reachable IP address can be found, the - first IP address of the hostname will be used anyway. - </p> -</dd> - - -<dt> -<term>tcpKeepAlive</term> -</dt> -<dd> -<p>(Java system property: <strong>zookeeper.tcpKeepAlive</strong>)</p> -<p> -<strong>New in 3.4.11:</strong> - Setting this to true sets the TCP keepAlive flag on the - sockets used by quorum members to perform elections. - This will allow for connections between quorum members to - remain up when there is network infrastructure that may - otherwise break them. Some NATs and firewalls may terminate - or lose state for long running or idle connections.</p> -<p> Enabling this option relies on OS level settings to work - properly, check your operating system's options regarding TCP - keepalive for more information. Defaults to - <strong>false</strong>. - </p> -</dd> - - -</dl> -<p></p> -<a name="sc_authOptions"></a> -<h4>Authentication & 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:<password>". Provide the - generated "super:<data>" 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:<password>". 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> - - -<dt> -<term>isro</term> -</dt> -<dd> -<p> -<strong>New in 3.4.0:</strong> Tests if - server is running in read-only mode. The server will respond with - "ro" if in read-only mode or "rw" if not in read-only mode.</p> -</dd> - - -<dt> -<term>gtmk</term> -</dt> -<dd> -<p>Gets the current trace mask as a 64-bit signed long value in - decimal format. See <span class="codefrag command">stmk</span> for an explanation of - the possible values.</p> -</dd> - - -<dt> -<term>stmk</term> -</dt> -<dd> -<p>Sets the current trace mask. The trace mask is 64 bits, - where each bit enables or disables a specific category of trace - logging on the server. Log4J must be configured to enable - <span class="codefrag command">TRACE</span> level first in order to see trace logging - messages. The bits of the trace mask correspond to the following - trace logging categories.</p> -<table class="ForrestTable" cellspacing="1" cellpadding="4"> -<caption>Trace Mask Bit Values</caption> - -<title>Trace Mask Bit Values</title> - - -<tr> - -<td>0b0000000000</td> - <td>Unused, reserved for future use.</td> - -</tr> - -<tr> - -<td>0b0000000010</td> - <td>Logs client requests, excluding ping - requests.</td> - -</tr> - -<tr> - -<td>0b0000000100</td> - <td>Unused, reserved for future use.</td> - -</tr> - -<tr> - -<td>0b0000001000</td> - <td>Logs client ping requests.</td> - -</tr> - -<tr> - -<td>0b0000010000</td> - <td>Logs packets received from the quorum peer that is - the current leader, excluding ping requests.</td> - -</tr> - -<tr> - -<td>0b0000100000</td> - <td>Logs addition, removal and validation of client - sessions.</td> - -</tr> - -<tr> - -<td>0b0001000000</td> - <td>Logs delivery of watch events to client - sessions.</td> - -</tr> - -<tr> - -<td>0b0010000000</td> - <td>Logs ping packets received from the quorum peer - that is the current leader.</td> - -</tr> - -<tr> - -<td>0b0100000000</td> - <td>Unused, reserved for future use.</td> - -</tr> - -<tr> - -<td>0b1000000000</td> - <td>Unused, reserved for future use.</td> - -</tr> - - -</table> -<p>All remaining bits in the 64-bit value are unused and - reserved for future use. Multiple trace logging categories are - specified by calculating the bitwise OR of the documented values. - The default trace mask is 0b0100110010. Thus, by default, trace - logging includes client requests, packets received from the - leader and sessions.</p> -<p>To set a different trace mask, send a request containing the - <span class="codefrag command">stmk</span> four-letter word followed by the trace - mask represented as a 64-bit signed long value. This example uses - the Perl <span class="codefrag command">pack</span> function to construct a trace - mask that enables all trace logging categories described above and - convert it to a 64-bit signed long value with big-endian byte - order. The result is appended to <span class="codefrag command">stmk</span> and sent - to the server using netcat. The server responds with the new - trace mask in decimal format.</p> -<pre class="code">$ perl -e "print 'stmk', pack('q>', 0b0011111010)" | nc localhost 2181 -250 - </pre> -</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="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 -zk_fsync_threshold_exceed_count 0 -</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.<zxid></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 when the number of transactions written to the - current log file reaches a (variable) threshold. The threshold is - computed using the same parameter which influences the frequency of - snapshotting (see snapCount above). 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, all log - files following it, and the last log file preceding it. The latter - requirement is necessary to include updates which happened after this - snapshot was started but went into the existing log file at that time. - This is possible because snapshotting and rolling over of logs - proceed somewhat independently in ZooKeeper. 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> -<div class="note"> -<div class="label">Note</div> -<div class="content"> - -<p>The data stored in these files is not encrypted. In the case of - storing sensitive data in ZooKeeper, necessary measures need to be - taken to prevent unauthorized access. Such measures are external to - ZooKeeper (e.g., control access to the files) and depend on the - individual settings in which it is being deployed. </p> - -</div> -</div> -<a name="Recovery+-+TxnLogToolkit"></a> -<h4>Recovery - TxnLogToolkit</h4> -<p>TxnLogToolkit is a command line tool shipped with ZooKeeper which - is capable of recovering transaction log entries with broken CRC.</p> -<p>Running it without any command line parameters or with the "-h,--help" - argument, it outputs the following help page:</p> -<pre class="code"> - $ bin/zkTxnLogToolkit.sh - - usage: TxnLogToolkit [-dhrv] txn_log_file_name - -d,--dump Dump mode. Dump all entries of the log file. (this is the default) - -h,--help Print help message - -r,--recover Recovery mode. Re-calculate CRC for broken entries. - -v,--verbose Be verbose in recovery mode: print all entries, not just fixed ones. - -y,--yes Non-interactive mode: repair all CRC errors without asking - </pre> -<p>The default behaviour is safe: it dumps the entries of the given - transaction log file to the screen: (same as using '-d,--dump' parameter)</p> -<pre class="code"> - $ bin/zkTxnLogToolkit.sh log.100000001 - ZooKeeper Transactional Log File with dbid 0 txnlog format version 2 - 4/5/18 2:15:58 PM CEST session 0x16295bafcc40000 cxid 0x0 zxid 0x100000001 createSession 30000 - <strong>CRC ERROR - 4/5/18 2:16:05 PM CEST session 0x16295bafcc40000 cxid 0x1 zxid 0x100000002 closeSession null</strong> - 4/5/18 2:16:05 PM CEST session 0x16295bafcc40000 cxid 0x1 zxid 0x100000002 closeSession null - 4/5/18 2:16:12 PM CEST session 0x26295bafcc90000 cxid 0x0 zxid 0x100000003 createSession 30000 - 4/5/18 2:17:34 PM CEST session 0x26295bafcc90000 cxid 0x0 zxid 0x200000001 closeSession null - 4/5/18 2:17:34 PM CEST session 0x16295bd23720000 cxid 0x0 zxid 0x200000002 createSession 30000 - 4/5/18 2:18:02 PM CEST session 0x16295bd23720000 cxid 0x2 zxid 0x200000003 create '/andor,#626262,v{s{31,s{'world,'anyone}}},F,1 - EOF reached after 6 txns. - </pre> -<p>There's a CRC error in the 2nd entry of the above transaction log file. In <strong>dump</strong> - mode, the toolkit only prints this information to the screen without touching the original file. In - <strong>recovery</strong> mode (-r,--recover flag) the original file still remains - untouched and all transactions will be copied over to a new txn log file with ".fixed" suffix. It recalculates - CRC values and copies the calculated value, if it doesn't match the original txn entry. - By default, the tool works interactively: it asks for confirmation whenever CRC error encountered.</p> -<pre class="code"> - $ bin/zkTxnLogToolkit.sh -r log.100000001 - ZooKeeper Transactional Log File with dbid 0 txnlog format version 2 - CRC ERROR - 4/5/18 2:16:05 PM CEST session 0x16295bafcc40000 cxid 0x1 zxid 0x100000002 closeSession null - Would you like to fix it (Yes/No/Abort) ? - </pre> -<p>Answering <strong>Yes</strong> means the newly calculated CRC value will be outputted - to the new file. <strong>No</strong> means that the original CRC value will be copied over. - <strong>Abort</strong> will abort the entire operation and exits. - (In this case the ".fixed" will not be deleted and left in a half-complete state: contains only entries which - have already been processed or only the header if the operation was aborted at the first entry.)</p> -<pre class="code"> - $ bin/zkTxnLogToolkit.sh -r log.100000001 - ZooKeeper Transactional Log File with dbid 0 txnlog format version 2 - CRC ERROR - 4/5/18 2:16:05 PM CEST session 0x16295bafcc40000 cxid 0x1 zxid 0x100000002 closeSession null - Would you like to fix it (Yes/No/Abort) ? y - EOF reached after 6 txns. - Recovery file log.100000001.fixed has been written with 1 fixed CRC error(s) - </pre> -<p>The default behaviour of recovery is to be silent: only entries with CRC error get printed to the screen. - One can turn on verbose mode with the -v,--verbose parameter to see all records. - Interactive mode can be turned off with the -y,--yes parameter. In this case all CRC errors will be fixed - in the new transaction file.</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 transaction 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> - - -<dt> -<term>Publicly accessible deployment</term> -</dt> -<dd> -<p> - A ZooKeeper ensemble is expected to operate in a trusted computing environment. - It is thus recommended to deploy ZooKeeper behind a firewall. - </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> - -<p align="right"> -<font size="-2"></font> -</p> -</div> -<!--+ - |end content - +--> -<div class="clearboth"> </div> -</div> -<div id="footer"> -<!--+ - |start bottomstrip - +--> -<div class="lastmodified"> -<script type="text/javascript"><!-- -document.write("Last Published: " + document.lastModified); -// --></script> -</div> -<div class="copyright"> - Copyright © - <a href="http://www.apache.org/licenses/";>The Apache Software Foundation.</a> -</div> -<!--+ - |end bottomstrip - +--> -</div> -</body> -</html>
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The basic idea is - very simple. First, we split servers into groups, and add a line for each group listing - the servers that form this group. Next we have to assign a weight to each server. - </p> - - -<p> - The following example shows how to configure a system with three groups of three servers - each, and we assign a weight of 1 to each server: - </p> - - -<pre class="code"> - group.1=1:2:3 - group.2=4:5:6 - group.3=7:8:9 - - weight.1=1 - weight.2=1 - weight.3=1 - weight.4=1 - weight.5=1 - weight.6=1 - weight.7=1 - weight.8=1 - weight.9=1 - </pre> - - -<p> - When running the system, we are able to form a quorum once we have a majority of votes from - a majority of non-zero-weight groups. Groups that have zero weight are discarded and not - considered when forming quorums. Looking at the example, we are able to form a quorum once - we have votes from at least two servers from each of two different groups. - </p> - -<p align="right"> -<font size="-2"></font> -</p> -</div> -<!--+ - |end content - +--> -<div class="clearboth"> </div> -</div> -<div id="footer"> -<!--+ - |start bottomstrip - +--> -<div class="lastmodified"> -<script type="text/javascript"><!-- -document.write("Last Published: " + document.lastModified); -// --></script> -</div> -<div class="copyright"> - Copyright © - <a href="http://www.apache.org/licenses/";>The Apache Software Foundation.</a> -</div> -<!--+ - |end bottomstrip - +--> -</div> -</body> -</html> http://git-wip-us.apache.org/repos/asf/zookeeper/blob/cf24deb2/docs/zookeeperHierarchicalQuorums.pdf ---------------------------------------------------------------------- diff --git a/docs/zookeeperHierarchicalQuorums.pdf b/docs/zookeeperHierarchicalQuorums.pdf deleted file mode 100644 index fd20251..0000000 Binary files a/docs/zookeeperHierarchicalQuorums.pdf and /dev/null differ
