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Posted to commits@zookeeper.apache.org by mi...@apache.org on 2015/09/03 07:07:52 UTC
svn commit: r1700930 [49/50] - in /zookeeper/site/trunk: content/
content/doc/r3.5.1-alpha/ content/doc/r3.5.1-alpha/api/
content/doc/r3.5.1-alpha/api/org/ content/doc/r3.5.1-alpha/api/org/apache/
content/doc/r3.5.1-alpha/api/org/apache/zookeeper/ cont...
Added: zookeeper/site/trunk/content/doc/r3.5.1-alpha/zookeeperQuotas.html
URL: http://svn.apache.org/viewvc/zookeeper/site/trunk/content/doc/r3.5.1-alpha/zookeeperQuotas.html?rev=1700930&view=auto
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--- zookeeper/site/trunk/content/doc/r3.5.1-alpha/zookeeperQuotas.html (added)
+++ zookeeper/site/trunk/content/doc/r3.5.1-alpha/zookeeperQuotas.html Thu Sep 3 05:07:48 2015
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+ PDF</a>
+</div>
+<h1>ZooKeeper Quota'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="#zookeeper_quotas">Quotas</a>
+<ul class="minitoc">
+<li>
+<a href="#Setting+Quotas">Setting Quotas</a>
+</li>
+<li>
+<a href="#Listing+Quotas">Listing Quotas</a>
+</li>
+<li>
+<a href="#Deleting+Quotas"> Deleting Quotas</a>
+</li>
+</ul>
+</li>
+</ul>
+</div>
+</div>
+
+
+
+
+<a name="zookeeper_quotas"></a>
+<h2 class="h3">Quotas</h2>
+<div class="section">
+<p> ZooKeeper has both namespace and bytes quotas. You can use the ZooKeeperMain class to setup quotas.
+ ZooKeeper prints <em>WARN</em> messages if users exceed the quota assigned to them. The messages
+ are printed in the log of the ZooKeeper.
+ </p>
+<p>
+<span class="codefrag computeroutput">$java -cp zookeeper.jar:src/java/lib/log4j-1.2.16.jar:src/java/lib/jline-2.11.jar:conf \
+ org.apache.zookeeper.ZooKeeperMain -server host:port</span>
+</p>
+<p> The above command gives you a command line option of using quotas.</p>
+<a name="Setting+Quotas"></a>
+<h3 class="h4">Setting Quotas</h3>
+<p>You can use
+ <em>setquota</em> to set a quota on a ZooKeeper node. It has an option of setting quota with
+ -n (for namespace)
+ and -b (for bytes). </p>
+<p> The ZooKeeper quota are stored in ZooKeeper itself in /zookeeper/quota. To disable other people from
+ changing the quota's set the ACL for /zookeeper/quota such that only admins are able to read and write to it.
+ </p>
+<a name="Listing+Quotas"></a>
+<h3 class="h4">Listing Quotas</h3>
+<p> You can use
+ <em>listquota</em> to list a quota on a ZooKeeper node.
+ </p>
+<a name="Deleting+Quotas"></a>
+<h3 class="h4"> Deleting Quotas</h3>
+<p> You can use
+ <em>delquota</em> to delete quota on a ZooKeeper node.
+ </p>
+</div>
+
+<p align="right">
+<font size="-2"></font>
+</p>
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Added: zookeeper/site/trunk/content/doc/r3.5.1-alpha/zookeeperQuotas.pdf
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--- zookeeper/site/trunk/content/doc/r3.5.1-alpha/zookeeperReconfig.html (added)
+++ zookeeper/site/trunk/content/doc/r3.5.1-alpha/zookeeperReconfig.html Thu Sep 3 05:07:48 2015
@@ -0,0 +1,1098 @@
+<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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+</div>
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+ +-->
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+ |start Project Logo
+ +-->
+<div class="projectlogo">
+<a href="http://zookeeper.apache.org/"><img class="logoImage" alt="ZooKeeper" src="images/zookeeper_small.gif" title="ZooKeeper: distributed coordination"></a>
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+</div>
+<div onclick="SwitchMenu('menu_selected_1.3', 'skin/')" id="menu_selected_1.3Title" class="menutitle" style="background-image: url('skin/images/chapter_open.gif');">Admin & Ops</div>
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+<a href="zookeeperObservers.html">Observers Guide</a>
+</div>
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+<div class="menupagetitle">Dynamic Reconfiguration</div>
+</div>
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+<a class="dida" href="zookeeperReconfig.pdf"><img alt="PDF -icon" src="skin/images/pdfdoc.gif" class="skin"><br>
+ PDF</a>
+</div>
+<h1>ZooKeeper Dynamic Reconfiguration</h1>
+<div id="front-matter">
+<div id="minitoc-area">
+<ul class="minitoc">
+<li>
+<a href="#ch_reconfig_intro">Overview</a>
+</li>
+<li>
+<a href="#ch_reconfig_format">Changes to Configuration Format</a>
+<ul class="minitoc">
+<li>
+<a href="#sc_reconfig_clientport">Specifying the client port</a>
+</li>
+<li>
+<a href="#sc_reconfig_standaloneEnabled">The standaloneEnabled flag</a>
+</li>
+<li>
+<a href="#sc_reconfig_file">Dynamic configuration file</a>
+</li>
+<li>
+<a href="#sc_reconfig_backward">Backward compatibility</a>
+</li>
+</ul>
+</li>
+<li>
+<a href="#ch_reconfig_upgrade">Upgrading to 3.5.0</a>
+</li>
+<li>
+<a href="#ch_reconfig_dyn">Dynamic Reconfiguration of the ZooKeeper Ensemble</a>
+<ul class="minitoc">
+<li>
+<a href="#sc_reconfig_retrieving">Retrieving the current dynamic configuration</a>
+</li>
+<li>
+<a href="#sc_reconfig_modifying">Modifying the current dynamic configuration</a>
+<ul class="minitoc">
+<li>
+<a href="#sc_reconfig_general">General</a>
+</li>
+<li>
+<a href="#sc_reconfig_incremental">Incremental mode</a>
+</li>
+<li>
+<a href="#sc_reconfig_nonincremental">Non-incremental mode</a>
+</li>
+<li>
+<a href="#sc_reconfig_conditional">Conditional reconfig</a>
+</li>
+<li>
+<a href="#sc_reconfig_errors">Error conditions</a>
+</li>
+<li>
+<a href="#sc_reconfig_additional">Additional comments</a>
+</li>
+</ul>
+</li>
+</ul>
+</li>
+<li>
+<a href="#ch_reconfig_rebalancing">Rebalancing Client Connections</a>
+</li>
+</ul>
+</div>
+</div>
+
+
+
+
+<a name="ch_reconfig_intro"></a>
+<h2 class="h3">Overview</h2>
+<div class="section">
+<p>Prior to the 3.5.0 release, the membership and all other configuration
+ parameters of Zookeeper were static - loaded during boot and immutable at
+ runtime. Operators resorted to ''rolling restarts'' - a manually intensive
+ and error-prone method of changing the configuration that has caused data
+ loss and inconsistency in production.</p>
+<p>Starting with 3.5.0, “rolling restarts” are no longer needed!
+ ZooKeeper comes with full support for automated configuration changes: the
+ set of Zookeeper servers, their roles (participant / observer), all ports,
+ and even the quorum system can be changed dynamically, without service
+ interruption and while maintaining data consistency. Reconfigurations are
+ performed immediately, just like other operations in ZooKeeper. Multiple
+ changes can be done using a single reconfiguration command. The dynamic
+ reconfiguration functionality does not limit operation concurrency, does
+ not require client operations to be stopped during reconfigurations, has a
+ very simple interface for administrators and no added complexity to other
+ client operations.</p>
+<p>New client-side features allow clients to find out about configuration
+ changes and to update the connection string (list of servers and their
+ client ports) stored in their ZooKeeper handle. A probabilistic algorithm
+ is used to rebalance clients across the new configuration servers while
+ keeping the extent of client migrations proportional to the change in
+ ensemble membership.</p>
+<p>This document provides the administrator manual for reconfiguration.
+ For a detailed description of the reconfiguration algorithms, performance
+ measurements, and more, please see our paper:</p>
+<dl>
+
+<dt>
+<term>Shraer, A., Reed, B., Malkhi, D., Junqueira, F. Dynamic
+ Reconfiguration of Primary/Backup Clusters. In <em>USENIX Annual
+ Technical Conference (ATC) </em>(2012), 425-437</term>
+</dt>
+<dd>
+<p>Links: <a href="https://www.usenix.org/system/files/conference/atc12/atc12-final74.pdf">paper (pdf)</a>, <a href="https://www.usenix.org/sites/default/files/conference/protected-files/shraer_atc12_slides.pdf">slides (pdf)</a>, <a href="https://www.usenix.org/conference/atc12/technical-sessions/presentation/shraer">video</a>, <a href="http://www.slideshare.net/Hadoop_Summit/dynamic-reconfiguration-of-zookeeper">hadoop summit slides</a>
+</p>
+</dd>
+
+</dl>
+</div>
+
+<a name="ch_reconfig_format"></a>
+<h2 class="h3">Changes to Configuration Format</h2>
+<div class="section">
+<a name="sc_reconfig_clientport"></a>
+<h3 class="h4">Specifying the client port</h3>
+<p>A client port of a server is the port on which the server accepts
+ client connection requests. Starting with 3.5.0 the
+ <em>clientPort</em> and <em>clientPortAddress
+ </em> configuration parameters should no longer be used. Instead,
+ this information is now part of the server keyword specification, which
+ becomes as follows:</p>
+<p>
+<span class="codefrag computeroutput">server.<positive id> = <address1>:<port1>:<port2>[:role];[<client port address>:]<client port></span>
+</p>
+<p>The client port specification is to the right of the semicolon. The
+ client port address is optional, and if not specified it defaults to
+ "0.0.0.0". As usual, role is also optional, it can be
+ <em>participant</em> or <em>observer</em>
+ (<em>participant</em> by default).</p>
+<p> Examples of legal server statements: </p>
+<ul>
+
+<li>
+
+<p>
+<span class="codefrag computeroutput">server.5 = 125.23.63.23:1234:1235;1236</span>
+</p>
+
+</li>
+
+<li>
+
+<p>
+<span class="codefrag computeroutput">server.5 = 125.23.63.23:1234:1235:participant;1236</span>
+</p>
+
+</li>
+
+<li>
+
+<p>
+<span class="codefrag computeroutput">server.5 = 125.23.63.23:1234:1235:observer;1236</span>
+</p>
+
+</li>
+
+<li>
+
+<p>
+<span class="codefrag computeroutput">server.5 = 125.23.63.23:1234:1235;125.23.63.24:1236</span>
+</p>
+
+</li>
+
+<li>
+
+<p>
+<span class="codefrag computeroutput">server.5 = 125.23.63.23:1234:1235:participant;125.23.63.23:1236</span>
+</p>
+
+</li>
+
+</ul>
+<a name="sc_reconfig_standaloneEnabled"></a>
+<h3 class="h4">The standaloneEnabled flag</h3>
+<p>Prior to 3.5.0, one could run ZooKeeper in Standalone mode or in a
+ Distributed mode. These are separate implementation stacks, and
+ switching between them during run time is not possible. By default (for
+ backward compatibility) <em>standaloneEnabled</em> is set to
+ <em>true</em>. The consequence of using this default is that
+ if started with a single server the ensemble will not be allowed to
+ grow, and if started with more than one server it will not be allowed to
+ shrink to contain fewer than two participants.</p>
+<p>Setting the flag to <em>false</em> instructs the system
+ to run the Distributed software stack even if there is only a single
+ participant in the ensemble. To achieve this the (static) configuration
+ file should contain:</p>
+<p>
+<span class="codefrag computeroutput">standaloneEnabled=false</span>
+</p>
+<p>With this setting it is possible to start a ZooKeeper ensemble
+ containing a single participant and to dynamically grow it by adding
+ more servers. Similarly, it is possible to shrink an ensemble so that
+ just a single participant remains, by removing servers.</p>
+<p>Since running the Distributed mode allows more flexibility, we
+ recommend setting the flag to <em>false</em>. We expect that
+ the legacy Standalone mode will be deprecated in the future.</p>
+<a name="sc_reconfig_file"></a>
+<h3 class="h4">Dynamic configuration file</h3>
+<p>Starting with 3.5.0 we're distinguishing between dynamic
+ configuration parameters, which can be changed during runtime, and
+ static configuration parameters, which are read from a configuration
+ file when a server boots and don't change during its execution. For now,
+ the following configuration keywords are considered part of the dynamic
+ configuration: <em>server</em>, <em>group</em>
+ and <em>weight</em>.</p>
+<p>Dynamic configuration parameters are stored in a separate file on
+ the server (which we call the dynamic configuration file). This file is
+ linked from the static config file using the new
+ <em>dynamicConfigFile</em> keyword.</p>
+<p>
+<strong>Example</strong>
+</p>
+<div class="note example">
+<div class="label">zoo_replicated1.cfg</div>
+<div class="content">
+
+<title>zoo_replicated1.cfg</title>
+
+<pre class="code">tickTime=2000
+dataDir=/zookeeper/data/zookeeper1
+initLimit=5
+syncLimit=2
+dynamicConfigFile=/zookeeper/conf/zoo_replicated1.cfg.dynamic</pre>
+
+</div>
+</div>
+<div class="note example">
+<div class="label">zoo_replicated1.cfg.dynamic</div>
+<div class="content">
+
+<title>zoo_replicated1.cfg.dynamic</title>
+
+<pre class="code">server.1=125.23.63.23:2780:2783:participant;2791
+server.2=125.23.63.24:2781:2784:participant;2792
+server.3=125.23.63.25:2782:2785:participant;2793</pre>
+
+</div>
+</div>
+<p>When the ensemble configuration changes, the static configuration
+ parameters remain the same. The dynamic parameters are pushed by
+ ZooKeeper and overwrite the dynamic configuration files on all servers.
+ Thus, the dynamic configuration files on the different servers are
+ usually identical (they can only differ momentarily when a
+ reconfiguration is in progress, or if a new configuration hasn't
+ propagated yet to some of the servers). Once created, the dynamic
+ configuration file should not be manually altered. Changed are only made
+ through the new reconfiguration commands outlined below. Note that
+ changing the config of an offline cluster could result in an
+ inconsistency with respect to configuration information stored in the
+ ZooKeeper log (and the special configuration znode, populated from the
+ log) and is therefore highly discouraged.</p>
+<p>
+<strong>Example 2</strong>
+</p>
+<p>Users may prefer to initially specify a single configuration file.
+ The following is thus also legal:</p>
+<div class="note example">
+<div class="label">zoo_replicated1.cfg</div>
+<div class="content">
+
+<title>zoo_replicated1.cfg</title>
+
+<pre class="code">tickTime=2000
+dataDir=/zookeeper/data/zookeeper1
+initLimit=5
+syncLimit=2
+clientPort=<strong>2791</strong> // note that this line is now redundant and therefore not recommended
+server.1=125.23.63.23:2780:2783:participant;<strong>2791</strong>
+server.2=125.23.63.24:2781:2784:participant;2792
+server.3=125.23.63.25:2782:2785:participant;2793</pre>
+
+</div>
+</div>
+<p>The configuration files on each server will be automatically split
+ into dynamic and static files, if they are not already in this format.
+ So the configuration file above will be automatically transformed into
+ the two files in Example 1. Note that the clientPort and
+ clientPortAddress lines (if specified) will be automatically removed
+ during this process, if they are redundant (as in the example above).
+ The original static configuration file is backed up (in a .bak
+ file).</p>
+<a name="sc_reconfig_backward"></a>
+<h3 class="h4">Backward compatibility</h3>
+<p>We still support the old configuration format. For example, the
+ following configuration file is acceptable (but not recommended):</p>
+<div class="note example">
+<div class="label">zoo_replicated1.cfg</div>
+<div class="content">
+
+<title>zoo_replicated1.cfg</title>
+
+<pre class="code">tickTime=2000
+dataDir=/zookeeper/data/zookeeper1
+initLimit=5
+syncLimit=2
+clientPort=2791
+server.1=125.23.63.23:2780:2783:participant
+server.2=125.23.63.24:2781:2784:participant
+server.3=125.23.63.25:2782:2785:participant</pre>
+
+</div>
+</div>
+<p>During boot, a dynamic configuration file is created and contains
+ the dynamic part of the configuration as explained earlier. In this
+ case, however, the line "clientPort=2791" will remain in the static
+ configuration file of server 1 since it is not redundant -- it was not
+ specified as part of the "server.1=..." using the format explained in
+ the section <a href="#ch_reconfig_format">Changes to Configuration Format</a>. If a reconfiguration
+ is invoked that sets the client port of server 1, we remove
+ "clientPort=2791" from the static configuration file (the dynamic file
+ now contain this information as part of the specification of server
+ 1).</p>
+</div>
+
+<a name="ch_reconfig_upgrade"></a>
+<h2 class="h3">Upgrading to 3.5.0</h2>
+<div class="section">
+<p>Upgrading a running ZooKeeper ensemble to 3.5.0 should be done only
+ after upgrading your ensemble to the 3.4.6 release. Note that this is only
+ necessary for rolling upgrades (if you're fine with shutting down the
+ system completely, you don't have to go through 3.4.6). If you attempt a
+ rolling upgrade without going through 3.4.6 (for example from 3.4.5), you
+ may get the following error:</p>
+<pre class="code">2013-01-30 11:32:10,663 [myid:2] - INFO [localhost/127.0.0.1:2784:QuorumCnxManager$Listener@498] - Received connection request /127.0.0.1:60876
+2013-01-30 11:32:10,663 [myid:2] - WARN [localhost/127.0.0.1:2784:QuorumCnxManager@349] - Invalid server id: -65536</pre>
+<p>During a rolling upgrade, each server is taken down in turn and
+ rebooted with the new 3.5.0 binaries. Before starting the server with
+ 3.5.0 binaries, we highly recommend updating the configuration file so
+ that all server statements "server.x=..." contain client ports (see the
+ section <a href="#sc_reconfig_clientport">Specifying the client port</a>). As explained earlier
+ you may leave the configuration in a single file, as well as leave the
+ clientPort/clientPortAddress statements (although if you specify client
+ ports in the new format, these statements are now redundant).</p>
+</div>
+
+<a name="ch_reconfig_dyn"></a>
+<h2 class="h3">Dynamic Reconfiguration of the ZooKeeper Ensemble</h2>
+<div class="section">
+<p>The ZooKeeper Java and C API were extended with getConfig and reconfig
+ commands that facilitate reconfiguration. Both commands have a synchronous
+ (blocking) variant and an asynchronous one. We demonstrate these commands
+ here using the Java CLI, but note that you can similarly use the C CLI or
+ invoke the commands directly from a program just like any other ZooKeeper
+ command.</p>
+<a name="sc_reconfig_retrieving"></a>
+<h3 class="h4">Retrieving the current dynamic configuration</h3>
+<p>The dynamic configuration is stored in a special znode
+ ZooDefs.CONFIG_NODE = /zookeeper/config. The new
+ <span class="codefrag command">config</span> CLI command reads this znode (currently it is
+ simply a wrapper to <span class="codefrag command">get /zookeeper/config</span>). As with
+ normal reads, to retrieve the latest committed value you should do a
+ <span class="codefrag command">sync</span> first.</p>
+<pre class="code">[zk: 127.0.0.1:2791(CONNECTED) 3] config
+server.1=localhost:2780:2783:participant;localhost:2791
+server.2=localhost:2781:2784:participant;localhost:2792
+server.3=localhost:2782:2785:participant;localhost:2793
+<strong>version=400000003</strong>
+</pre>
+<p>Notice the last line of the output. This is the configuration
+ version. The version equals to the zxid of the reconfiguration command
+ which created this configuration. The version of the first established
+ configuration equals to the zxid of the NEWLEADER message sent by the
+ first successfully established leader. When a configuration is written
+ to a dynamic configuration file, the version automatically becomes part
+ of the filename and the static configuration file is updated with the
+ path to the new dynamic configuration file. Configuration files
+ corresponding to earlier versions are retained for backup
+ purposes.</p>
+<p>During boot time the version (if it exists) is extracted from the
+ filename. The version should never be altered manually by users or the
+ system administrator. It is used by the system to know which
+ configuration is most up-to-date. Manipulating it manually can result in
+ data loss and inconsistency.</p>
+<p>Just like a <span class="codefrag command">get</span> command, the
+ <span class="codefrag command">config</span> CLI command accepts the <span class="codefrag option">-w</span>
+ flag for setting a watch on the znode, and <span class="codefrag option">-s</span> flag for
+ displaying the Stats of the znode. It additionally accepts a new flag
+ <span class="codefrag option">-c</span> which outputs only the version and the client
+ connection string corresponding to the current configuration. For
+ example, for the configuration above we would get:</p>
+<pre class="code">[zk: 127.0.0.1:2791(CONNECTED) 17] config -c
+400000003 localhost:2791,localhost:2793,localhost:2792</pre>
+<p>Note that when using the API directly, this command is called
+ <span class="codefrag command">getConfig</span>.</p>
+<p>As any read command it returns the configuration known to the
+ follower to which your client is connected, which may be slightly
+ out-of-date. One can use the <span class="codefrag command">sync</span> command for
+ stronger guarantees. For example using the Java API:</p>
+<pre class="code">zk.sync(ZooDefs.CONFIG_NODE, void_callback, context);
+zk.getConfig(watcher, callback, context);</pre>
+<p>Note: in 3.5.0 it doesn't really matter which path is passed to the
+ <span class="codefrag command">sync() </span> command as all the server's state is brought
+ up to date with the leader (so one could use a different path instead of
+ ZooDefs.CONFIG_NODE). However, this may change in the future.</p>
+<a name="sc_reconfig_modifying"></a>
+<h3 class="h4">Modifying the current dynamic configuration</h3>
+<p>Modifying the configuration is done through the
+ <span class="codefrag command">reconfig</span> command. There are two modes of
+ reconfiguration: incremental and non-incremental (bulk). The
+ non-incremental simply specifies the new dynamic configuration of the
+ system. The incremental specifies changes to the current configuration.
+ The <span class="codefrag command">reconfig</span> command returns the new
+ configuration.</p>
+<p>A few examples are in: <span class="codefrag filename">ReconfigTest.java</span>,
+ <span class="codefrag filename">ReconfigRecoveryTest.java</span> and
+ <span class="codefrag filename">TestReconfigServer.cc</span>.</p>
+<a name="sc_reconfig_general"></a>
+<h4>General</h4>
+<p>
+<strong>Removing servers:</strong> Any server can
+ be removed, including the leader (although removing the leader will
+ result in a short unavailability, see Figures 6 and 8 in the <a href="https://www.usenix.org/conference/usenixfederatedconferencesweek/dynamic-recon%EF%AC%81guration-primarybackup-clusters">paper</a>). The server will not be shut-down automatically.
+ Instead, it becomes a "non-voting follower". This is somewhat similar
+ to an observer in that its votes don't count towards the Quorum of
+ votes necessary to commit operations. However, unlike a non-voting
+ follower, an observer doesn't actually see any operation proposals and
+ does not ACK them. Thus a non-voting follower has a more significant
+ negative effect on system throughput compared to an observer.
+ Non-voting follower mode should only be used as a temporary mode,
+ before shutting the server down, or adding it as a follower or as an
+ observer to the ensemble. We do not shut the server down automatically
+ for two main reasons. The first reason is that we do not want all the
+ clients connected to this server to be immediately disconnected,
+ causing a flood of connection requests to other servers. Instead, it
+ is better if each client decides when to migrate independently. The
+ second reason is that removing a server may sometimes (rarely) be
+ necessary in order to change it from "observer" to "participant" (this
+ is explained in the section <a href="#sc_reconfig_additional">Additional comments</a>).</p>
+<p>Note that the new configuration should have some minimal number of
+ participants in order to be considered legal. If the proposed change
+ would leave the cluster with less than 2 participants and standalone
+ mode is enabled (standaloneEnabled=true, see the section <a href="#sc_reconfig_standaloneEnabled">The standaloneEnabled flag</a>), the reconfig will not be
+ processed (BadArgumentsException). If standalone mode is disabled
+ (standaloneEnabled=false) then its legal to remain with 1 or more
+ participants.</p>
+<p>
+<strong>Adding servers:</strong> Before a
+ reconfiguration is invoked, the administrator must make sure that a
+ quorum (majority) of participants from the new configuration are
+ already connected and synced with the current leader. To achieve this
+ we need to connect a new joining server to the leader before it is
+ officially part of the ensemble. This is done by starting the joining
+ server using an initial list of servers which is technically not a
+ legal configuration of the system but (a) contains the joiner, and (b)
+ gives sufficient information to the joiner in order for it to find and
+ connect to the current leader. We list a few different options of
+ doing this safely.</p>
+<ol>
+
+<li>
+
+<p>Initial configuration of joiners is comprised of servers in
+ the last committed configuration and one or more joiners, where
+ <strong>joiners are listed as observers.</strong>
+ For example, if servers D and E are added at the same time to (A,
+ B, C) and server C is being removed, the initial configuration of
+ D could be (A, B, C, D) or (A, B, C, D, E), where D and E are
+ listed as observers. Similarly, the configuration of E could be
+ (A, B, C, E) or (A, B, C, D, E), where D and E are listed as
+ observers. <strong>Note that listing the joiners as
+ observers will not actually make them observers - it will only
+ prevent them from accidentally forming a quorum with other
+ joiners.</strong> Instead, they will contact the servers in the
+ current configuration and adopt the last committed configuration
+ (A, B, C), where the joiners are absent. Configuration files of
+ joiners are backed up and replaced automatically as this happens.
+ After connecting to the current leader, joiners become non-voting
+ followers until the system is reconfigured and they are added to
+ the ensemble (as participant or observer, as appropriate).</p>
+
+</li>
+
+<li>
+
+<p>Initial configuration of each joiner is comprised of servers
+ in the last committed configuration + <strong>the
+ joiner itself, listed as a participant.</strong> For example, to
+ add a new server D to a configuration consisting of servers (A, B,
+ C), the administrator can start D using an initial configuration
+ file consisting of servers (A, B, C, D). If both D and E are added
+ at the same time to (A, B, C), the initial configuration of D
+ could be (A, B, C, D) and the configuration of E could be (A, B,
+ C, E). Similarly, if D is added and C is removed at the same time,
+ the initial configuration of D could be (A, B, C, D). Never list
+ more than one joiner as participant in the initial configuration
+ (see warning below).</p>
+
+</li>
+
+<li>
+
+<p>Whether listing the joiner as an observer or as participant,
+ it is also fine not to list all the current configuration servers,
+ as long as the current leader is in the list. For example, when
+ adding D we could start D with a configuration file consisting of
+ just (A, D) if A is the current leader. however this is more
+ fragile since if A fails before D officially joins the ensemble, D
+ doesn’t know anyone else and therefore the administrator will have
+ to intervene and restart D with another server list.</p>
+
+</li>
+
+</ol>
+<div class="note">
+<div class="label">Warning</div>
+<div class="content">
+
+<title>Warning</title>
+
+<p>Never specify more than one joining server in the same initial
+ configuration as participants. Currently, the joining servers don’t
+ know that they are joining an existing ensemble; if multiple joiners
+ are listed as participants they may form an independent quorum
+ creating a split-brain situation such as processing operations
+ independently from your main ensemble. It is OK to list multiple
+ joiners as observers in an initial config.</p>
+
+</div>
+</div>
+<p>Finally, note that once connected to the leader, a joiner adopts
+ the last committed configuration, in which it is absent (the initial
+ config of the joiner is backed up before being rewritten). If the
+ joiner restarts in this state, it will not be able to boot since it is
+ absent from its configuration file. In order to start it you’ll once
+ again have to specify an initial configuration.</p>
+<p>
+<strong>Modifying server parameters:</strong> One
+ can modify any of the ports of a server, or its role
+ (participant/observer) by adding it to the ensemble with different
+ parameters. This works in both the incremental and the bulk
+ reconfiguration modes. It is not necessary to remove the server and
+ then add it back; just specify the new parameters as if the server is
+ not yet in the system. The server will detect the configuration change
+ and perform the necessary adjustments. See an example in the section
+ <a href="#sc_reconfig_incremental">Incremental mode</a> and an exception to this
+ rule in the section <a href="#sc_reconfig_additional">Additional comments</a>.</p>
+<p>It is also possible to change the Quorum System used by the
+ ensemble (for example, change the Majority Quorum System to a
+ Hierarchical Quorum System on the fly). This, however, is only allowed
+ using the bulk (non-incremental) reconfiguration mode. In general,
+ incremental reconfiguration only works with the Majority Quorum
+ System. Bulk reconfiguration works with both Hierarchical and Majority
+ Quorum Systems.</p>
+<p>
+<strong>Performance Impact:</strong> There is
+ practically no performance impact when removing a follower, since it
+ is not being automatically shut down (the effect of removal is that
+ the server's votes are no longer being counted). When adding a server,
+ there is no leader change and no noticeable performance disruption.
+ For details and graphs please see Figures 6, 7 and 8 in the <a href="https://www.usenix.org/conference/usenixfederatedconferencesweek/dynamic-recon%EF%AC%81guration-primarybackup-clusters">paper</a>.</p>
+<p>The most significant disruption will happen when a leader change
+ is caused, in one of the following cases:</p>
+<ol>
+
+<li>
+
+<p>Leader is removed from the ensemble.</p>
+
+</li>
+
+<li>
+
+<p>Leader's role is changed from participant to observer.</p>
+
+</li>
+
+<li>
+
+<p>The port used by the leader to send transactions to others
+ (quorum port) is modified.</p>
+
+</li>
+
+</ol>
+<p>In these cases we perform a leader hand-off where the old leader
+ nominates a new leader. The resulting unavailability is usually
+ shorter than when a leader crashes since detecting leader failure is
+ unnecessary and electing a new leader can usually be avoided during a
+ hand-off (see Figures 6 and 8 in the <a href="https://www.usenix.org/conference/usenixfederatedconferencesweek/dynamic-recon%EF%AC%81guration-primarybackup-clusters">paper</a>).</p>
+<p>When the client port of a server is modified, it does not drop
+ existing client connections. New connections to the server will have
+ to use the new client port.</p>
+<p>
+<strong>Progress guarantees:</strong> Up to the
+ invocation of the reconfig operation, a quorum of the old
+ configuration is required to be available and connected for ZooKeeper
+ to be able to make progress. Once reconfig is invoked, a quorum of
+ both the old and of the new configurations must be available. The
+ final transition happens once (a) the new configuration is activated,
+ and (b) all operations scheduled before the new configuration is
+ activated by the leader are committed. Once (a) and (b) happen, only a
+ quorum of the new configuration is required. Note, however, that
+ neither (a) nor (b) are visible to a client. Specifically, when a
+ reconfiguration operation commits, it only means that an activation
+ message was sent out by the leader. It does not necessarily mean that
+ a quorum of the new configuration got this message (which is required
+ in order to activate it) or that (b) has happened. If one wants to
+ make sure that both (a) and (b) has already occurred (for example, in
+ order to know that it is safe to shut down old servers that were
+ removed), one can simply invoke an update
+ (<span class="codefrag command">set-data</span>, or some other quorum operation, but not
+ a <span class="codefrag command">sync</span>) and wait for it to commit. An alternative
+ way to achieve this was to introduce another round to the
+ reconfiguration protocol (which, for simplicity and compatibility with
+ Zab, we decided to avoid).</p>
+<a name="sc_reconfig_incremental"></a>
+<h4>Incremental mode</h4>
+<p>The incremental mode allows adding and removing servers to the
+ current configuration. Multiple changes are allowed. For
+ example:</p>
+<p>
+<span class="codefrag userinput">> reconfig -remove 3 -add
+ server.5=125.23.63.23:1234:1235;1236</span>
+</p>
+<p>Both the add and the remove options get a list of comma separated
+ arguments (no spaces):</p>
+<p>
+<span class="codefrag userinput">> reconfig -remove 3,4 -add
+ server.5=localhost:2111:2112;2113,6=localhost:2114:2115:observer;2116</span>
+</p>
+<p>The format of the server statement is exactly the same as
+ described in the section <a href="#sc_reconfig_clientport">Specifying the client port</a> and
+ includes the client port. Notice that here instead of "server.5=" you
+ can just say "5=". In the example above, if server 5 is already in the
+ system, but has different ports or is not an observer, it is updated
+ and once the configuration commits becomes an observer and starts
+ using these new ports. This is an easy way to turn participants into
+ observers and vise versa or change any of their ports, without
+ rebooting the server.</p>
+<p>ZooKeeper supports two types of Quorum Systems – the simple
+ Majority system (where the leader commits operations after receiving
+ ACKs from a majority of voters) and a more complex Hierarchical
+ system, where votes of different servers have different weights and
+ servers are divided into voting groups. Currently, incremental
+ reconfiguration is allowed only if the last proposed configuration
+ known to the leader uses a Majority Quorum System
+ (BadArgumentsException is thrown otherwise).</p>
+<p>Incremental mode - examples using the Java API:</p>
+<pre class="code">List<String> leavingServers = new ArrayList<String>();
+leavingServers.add("1");
+leavingServers.add("2");
+byte[] config = zk.reconfig(null, leavingServers, null, -1, new Stat());</pre>
+<pre class="code">List<String> leavingServers = new ArrayList<String>();
+List<String> joiningServers = new ArrayList<String>();
+leavingServers.add("1");
+joiningServers.add("server.4=localhost:1234:1235;1236");
+byte[] config = zk.reconfig(joiningServers, leavingServers, null, -1, new Stat());
+
+String configStr = new String(config);
+System.out.println(configStr);</pre>
+<p>There is also an asynchronous API, and an API accepting comma
+ separated Strings instead of List<String>. See
+ src/java/main/org/apache/zookeeper/ZooKeeper.java.</p>
+<a name="sc_reconfig_nonincremental"></a>
+<h4>Non-incremental mode</h4>
+<p>The second mode of reconfiguration is non-incremental, whereby a
+ client gives a complete specification of the new dynamic system
+ configuration. The new configuration can either be given in place or
+ read from a file:</p>
+<p>
+<span class="codefrag userinput">> reconfig -file newconfig.cfg
+ </span>//newconfig.cfg is a dynamic config file, see <a href="#sc_reconfig_file">Dynamic configuration file</a>
+</p>
+<p>
+<span class="codefrag userinput">> reconfig -members
+ server.1=125.23.63.23:2780:2783:participant;2791,server.2=125.23.63.24:2781:2784:participant;2792,server.3=125.23.63.25:2782:2785:participant;2793</span>
+</p>
+<p>The new configuration may use a different Quorum System. For
+ example, you may specify a Hierarchical Quorum System even if the
+ current ensemble uses a Majority Quorum System.</p>
+<p>Bulk mode - example using the Java API:</p>
+<pre class="code">ArrayList<String> newMembers = new ArrayList<String>();
+newMembers.add("server.1=1111:1234:1235;1236");
+newMembers.add("server.2=1112:1237:1238;1239");
+newMembers.add("server.3=1114:1240:1241:observer;1242");
+
+byte[] config = zk.reconfig(null, null, newMembers, -1, new Stat());
+
+String configStr = new String(config);
+System.out.println(configStr);</pre>
+<p>There is also an asynchronous API, and an API accepting comma
+ separated String containing the new members instead of
+ List<String>. See
+ src/java/main/org/apache/zookeeper/ZooKeeper.java.</p>
+<a name="sc_reconfig_conditional"></a>
+<h4>Conditional reconfig</h4>
+<p>Sometimes (especially in non-incremental mode) a new proposed
+ configuration depends on what the client "believes" to be the current
+ configuration, and should be applied only to that configuration.
+ Specifically, the <span class="codefrag command">reconfig</span> succeeds only if the
+ last configuration at the leader has the specified version.</p>
+<p>
+<span class="codefrag userinput">> reconfig -file <filename> -v <version></span>
+</p>
+<p>In the previously listed Java examples, instead of -1 one could
+ specify a configuration version to condition the
+ reconfiguration.</p>
+<a name="sc_reconfig_errors"></a>
+<h4>Error conditions</h4>
+<p>In addition to normal ZooKeeper error conditions, a
+ reconfiguration may fail for the following reasons:</p>
+<ol>
+
+<li>
+
+<p>another reconfig is currently in progress
+ (ReconfigInProgress)</p>
+
+</li>
+
+<li>
+
+<p>the proposed change would leave the cluster with less than 2
+ participants, in case standalone mode is enabled, or, if
+ standalone mode is disabled then its legal to remain with 1 or
+ more participants (BadArgumentsException)</p>
+
+</li>
+
+<li>
+
+<p>no quorum of the new configuration was connected and
+ up-to-date with the leader when the reconfiguration processing
+ began (NewConfigNoQuorum)</p>
+
+</li>
+
+<li>
+
+<p>
+<span class="codefrag userinput">-v x</span> was specified, but the version
+ <span class="codefrag userinput">y</span> of the latest configuration is not
+ <span class="codefrag userinput">x</span> (BadVersionException)</p>
+
+</li>
+
+<li>
+
+<p>an incremental reconfiguration was requested but the last
+ configuration at the leader uses a Quorum System which is
+ different from the Majority system (BadArgumentsException)</p>
+
+</li>
+
+<li>
+
+<p>syntax error (BadArgumentsException)</p>
+
+</li>
+
+<li>
+
+<p>I/O exception when reading the configuration from a file
+ (BadArgumentsException)</p>
+
+</li>
+
+</ol>
+<p>Most of these are illustrated by test-cases in
+ <span class="codefrag filename">ReconfigFailureCases.java</span>.</p>
+<a name="sc_reconfig_additional"></a>
+<h4>Additional comments</h4>
+<p>
+<strong>Liveness:</strong> To better understand
+ the difference between incremental and non-incremental
+ reconfiguration, suppose that client C1 adds server D to the system
+ while a different client C2 adds server E. With the non-incremental
+ mode, each client would first invoke <span class="codefrag command">config</span> to find
+ out the current configuration, and then locally create a new list of
+ servers by adding its own suggested server. The new configuration can
+ then be submitted using the non-incremental
+ <span class="codefrag command">reconfig</span> command. After both reconfigurations
+ complete, only one of E or D will be added (not both), depending on
+ which client's request arrives second to the leader, overwriting the
+ previous configuration. The other client can repeat the process until
+ its change takes effect. This method guarantees system-wide progress
+ (i.e., for one of the clients), but does not ensure that every client
+ succeeds. To have more control C2 may request to only execute the
+ reconfiguration in case the version of the current configuration
+ hasn't changed, as explained in the section <a href="#sc_reconfig_conditional">Conditional reconfig</a>. In this way it may avoid blindly
+ overwriting the configuration of C1 if C1's configuration reached the
+ leader first.</p>
+<p>With incremental reconfiguration, both changes will take effect as
+ they are simply applied by the leader one after the other to the
+ current configuration, whatever that is (assuming that the second
+ reconfig request reaches the leader after it sends a commit message
+ for the first reconfig request -- currently the leader will refuse to
+ propose a reconfiguration if another one is already pending). Since
+ both clients are guaranteed to make progress, this method guarantees
+ stronger liveness. In practice, multiple concurrent reconfigurations
+ are probably rare. Non-incremental reconfiguration is currently the
+ only way to dynamically change the Quorum System. Incremental
+ configuration is currently only allowed with the Majority Quorum
+ System.</p>
+<p>
+<strong>Changing an observer into a
+ follower:</strong> Clearly, changing a server that participates in
+ voting into an observer may fail if error (2) occurs, i.e., if fewer
+ than the minimal allowed number of participants would remain. However,
+ converting an observer into a participant may sometimes fail for a
+ more subtle reason: Suppose, for example, that the current
+ configuration is (A, B, C, D), where A is the leader, B and C are
+ followers and D is an observer. In addition, suppose that B has
+ crashed. If a reconfiguration is submitted where D is said to become a
+ follower, it will fail with error (3) since in this configuration, a
+ majority of voters in the new configuration (any 3 voters), must be
+ connected and up-to-date with the leader. An observer cannot
+ acknowledge the history prefix sent during reconfiguration, and
+ therefore it does not count towards these 3 required servers and the
+ reconfiguration will be aborted. In case this happens, a client can
+ achieve the same task by two reconfig commands: first invoke a
+ reconfig to remove D from the configuration and then invoke a second
+ command to add it back as a participant (follower). During the
+ intermediate state D is a non-voting follower and can ACK the state
+ transfer performed during the second reconfig comand.</p>
+</div>
+
+<a name="ch_reconfig_rebalancing"></a>
+<h2 class="h3">Rebalancing Client Connections</h2>
+<div class="section">
+<p>When a ZooKeeper cluster is started, if each client is given the same
+ connection string (list of servers), the client will randomly choose a
+ server in the list to connect to, which makes the expected number of
+ client connections per server the same for each of the servers. We
+ implemented a method that preserves this property when the set of servers
+ changes through reconfiguration. See Sections 4 and 5.1 in the <a href="https://www.usenix.org/conference/usenixfederatedconferencesweek/dynamic-recon%EF%AC%81guration-primarybackup-clusters">paper</a>.</p>
+<p>In order for the method to work, all clients must subscribe to
+ configuration changes (by setting a watch on /zookeeper/config either
+ directly or through the <span class="codefrag command">getConfig</span> API command). When
+ the watch is triggered, the client should read the new configuration by
+ invoking <span class="codefrag command">sync</span> and <span class="codefrag command">getConfig</span> and if
+ the configuration is indeed new invoke the
+ <span class="codefrag command">updateServerList</span> API command. To avoid mass client
+ migration at the same time, it is better to have each client sleep a
+ random short period of time before invoking
+ <span class="codefrag command">updateServerList</span>.</p>
+<p>A few examples can be found in:
+ <span class="codefrag filename">StaticHostProviderTest.java</span> and
+ <span class="codefrag filename">TestReconfig.cc</span>
+</p>
+<p>Example (this is not a recipe, but a simplified example just to
+ explain the general idea):</p>
+<pre class="code">
+public void process(WatchedEvent event) {
+ synchronized (this) {
+ if (event.getType() == EventType.None) {
+ connected = (event.getState() == KeeperState.SyncConnected);
+ notifyAll();
+ } else if (event.getPath()!=null && event.getPath().equals(ZooDefs.CONFIG_NODE)) {
+ // in prod code never block the event thread!
+ zk.sync(ZooDefs.CONFIG_NODE, this, null);
+ zk.getConfig(this, this, null);
+ }
+ }
+}
+public void processResult(int rc, String path, Object ctx, byte[] data, Stat stat) {
+ if (path!=null && path.equals(ZooDefs.CONFIG_NODE)) {
+ String config[] = ConfigUtils.getClientConfigStr(new String(data)).split(" "); // similar to config -c
+ long version = Long.parseLong(config[0], 16);
+ if (this.configVersion == null){
+ this.configVersion = version;
+ } else if (version > this.configVersion) {
+ hostList = config[1];
+ try {
+ // the following command is not blocking but may cause the client to close the socket and
+ // migrate to a different server. In practice its better to wait a short period of time, chosen
+ // randomly, so that different clients migrate at different times
+ zk.updateServerList(hostList);
+ } catch (IOException e) {
+ System.err.println("Error updating server list");
+ e.printStackTrace();
+ }
+ this.configVersion = version;
+} } }</pre>
+</div>
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+</div>
+<h1>ZooKeeper Getting Started Guide</h1>
+<div id="front-matter">
+<div id="minitoc-area">
+<ul class="minitoc">
+<li>
+<a href="#ch_GettingStarted">Getting Started: Coordinating Distributed Applications with
+ ZooKeeper</a>
+<ul class="minitoc">
+<li>
+<a href="#sc_Prerequisites">Pre-requisites</a>
+</li>
+<li>
+<a href="#sc_Download">Download</a>
+</li>
+<li>
+<a href="#sc_InstallingSingleMode">Standalone Operation</a>
+</li>
+<li>
+<a href="#sc_FileManagement">Managing ZooKeeper Storage</a>
+</li>
+<li>
+<a href="#sc_ConnectingToZooKeeper">Connecting to ZooKeeper</a>
+</li>
+<li>
+<a href="#sc_ProgrammingToZooKeeper">Programming to ZooKeeper</a>
+</li>
+<li>
+<a href="#sc_RunningReplicatedZooKeeper">Running Replicated ZooKeeper</a>
+</li>
+<li>
+<a href="#Other+Optimizations">Other Optimizations</a>
+</li>
+</ul>
+</li>
+</ul>
+</div>
+</div>
+
+
+
+
+
+<a name="ch_GettingStarted"></a>
+<h2 class="h3">Getting Started: Coordinating Distributed Applications with
+ ZooKeeper</h2>
+<div class="section">
+<p>This document contains information to get you started quickly with
+ ZooKeeper. It is aimed primarily at developers hoping to try it out, and
+ contains simple installation instructions for a single ZooKeeper server, a
+ few commands to verify that it is running, and a simple programming
+ example. Finally, as a convenience, there are a few sections regarding
+ more complicated installations, for example running replicated
+ deployments, and optimizing the transaction log. However for the complete
+ instructions for commercial deployments, please refer to the <a href="zookeeperAdmin.html">ZooKeeper
+ Administrator's Guide</a>.</p>
+<a name="sc_Prerequisites"></a>
+<h3 class="h4">Pre-requisites</h3>
+<p>See <a href="zookeeperAdmin.html#sc_systemReq">
+ System Requirements</a> in the Admin guide.</p>
+<a name="sc_Download"></a>
+<h3 class="h4">Download</h3>
+<p>To get a ZooKeeper distribution, download a recent
+ <a href="http://zookeeper.apache.org/releases.html">
+ stable</a> release from one of the Apache Download
+ Mirrors.</p>
+<a name="sc_InstallingSingleMode"></a>
+<h3 class="h4">Standalone Operation</h3>
+<p>Setting up a ZooKeeper server in standalone mode is
+ straightforward. The server is contained in a single JAR file,
+ so installation consists of creating a configuration.</p>
+<p>Once you've downloaded a stable ZooKeeper release unpack
+ it and cd to the root</p>
+<p>To start ZooKeeper you need a configuration file. Here is a sample,
+ create it in <strong>conf/zoo.cfg</strong>:</p>
+<pre class="code">
+tickTime=2000
+dataDir=/var/lib/zookeeper
+clientPort=2181
+</pre>
+<p>This file can be called anything, but for the sake of this
+ discussion call
+ it <strong>conf/zoo.cfg</strong>. Change the
+ value of <strong>dataDir</strong> to specify an
+ existing (empty to start with) directory. Here are the meanings
+ for each of the fields:</p>
+<dl>
+
+<dt>
+<term>
+<strong>tickTime</strong>
+</term>
+</dt>
+<dd>
+<p>the basic time unit in milliseconds used by ZooKeeper. It is
+ used to do heartbeats and the minimum session timeout will be
+ twice the tickTime.</p>
+</dd>
+
+</dl>
+<dl>
+
+<dt>
+<term>
+<strong>dataDir</strong>
+</term>
+</dt>
+<dd>
+<p>the location to store the in-memory database snapshots and,
+ unless specified otherwise, the transaction log of updates to the
+ database.</p>
+</dd>
+
+
+<dt>
+<term>
+<strong>clientPort</strong>
+</term>
+</dt>
+<dd>
+<p>the port to listen for client connections</p>
+</dd>
+
+</dl>
+<p>Now that you created the configuration file, you can start
+ ZooKeeper:</p>
+<pre class="code">bin/zkServer.sh start</pre>
+<p>ZooKeeper logs messages using log4j -- more detail
+ available in the
+ <a href="zookeeperProgrammers.html#Logging">Logging</a>
+ section of the Programmer's Guide. You will see log messages
+ coming to the console (default) and/or a log file depending on
+ the log4j configuration.</p>
+<p>The steps outlined here run ZooKeeper in standalone mode. There is
+ no replication, so if ZooKeeper process fails, the service will go down.
+ This is fine for most development situations, but to run ZooKeeper in
+ replicated mode, please see <a href="#sc_RunningReplicatedZooKeeper">Running Replicated
+ ZooKeeper</a>.</p>
+<a name="sc_FileManagement"></a>
+<h3 class="h4">Managing ZooKeeper Storage</h3>
+<p>For long running production systems ZooKeeper storage must
+ be managed externally (dataDir and logs). See the section on
+ <a href="zookeeperAdmin.html#sc_maintenance">maintenance</a> for
+ more details.</p>
+<a name="sc_ConnectingToZooKeeper"></a>
+<h3 class="h4">Connecting to ZooKeeper</h3>
+<p>Once ZooKeeper is running, you have several options for connection
+ to it:</p>
+<ul>
+
+<li>
+
+<p>
+<strong>Java</strong>: Use</p>
+
+
+<pre class="code">bin/zkCli.sh -server 127.0.0.1:2181</pre>
+
+
+<p>This lets you perform simple, file-like operations.</p>
+
+</li>
+
+
+<li>
+
+<p>
+<strong>C</strong>: compile cli_mt
+ (multi-threaded) or cli_st (single-threaded) by running
+ <span class="codefrag command">make cli_mt</span> or <span class="codefrag command">make
+ cli_st</span> in
+ the <strong>src/c</strong> subdirectory in
+ the ZooKeeper sources. See the README contained within
+ <strong>src/c</strong> for full details.</p>
+
+
+<p>You can run the program
+ from <strong>src/c</strong> using:</p>
+
+
+<pre class="code">LD_LIBRARY_PATH=. cli_mt 127.0.0.1:2181</pre>
+
+
+<p>or</p>
+
+
+<pre class="code">LD_LIBRARY_PATH=. cli_st 127.0.0.1:2181</pre>
+
+<p>This will give you a simple shell to execute file
+ system like operations on ZooKeeper.</p>
+
+</li>
+
+</ul>
+<p>Once you have connected, you should see something like:
+ </p>
+<pre class="code">
+
+Connecting to localhost:2181
+log4j:WARN No appenders could be found for logger (org.apache.zookeeper.ZooKeeper).
+log4j:WARN Please initialize the log4j system properly.
+Welcome to ZooKeeper!
+JLine support is enabled
+[zkshell: 0]
+ </pre>
+<p>
+ From the shell, type <span class="codefrag command">help</span> to get a listing of commands that can be executed from the client, as in:
+ </p>
+<pre class="code">
+
+[zkshell: 0] help
+ZooKeeper host:port cmd args
+ get path [watch]
+ ls path [watch]
+ set path data [version]
+ delquota [-n|-b] path
+ quit
+ printwatches on|off
+ create path data acl
+ stat path [watch]
+ listquota path
+ history
+ setAcl path acl
+ getAcl path
+ sync path
+ redo cmdno
+ addauth scheme auth
+ delete path [version]
+ deleteall path
+ setquota -n|-b val path
+
+ </pre>
+<p>From here, you can try a few simple commands to get a feel for this simple command line interface. First, start by issuing the list command, as
+ in <span class="codefrag command">ls</span>, yielding:
+ </p>
+<pre class="code">
+
+[zkshell: 8] ls /
+[zookeeper]
+ </pre>
+<p>Next, create a new znode by running <span class="codefrag command">create /zk_test my_data</span>. This creates a new znode and associates the string "my_data" with the node.
+ You should see:</p>
+<pre class="code">
+
+[zkshell: 9] create /zk_test my_data
+Created /zk_test
+ </pre>
+<p> Issue another <span class="codefrag command">ls /</span> command to see what the directory looks like:
+ </p>
+<pre class="code">
+
+[zkshell: 11] ls /
+[zookeeper, zk_test]
+
+ </pre>
+<p>
+ Notice that the zk_test directory has now been created.
+ </p>
+<p>Next, verify that the data was associated with the znode by running the <span class="codefrag command">get</span> command, as in:
+ </p>
+<pre class="code">
+
+[zkshell: 12] get /zk_test
+my_data
+cZxid = 5
+ctime = Fri Jun 05 13:57:06 PDT 2009
+mZxid = 5
+mtime = Fri Jun 05 13:57:06 PDT 2009
+pZxid = 5
+cversion = 0
+dataVersion = 0
+aclVersion = 0
+ephemeralOwner = 0
+dataLength = 7
+numChildren = 0
+ </pre>
+<p>We can change the data associated with zk_test by issuing the <span class="codefrag command">set</span> command, as in:
+ </p>
+<pre class="code">
+
+[zkshell: 14] set /zk_test junk
+cZxid = 5
+ctime = Fri Jun 05 13:57:06 PDT 2009
+mZxid = 6
+mtime = Fri Jun 05 14:01:52 PDT 2009
+pZxid = 5
+cversion = 0
+dataVersion = 1
+aclVersion = 0
+ephemeralOwner = 0
+dataLength = 4
+numChildren = 0
+[zkshell: 15] get /zk_test
+junk
+cZxid = 5
+ctime = Fri Jun 05 13:57:06 PDT 2009
+mZxid = 6
+mtime = Fri Jun 05 14:01:52 PDT 2009
+pZxid = 5
+cversion = 0
+dataVersion = 1
+aclVersion = 0
+ephemeralOwner = 0
+dataLength = 4
+numChildren = 0
+ </pre>
+<p>
+ (Notice we did a <span class="codefrag command">get</span> after setting the data and it did, indeed, change.</p>
+<p>Finally, let's <span class="codefrag command">delete</span> the node by issuing:
+ </p>
+<pre class="code">
+
+[zkshell: 16] delete /zk_test
+[zkshell: 17] ls /
+[zookeeper]
+[zkshell: 18]
+</pre>
+<p>That's it for now. To explore more, continue with the rest of this document and see the <a href="zookeeperProgrammers.html">Programmer's Guide</a>. </p>
+<a name="sc_ProgrammingToZooKeeper"></a>
+<h3 class="h4">Programming to ZooKeeper</h3>
+<p>ZooKeeper has a Java bindings and C bindings. They are
+ functionally equivalent. The C bindings exist in two variants: single
+ threaded and multi-threaded. These differ only in how the messaging loop
+ is done. For more information, see the <a href="zookeeperProgrammers.html#ch_programStructureWithExample">Programming
+ Examples in the ZooKeeper Programmer's Guide</a> for
+ sample code using of the different APIs.</p>
+<a name="sc_RunningReplicatedZooKeeper"></a>
+<h3 class="h4">Running Replicated ZooKeeper</h3>
+<p>Running ZooKeeper in standalone mode is convenient for evaluation,
+ some development, and testing. But in production, you should run
+ ZooKeeper in replicated mode. A replicated group of servers in the same
+ application is called a <em>quorum</em>, and in replicated
+ mode, all servers in the quorum have copies of the same configuration
+ file. The file is similar to the one used in standalone mode, but with a
+ few differences. Here is an example:</p>
+<pre class="code">
+tickTime=2000
+dataDir=/var/lib/zookeeper
+clientPort=2181
+initLimit=5
+syncLimit=2
+server.1=zoo1:2888:3888
+server.2=zoo2:2888:3888
+server.3=zoo3:2888:3888
+</pre>
+<p>The new entry, <strong>initLimit</strong> is
+ timeouts ZooKeeper uses to limit the length of time the ZooKeeper
+ servers in quorum have to connect to a leader. The entry <strong>syncLimit</strong> limits how far out of date a server can
+ be from a leader.</p>
+<p>With both of these timeouts, you specify the unit of time using
+ <strong>tickTime</strong>. In this example, the timeout
+ for initLimit is 5 ticks at 2000 milleseconds a tick, or 10
+ seconds.</p>
+<p>The entries of the form <em>server.X</em> list the
+ servers that make up the ZooKeeper service. When the server starts up,
+ it knows which server it is by looking for the file
+ <em>myid</em> in the data directory. That file has the
+ contains the server number, in ASCII.</p>
+<p>Finally, note the two port numbers after each server
+ name: " 2888" and "3888". Peers use the former port to connect
+ to other peers. Such a connection is necessary so that peers
+ can communicate, for example, to agree upon the order of
+ updates. More specifically, a ZooKeeper server uses this port
+ to connect followers to the leader. When a new leader arises, a
+ follower opens a TCP connection to the leader using this
+ port. Because the default leader election also uses TCP, we
+ currently require another port for leader election. This is the
+ second port in the server entry.
+ </p>
+<div class="note">
+<div class="label">Note</div>
+<div class="content">
+
+<p>If you want to test multiple servers on a single
+ machine, specify the servername
+ as <em>localhost</em> with unique quorum &
+ leader election ports (i.e. 2888:3888, 2889:3889, 2890:3890 in
+ the example above) for each server.X in that server's config
+ file. Of course separate <em>dataDir</em>s and
+ distinct <em>clientPort</em>s are also necessary
+ (in the above replicated example, running on a
+ single <em>localhost</em>, you would still have
+ three config files).</p>
+
+</div>
+</div>
+<a name="Other+Optimizations"></a>
+<h3 class="h4">Other Optimizations</h3>
+<p>There are a couple of other configuration parameters that can
+ greatly increase performance:</p>
+<ul>
+
+<li>
+
+<p>To get low latencies on updates it is important to
+ have a dedicated transaction log directory. By default
+ transaction logs are put in the same directory as the data
+ snapshots and <em>myid</em> file. The dataLogDir
+ parameters indicates a different directory to use for the
+ transaction logs.</p>
+
+</li>
+
+
+<li>
+
+<p>
+<em>[tbd: what is the other config param?]</em>
+</p>
+
+</li>
+
+</ul>
+</div>
+
+<p align="right">
+<font size="-2"></font>
+</p>
+</div>
+<!--+
+ |end content
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