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Posted to commits@zookeeper.apache.org by ar...@apache.org on 2022/05/07 07:52:30 UTC
svn commit: r54311 [9/11] - in /dev/zookeeper/zookeeper-3.7.1-rc1: ./ website/ website/images/ website/skin/
Added: dev/zookeeper/zookeeper-3.7.1-rc1/website/zookeeperQuotas.html
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--- dev/zookeeper/zookeeper-3.7.1-rc1/website/zookeeperQuotas.html (added)
+++ dev/zookeeper/zookeeper-3.7.1-rc1/website/zookeeperQuotas.html Sat May 7 07:52:30 2022
@@ -0,0 +1,232 @@
+
+<!DOCTYPE html>
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+<!--
+Copyright 2002-2004 The Apache Software Foundation
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+//-->
+<h1>ZooKeeper Quota's Guide</h1>
+<h3>A Guide to Deployment and Administration</h3>
+<ul>
+<li><a href="#zookeeper_quotas">Quotas</a>
+<ul>
+<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>
+<p><a name="zookeeper_quotas"></a></p>
+<h2>Quotas</h2>
+<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>Notice: What the <code>namespace</code> quota means is the count quota which limits the number of children under the path(included itself).</p>
+<pre><code>$ bin/zkCli.sh -server host:port**
+</code></pre>
+<p>The above command gives you a command line option of using quotas.</p>
+<p><a name="Setting+Quotas"></a></p>
+<h3>Setting Quotas</h3>
+<ul>
+<li>
+<p>You can use <code>setquota</code> to set a quota on a ZooKeeper node. It has an option of setting quota with <code>-n</code> (for namespace/count) and <code>-b</code> (for bytes/data length).</p>
+</li>
+<li>
+<p>The ZooKeeper quota is stored in ZooKeeper itself in <strong>/zookeeper/quota</strong>. To disable other people from changing the quotas, users can set the ACL for <strong>/zookeeper/quota</strong> ,so that only admins are able to read and write to it.</p>
+</li>
+<li>
+<p>If the quota doesn't exist in the specified path,create the quota, otherwise update the quota.</p>
+</li>
+<li>
+<p>The Scope of the quota users set is all the nodes under the path specified (included itself).</p>
+</li>
+<li>
+<p>In order to simplify the calculation of quota in the current directory/hierarchy structure, a complete tree path(from root to leaf node) can be set only one quota. In the situation when setting a quota in a path which its parent or child node already has a quota. <code>setquota</code> will reject and tell the specified parent or child path, users can adjust allocations of quotas(delete/move-up/move-down the quota) according to specific circumstances.</p>
+</li>
+<li>
+<p>Combined with the Chroot, the quota will have a better isolation effectiveness between different applications.For example:</p>
+<pre><code class="language-bash"># Chroot is:
+192.168.0.1:2181,192.168.0.2:2181,192.168.0.3:2181/apps/app1
+setquota -n 100000 /apps/app1
+</code></pre>
+</li>
+<li>
+<p>Users cannot set the quota on the path under <strong>/zookeeper/quota</strong></p>
+</li>
+<li>
+<p>The quota supports the soft and hard quota. The soft quota just logs the warning info when exceeding the quota, but the hard quota also throws a <code>QuotaExceededException</code>. When setting soft and hard quota on the same path, the hard quota has the priority.</p>
+</li>
+</ul>
+<p><a name="Listing+Quotas"></a></p>
+<h3>Listing Quotas</h3>
+<p>You can use <em>listquota</em> to list a quota on a ZooKeeper node.</p>
+<p><a name="Deleting+Quotas"></a></p>
+<h3>Deleting Quotas</h3>
+<p>You can use <em>delquota</em> to delete quota on a ZooKeeper node.</p>
+</div>
+<div class="clearboth"> </div>
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\ No newline at end of file
Added: dev/zookeeper/zookeeper-3.7.1-rc1/website/zookeeperReconfig.html
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--- dev/zookeeper/zookeeper-3.7.1-rc1/website/zookeeperReconfig.html (added)
+++ dev/zookeeper/zookeeper-3.7.1-rc1/website/zookeeperReconfig.html Sat May 7 07:52:30 2022
@@ -0,0 +1,505 @@
+
+<!DOCTYPE html>
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+
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+
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+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+//-->
+<h1>ZooKeeper Dynamic Reconfiguration</h1>
+<ul>
+<li><a href="#ch_reconfig_intro">Overview</a></li>
+<li><a href="#ch_reconfig_format">Changes to Configuration Format</a>
+<ul>
+<li><a href="#sc_reconfig_clientport">Specifying the client port</a></li>
+<li><a href="#sc_multiaddress">Specifying multiple server addresses</a></li>
+<li><a href="#sc_reconfig_standaloneEnabled">The standaloneEnabled flag</a></li>
+<li><a href="#sc_reconfig_reconfigEnabled">The reconfigEnabled 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>
+<li><a href="#ch_reconfig_api">API</a></li>
+<li><a href="#sc_reconfig_access_control">Security</a></li>
+<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>
+<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>
+<p><a name="ch_reconfig_intro"></a></p>
+<h2>Overview</h2>
+<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, ârolling restartsâ 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>
+<ul>
+<li><em>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</em> : 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></li>
+</ul>
+<p><strong>Note:</strong> Starting with 3.5.3, the dynamic reconfiguration feature is disabled by default, and has to be explicitly turned on via <a href="zookeeperAdmin.html#sc_advancedConfiguration">reconfigEnabled</a> configuration option.</p>
+<p><a name="ch_reconfig_format"></a></p>
+<h2>Changes to Configuration Format</h2>
+<p><a name="sc_reconfig_clientport"></a></p>
+<h3>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>
+<pre><code>server.<positive id> = <address1>:<port1>:<port2>[:role];[<client port address>:]<client port>**
+</code></pre>
+<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>
+<pre><code>server.5 = 125.23.63.23:1234:1235;1236
+server.5 = 125.23.63.23:1234:1235:participant;1236
+server.5 = 125.23.63.23:1234:1235:observer;1236
+server.5 = 125.23.63.23:1234:1235;125.23.63.24:1236
+server.5 = 125.23.63.23:1234:1235:participant;125.23.63.23:1236
+</code></pre>
+<p><a name="sc_multiaddress"></a></p>
+<h3>Specifying multiple server addresses</h3>
+<p>Since ZooKeeper 3.6.0 it is possible to specify multiple addresses for each ZooKeeper server (see <a href="https://issues.apache.org/jira/projects/ZOOKEEPER/issues/ZOOKEEPER-3188">ZOOKEEPER-3188</a>). This helps to increase availability and adds network level resiliency to ZooKeeper. When multiple physical network interfaces are used for the servers, ZooKeeper is able to bind on all interfaces and runtime switching to a working interface in case a network error. The different addresses can be specified in the config using a pipe ('|') character.</p>
+<p>Examples for a valid configurations using multiple addresses:</p>
+<pre><code>server.2=zoo2-net1:2888:3888|zoo2-net2:2889:3889;2188
+server.2=zoo2-net1:2888:3888|zoo2-net2:2889:3889|zoo2-net3:2890:3890;2188
+server.2=zoo2-net1:2888:3888|zoo2-net2:2889:3889;zoo2-net1:2188
+server.2=zoo2-net1:2888:3888:observer|zoo2-net2:2889:3889:observer;2188
+</code></pre>
+<p><a name="sc_reconfig_standaloneEnabled"></a></p>
+<h3>The <em>standaloneEnabled</em> 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>
+<pre><code>standaloneEnabled=false**
+</code></pre>
+<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>
+<p><a name="sc_reconfig_reconfigEnabled"></a></p>
+<h3>The <em>reconfigEnabled</em> flag</h3>
+<p>Starting with 3.5.0 and prior to 3.5.3, there is no way to disable dynamic reconfiguration feature. We would like to offer the option of disabling reconfiguration feature because with reconfiguration enabled, we have a security concern that a malicious actor can make arbitrary changes to the configuration of a ZooKeeper ensemble, including adding a compromised server to the ensemble. We prefer to leave to the discretion of the user to decide whether to enable it or not and make sure that the appropriate security measure are in place. So in 3.5.3 the <a href="zookeeperAdmin.html#sc_advancedConfiguration">reconfigEnabled</a> configuration option is introduced such that the reconfiguration feature can be completely disabled and any attempts to reconfigure a cluster through reconfig API with or without authentication will fail by default, unless <strong>reconfigEnabled</strong> is set to <strong>true</strong>.</p>
+<p>To set the option to true, the configuration file (zoo.cfg) should contain:</p>
+<pre><code>reconfigEnabled=true
+</code></pre>
+<p><a name="sc_reconfig_file"></a></p>
+<h3>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>
+<h4>zoo_replicated1.cfg</h4>
+<pre><code>tickTime=2000
+dataDir=/zookeeper/data/zookeeper1
+initLimit=5
+syncLimit=2
+dynamicConfigFile=/zookeeper/conf/zoo_replicated1.cfg.dynamic
+</code></pre>
+<h4>zoo_replicated1.cfg.dynamic</h4>
+<pre><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
+</code></pre>
+<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>
+<h4>zoo_replicated1.cfg</h4>
+<pre><code>tickTime=2000
+dataDir=/zookeeper/data/zookeeper1
+initLimit=5
+syncLimit=2
+clientPort=
+</code></pre>
+<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>
+<p><a name="sc_reconfig_backward"></a></p>
+<h3>Backward compatibility</h3>
+<p>We still support the old configuration format. For example, the following configuration file is acceptable (but not recommended):</p>
+<h4>zoo_replicated1.cfg</h4>
+<pre><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
+</code></pre>
+<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>
+<p><a name="ch_reconfig_upgrade"></a></p>
+<h2>Upgrading to 3.5.0</h2>
+<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><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
+</code></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>
+<p><a name="ch_reconfig_dyn"></a></p>
+<h2>Dynamic Reconfiguration of the ZooKeeper Ensemble</h2>
+<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>
+<p><a name="ch_reconfig_api"></a></p>
+<h3>API</h3>
+<p>There are two sets of APIs for both Java and C client.</p>
+<ul>
+<li>
+<p><strong><em>Reconfiguration API</em></strong> : Reconfiguration API is used to reconfigure the ZooKeeper cluster. Starting with 3.5.3, reconfiguration Java APIs are moved into ZooKeeperAdmin class from ZooKeeper class, and use of this API requires ACL setup and user authentication (see <a href="#sc_reconfig_access_control">Security</a> for more information.).</p>
+</li>
+<li>
+<p><strong><em>Get Configuration API</em></strong> : Get configuration APIs are used to retrieve ZooKeeper cluster configuration information stored in /zookeeper/config znode. Use of this API does not require specific setup or authentication, because /zookeeper/config is readable to any users.</p>
+</li>
+</ul>
+<p><a name="sc_reconfig_access_control"></a></p>
+<h3>Security</h3>
+<p>Prior to <strong>3.5.3</strong>, there is no enforced security mechanism over reconfig so any ZooKeeper clients that can connect to ZooKeeper server ensemble will have the ability to change the state of a ZooKeeper cluster via reconfig. It is thus possible for a malicious client to add compromised server to an ensemble, e.g., add a compromised server, or remove legitimate servers. Cases like these could be security vulnerabilities on a case by case basis.</p>
+<p>To address this security concern, we introduced access control over reconfig starting from <strong>3.5.3</strong> such that only a specific set of users can use reconfig commands or APIs, and these users need be configured explicitly. In addition, the setup of ZooKeeper cluster must enable authentication so ZooKeeper clients can be authenticated.</p>
+<p>We also provide an escape hatch for users who operate and interact with a ZooKeeper ensemble in a secured environment (i.e. behind company firewall). For those users who want to use reconfiguration feature but don't want the overhead of configuring an explicit list of authorized user for reconfig access checks, they can set <a href="zookeeperAdmin.html#sc_authOptions">"skipACL"</a> to "yes" which will skip ACL check and allow any user to reconfigure cluster.</p>
+<p>Overall, ZooKeeper provides flexible configuration options for the reconfigure feature that allow a user to choose based on user's security requirement. We leave to the discretion of the user to decide appropriate security measure are in place.</p>
+<ul>
+<li>
+<p><strong><em>Access Control</em></strong> : The dynamic configuration is stored in a special znode ZooDefs.CONFIG_NODE = /zookeeper/config. This node by default is read only for all users, except super user and users that's explicitly configured for write access. Clients that need to use reconfig commands or reconfig API should be configured as users that have write access to CONFIG_NODE. By default, only the super user has full control including write access to CONFIG_NODE. Additional users can be granted write access through superuser by setting an ACL that has write permission associated with specified user. A few examples of how to setup ACLs and use reconfiguration API with authentication can be found in ReconfigExceptionTest.java and TestReconfigServer.cc.</p>
+</li>
+<li>
+<p><strong><em>Authentication</em></strong> : Authentication of users is orthogonal to the access control and is delegated to existing authentication mechanism supported by ZooKeeper's pluggable authentication schemes. See <a href="https://cwiki.apache.org/confluence/display/ZOOKEEPER/Zookeeper+and+SASL">ZooKeeper and SASL</a> for more details on this topic.</p>
+</li>
+<li>
+<p><strong><em>Disable ACL check</em></strong> : ZooKeeper supports <a href="zookeeperAdmin.html#sc_authOptions">"skipACL"</a> option such that ACL check will be completely skipped, if skipACL is set to "yes". In such cases any unauthenticated users can use reconfig API.</p>
+</li>
+</ul>
+<p><a name="sc_reconfig_retrieving"></a></p>
+<h3>Retrieving the current dynamic configuration</h3>
+<p>The dynamic configuration is stored in a special znode ZooDefs.CONFIG_NODE = /zookeeper/config. The new <code>config</code> CLI command reads this znode (currently it is simply a wrapper to <code>get /zookeeper/config</code>). As with normal reads, to retrieve the latest committed value you should do a <code>sync</code> first.</p>
+<pre><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
+</code></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 <code>get</code> command, the <code>config</code> CLI command accepts the <em>-w</em> flag for setting a watch on the znode, and <em>-s</em> flag for displaying the Stats of the znode. It additionally accepts a new flag <em>-c</em> 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><code>[zk: 127.0.0.1:2791(CONNECTED) 17] config -c
+400000003 localhost:2791,localhost:2793,localhost:2792
+</code></pre>
+<p>Note that when using the API directly, this command is called <code>getConfig</code>.</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 <code>sync</code> command for stronger guarantees. For example using the Java API:</p>
+<pre><code>zk.sync(ZooDefs.CONFIG_NODE, void_callback, context);
+zk.getConfig(watcher, callback, context);
+</code></pre>
+<p>Note: in 3.5.0 it doesn't really matter which path is passed to the <code>sync()</code> 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>
+<p><a name="sc_reconfig_modifying"></a></p>
+<h3>Modifying the current dynamic configuration</h3>
+<p>Modifying the configuration is done through the <code>reconfig</code> 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 <code>reconfig</code> command returns the new configuration.</p>
+<p>A few examples are in: <em>ReconfigTest.java</em>, <em>ReconfigRecoveryTest.java</em> and <em>TestReconfigServer.cc</em>.</p>
+<p><a name="sc_reconfig_general"></a></p>
+<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 <em>standaloneEnabled</em> flag</a>), the reconfig will not be processed (BadArgumentsException). If standalone mode is disabled (standaloneEnabled=false) then it's 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>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 f
ollowers until the system is reconfigured and they are added to the ensemble (as participant or observer, as appropriate).</li>
+<li>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).</li>
+<li>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 doesnât know anyone else and therefore the administrator will have to intervene and restart D with another server list.</li>
+</ol>
+<h6>Note</h6>
+<blockquote>
+<h5>Warning</h5>
+<p>Never specify more than one joining server in the same initial configuration as participants. Currently, the joining servers donât 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>
+</blockquote>
+<p>If the configuration of existing servers changes or they become unavailable before the joiner succeeds to connect and learn about configuration changes, the joiner may need to be restarted with an updated configuration file in order to be able to connect.</p>
+<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 youâll 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>Leader is removed from the ensemble.</li>
+<li>Leader's role is changed from participant to observer.</li>
+<li>The port used by the leader to send transactions to others (quorum port) is modified.</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 ord
er to know that it is safe to shut down old servers that were removed), one can simply invoke an update (<code>set-data</code>, or some other quorum operation, but not a <code>sync</code>) 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>
+<p><a name="sc_reconfig_incremental"></a></p>
+<h4>Incremental mode</h4>
+<p>The incremental mode allows adding and removing servers to the current configuration. Multiple changes are allowed. For example:</p>
+<pre><code>> reconfig -remove 3 -add
+server.5=125.23.63.23:1234:1235;1236
+</code></pre>
+<p>Both the add and the remove options get a list of comma separated arguments (no spaces):</p>
+<pre><code>> reconfig -remove 3,4 -add
+server.5=localhost:2111:2112;2113,6=localhost:2114:2115:observer;2116
+</code></pre>
+<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 vice versa or change any of their ports, without rebooting the server.</p>
+<p>ZooKeeper supports two types of Quorum Systems â 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><code>List<String> leavingServers = new ArrayList<String>();
+leavingServers.add("1");
+leavingServers.add("2");
+byte[] config = zk.reconfig(null, leavingServers, null, -1, new Stat());
+
+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);
+</code></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>
+<p><a name="sc_reconfig_nonincremental"></a></p>
+<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>
+<pre><code>> reconfig -file newconfig.cfg
+</code></pre>
+<p>//newconfig.cfg is a dynamic config file, see <a href="#sc_reconfig_file">Dynamic configuration file</a></p>
+<pre><code>> 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}}
+</code></pre>
+<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><code>List<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);
+</code></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>
+<p><a name="sc_reconfig_conditional"></a></p>
+<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 <code>reconfig</code> succeeds only if the last configuration at the leader has the specified version.</p>
+<pre><code>> reconfig -file <filename> -v <version>
+</code></pre>
+<p>In the previously listed Java examples, instead of -1 one could specify a configuration version to condition the reconfiguration.</p>
+<p><a name="sc_reconfig_errors"></a></p>
+<h4>Error conditions</h4>
+<p>In addition to normal ZooKeeper error conditions, a reconfiguration may fail for the following reasons:</p>
+<ol>
+<li>another reconfig is currently in progress (ReconfigInProgress)</li>
+<li>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)</li>
+<li>no quorum of the new configuration was connected and up-to-date with the leader when the reconfiguration processing began (NewConfigNoQuorum)</li>
+<li><code>-v x</code> was specified, but the version <code>y</code> of the latest configuration is not <code>x</code> (BadVersionException)</li>
+<li>an incremental reconfiguration was requested but the last configuration at the leader uses a Quorum System which is different from the Majority system (BadArgumentsException)</li>
+<li>syntax error (BadArgumentsException)</li>
+<li>I/O exception when reading the configuration from a file (BadArgumentsException)</li>
+</ol>
+<p>Most of these are illustrated by test-cases in <em>ReconfigFailureCases.java</em>.</p>
+<p><a name="sc_reconfig_additional"></a></p>
+<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 <code>config</code> 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 <code>reconfig</code> 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 command.</p>
+<p><a name="ch_reconfig_rebalancing"></a></p>
+<h2>Rebalancing Client Connections</h2>
+<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 <code>getConfig</code> API command). When the watch is triggered, the client should read the new configuration by invoking <code>sync</code> and <code>getConfig</code> and if the configuration is indeed new invoke the <code>updateServerList</code> 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 <code>updateServerList</code>.</p>
+<p>A few examples can be found in: <em>StaticHostProviderTest.java</em> and <em>TestReconfig.cc</em></p>
+<p>Example (this is not a recipe, but a simplified example just to explain the general idea):</p>
+<pre><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 it's 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;
+ }
+ }
+}
+</code></pre>
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+<h1>ZooKeeper Getting Started Guide</h1>
+<ul>
+<li><a href="#getting-started-coordinating-distributed-applications-with-zooKeeper">Getting Started: Coordinating Distributed Applications with ZooKeeper</a>
+<ul>
+<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>
+<p><a name="getting-started-coordinating-distributed-applications-with-zooKeeper"></a></p>
+<h2>Getting Started: Coordinating Distributed Applications with ZooKeeper</h2>
+<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>
+<p><a name="sc_Prerequisites"></a></p>
+<h3>Pre-requisites</h3>
+<p>See <a href="zookeeperAdmin.html#sc_systemReq">System Requirements</a> in the Admin guide.</p>
+<p><a name="sc_Download"></a></p>
+<h3>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>
+<p><a name="sc_InstallingSingleMode"></a></p>
+<h3>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><code>tickTime=2000
+dataDir=/var/lib/zookeeper
+clientPort=2181
+</code></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>
+<ul>
+<li>
+<p><strong><em>tickTime</em></strong> : 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>
+</li>
+<li>
+<p><strong><em>dataDir</em></strong> : the location to store the in-memory database snapshots and, unless specified otherwise, the transaction log of updates to the database.</p>
+</li>
+<li>
+<p><strong><em>clientPort</em></strong> : the port to listen for client connections</p>
+</li>
+</ul>
+<p>Now that you created the configuration file, you can start ZooKeeper:</p>
+<pre><code>bin/zkServer.sh start
+</code></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>
+<p><a name="sc_FileManagement"></a></p>
+<h3>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>
+<p><a name="sc_ConnectingToZooKeeper"></a></p>
+<h3>Connecting to ZooKeeper</h3>
+<pre><code>$ bin/zkCli.sh -server 127.0.0.1:2181
+</code></pre>
+<p>This lets you perform simple, file-like operations.</p>
+<p>Once you have connected, you should see something like:</p>
+<pre><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]
+</code></pre>
+<p>From the shell, type <code>help</code> to get a listing of commands that can be executed from the client, as in:</p>
+<pre><code>[zkshell: 0] help
+ZooKeeper -server host:port cmd args
+addauth scheme auth
+close
+config [-c] [-w] [-s]
+connect host:port
+create [-s] [-e] [-c] [-t ttl] path [data] [acl]
+delete [-v version] path
+deleteall path
+delquota [-n|-b] path
+get [-s] [-w] path
+getAcl [-s] path
+getAllChildrenNumber path
+getEphemerals path
+history
+listquota path
+ls [-s] [-w] [-R] path
+ls2 path [watch]
+printwatches on|off
+quit
+reconfig [-s] [-v version] [[-file path] | [-members serverID=host:port1:port2;port3[,...]*]] | [-add serverId=host:port1:port2;port3[,...]]* [-remove serverId[,...]*]
+redo cmdno
+removewatches path [-c|-d|-a] [-l]
+rmr path
+set [-s] [-v version] path data
+setAcl [-s] [-v version] [-R] path acl
+setquota -n|-b val path
+stat [-w] path
+sync path
+</code></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 <code>ls</code>, yielding:</p>
+<pre><code>[zkshell: 8] ls /
+[zookeeper]
+</code></pre>
+<p>Next, create a new znode by running <code>create /zk_test my_data</code>. This creates a new znode and associates the string "my_data" with the node. You should see:</p>
+<pre><code>[zkshell: 9] create /zk_test my_data
+Created /zk_test
+</code></pre>
+<p>Issue another <code>ls /</code> command to see what the directory looks like:</p>
+<pre><code>[zkshell: 11] ls /
+[zookeeper, zk_test]
+</code></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 <code>get</code> command, as in:</p>
+<pre><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
+</code></pre>
+<p>We can change the data associated with zk_test by issuing the <code>set</code> command, as in:</p>
+<pre><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
+</code></pre>
+<p>(Notice we did a <code>get</code> after setting the data and it did, indeed, change.</p>
+<p>Finally, let's <code>delete</code> the node by issuing:</p>
+<pre><code>[zkshell: 16] delete /zk_test
+[zkshell: 17] ls /
+[zookeeper]
+[zkshell: 18]
+</code></pre>
+<p>That's it for now. To explore more, see the <a href="zookeeperCLI.html">Zookeeper CLI</a>.</p>
+<p><a name="sc_ProgrammingToZooKeeper"></a></p>
+<h3>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 the different APIs.</p>
+<p><a name="sc_RunningReplicatedZooKeeper"></a></p>
+<h3>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.</p>
+<h6>Note</h6>
+<blockquote>
+<p>For replicated mode, a minimum of three servers are required, and it is strongly recommended that you have an odd number of servers. If you only have two servers, then you are in a situation where if one of them fails, there are not enough machines to form a majority quorum. Two servers are inherently <strong>less</strong> stable than a single server, because there are two single points of failure.</p>
+</blockquote>
+<p>The required <strong>conf/zoo.cfg</strong> file for replicated mode is similar to the one used in standalone mode, but with a few differences. Here is an example:</p>
+<pre><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
+</code></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 milliseconds 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>
+<h6>Note</h6>
+<blockquote>
+<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 _dataDir_s and distinct _clientPort_s are also necessary (in the above replicated example, running on a single <em>localhost</em>, you would still have three config files).</p>
+<p>Please be aware that setting up multiple servers on a single machine will not create any redundancy. If something were to happen which caused the machine to die, all of the zookeeper servers would be offline. Full redundancy requires that each server have its own machine. It must be a completely separate physical server. Multiple virtual machines on the same physical host are still vulnerable to the complete failure of that host.</p>
+<p>If you have multiple network interfaces in your ZooKeeper machines, you can also instruct ZooKeeper to bind on all of your interfaces and automatically switch to a healthy interface in case of a network failure. For details, see the <a href="zookeeperAdmin.html#id_multi_address">Configuration Parameters</a>.</p>
+</blockquote>
+<p><a name="other-optimizations"></a></p>
+<h3>Other Optimizations</h3>
+<p>There are a couple of other configuration parameters that can greatly increase performance:</p>
+<ul>
+<li>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.</li>
+</ul>
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