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- - - -<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> -<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> -</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_reconfigEnabled"></a> -<h3 class="h4">The reconfigEnabled 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> -<p> -<span class="codefrag computeroutput">reconfigEnabled=true</span> -</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="ch_reconfig_api"></a> -<h3 class="h4">API</h3> -<p>There are two sets of APIs for both Java and C client. - </p> -<dl> - -<dt> -<term> -<strong>Reconfiguration API</strong> -</term> -</dt> -<dd> -<p>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> -<p>Note: for temporary backward compatibility, the reconfig() APIs will remain in ZooKeeper.java - where they were for a few alpha versions of 3.5.x. However, these APIs are deprecated and users - should move to the reconfigure() APIs in ZooKeeperAdmin.java. - </p> -</dd> - - -<dt> -<term> -<strong>Get Configuration API</strong> -</term> -</dt> -<dd> -<p>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> -</dd> - -</dl> -<a name="sc_reconfig_access_control"></a> -<h3 class="h4">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 provides 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> -<dl> - -<dt> -<term> -<strong>Access Control</strong> -</term> -</dt> -<dd> -<p>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. - </p> -<p>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. - </p> -<p>A few examples of how to setup ACLs and use reconfiguration API with authentication can be found in - ReconfigExceptionTest.java and TestReconfigServer.cc.</p> -</dd> - - -<dt> -<term> -<strong>Authentication</strong> -</term> -</dt> -<dd> -<p>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> -</dd> - - -<dt> -<term> -<strong>Disable ACL check</strong> -</term> -</dt> -<dd> -<p> - 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> -</dd> - -</dl> -<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>If the configuration of existing servers changes or they become unavailable - before the joiner succeeds to connect and learn obout 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 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 - zookeeper-server/src/main/java/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 - zookeeper-server/src/main/java/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> - -<p align="right"> -<font size="-2"></font> -</p> -</div> -<!--+ - |end content - +--> -<div class="clearboth"> </div> -</div> -<div id="footer"> -<!--+ - |start bottomstrip - +--> -<div class="lastmodified"> -<script type="text/javascript"><!-- -document.write("Last Published: " + document.lastModified); -// --></script> -</div> -<div class="copyright"> - Copyright © - <a href="http://www.apache.org/licenses/";>The Apache Software Foundation.</a> -</div> -<!--+ - |end bottomstrip - +--> -</div> -</body> -</html> http://git-wip-us.apache.org/repos/asf/zookeeper/blob/ab59048a/docs/zookeeperReconfig.pdf ---------------------------------------------------------------------- diff --git a/docs/zookeeperReconfig.pdf b/docs/zookeeperReconfig.pdf deleted file mode 100644 index d91638a..0000000 Binary files a/docs/zookeeperReconfig.pdf and /dev/null differ
