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Posted to commits@zookeeper.apache.org by an...@apache.org on 2018/11/06 17:50:27 UTC
[08/16] zookeeper git commit: ZOOKEEPER-3155: Remove Forrest XMLs and their build process from the …
http://git-wip-us.apache.org/repos/asf/zookeeper/blob/e5fc1228/docs/zookeeperQuotas.html
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-<li>
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-</li>
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-<h2 class="h3">Quotas</h2>
-<div class="section">
-<p> ZooKeeper has both namespace and bytes quotas. You can use the ZooKeeperMain class to setup quotas.
- ZooKeeper prints <em>WARN</em> messages if users exceed the quota assigned to them. The messages
- are printed in the log of the ZooKeeper.
- </p>
-<p>
-<span class="codefrag computeroutput">$ bin/zkCli.sh -server host:port</span>
-</p>
-<p> The above command gives you a command line option of using quotas.</p>
-<a name="Setting+Quotas"></a>
-<h3 class="h4">Setting Quotas</h3>
-<p>You can use
- <em>setquota</em> to set a quota on a ZooKeeper node. It has an option of setting quota with
- -n (for namespace)
- and -b (for bytes). </p>
-<p> The ZooKeeper quota are stored in ZooKeeper itself in /zookeeper/quota. To disable other people from
- changing the quota's set the ACL for /zookeeper/quota such that only admins are able to read and write to it.
- </p>
-<a name="Listing+Quotas"></a>
-<h3 class="h4">Listing Quotas</h3>
-<p> You can use
- <em>listquota</em> to list a quota on a ZooKeeper node.
- </p>
-<a name="Deleting+Quotas"></a>
-<h3 class="h4"> Deleting Quotas</h3>
-<p> You can use
- <em>delquota</em> to delete quota on a ZooKeeper node.
- </p>
-</div>
-
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http://git-wip-us.apache.org/repos/asf/zookeeper/blob/e5fc1228/docs/zookeeperQuotas.pdf
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-<a href="#ch_reconfig_intro">Overview</a>
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-<a href="#ch_reconfig_format">Changes to Configuration Format</a>
-<ul class="minitoc">
-<li>
-<a href="#sc_reconfig_clientport">Specifying the client port</a>
-</li>
-<li>
-<a href="#sc_reconfig_standaloneEnabled">The standaloneEnabled flag</a>
-</li>
-<li>
-<a href="#sc_reconfig_reconfigEnabled">The reconfigEnabled flag</a>
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-<a href="#sc_reconfig_file">Dynamic configuration file</a>
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-<a href="#sc_reconfig_backward">Backward compatibility</a>
-</li>
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-<a href="#ch_reconfig_upgrade">Upgrading to 3.5.0</a>
-</li>
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-<a href="#ch_reconfig_dyn">Dynamic Reconfiguration of the ZooKeeper Ensemble</a>
-<ul class="minitoc">
-<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 class="minitoc">
-<li>
-<a href="#sc_reconfig_general">General</a>
-</li>
-<li>
-<a href="#sc_reconfig_incremental">Incremental mode</a>
-</li>
-<li>
-<a href="#sc_reconfig_nonincremental">Non-incremental mode</a>
-</li>
-<li>
-<a href="#sc_reconfig_conditional">Conditional reconfig</a>
-</li>
-<li>
-<a href="#sc_reconfig_errors">Error conditions</a>
-</li>
-<li>
-<a href="#sc_reconfig_additional">Additional comments</a>
-</li>
-</ul>
-</li>
-</ul>
-</li>
-<li>
-<a href="#ch_reconfig_rebalancing">Rebalancing Client Connections</a>
-</li>
-</ul>
-</div>
-</div>
-
-
-
-
-<a name="ch_reconfig_intro"></a>
-<h2 class="h3">Overview</h2>
-<div class="section">
-<p>Prior to the 3.5.0 release, the membership and all other configuration
- parameters of Zookeeper were static - loaded during boot and immutable at
- runtime. Operators resorted to ''rolling restarts'' - a manually intensive
- and error-prone method of changing the configuration that has caused data
- loss and inconsistency in production.</p>
-<p>Starting with 3.5.0, “rolling restarts” are no longer needed!
- ZooKeeper comes with full support for automated configuration changes: the
- set of Zookeeper servers, their roles (participant / observer), all ports,
- and even the quorum system can be changed dynamically, without service
- interruption and while maintaining data consistency. Reconfigurations are
- performed immediately, just like other operations in ZooKeeper. Multiple
- changes can be done using a single reconfiguration command. The dynamic
- reconfiguration functionality does not limit operation concurrency, does
- not require client operations to be stopped during reconfigurations, has a
- very simple interface for administrators and no added complexity to other
- client operations.</p>
-<p>New client-side features allow clients to find out about configuration
- changes and to update the connection string (list of servers and their
- client ports) stored in their ZooKeeper handle. A probabilistic algorithm
- is used to rebalance clients across the new configuration servers while
- keeping the extent of client migrations proportional to the change in
- ensemble membership.</p>
-<p>This document provides the administrator manual for reconfiguration.
- For a detailed description of the reconfiguration algorithms, performance
- measurements, and more, please see our paper:</p>
-<dl>
-
-<dt>
-<term>Shraer, A., Reed, B., Malkhi, D., Junqueira, F. Dynamic
- Reconfiguration of Primary/Backup Clusters. In <em>USENIX Annual
- Technical Conference (ATC) </em>(2012), 425-437</term>
-</dt>
-<dd>
-<p>Links: <a href="https://www.usenix.org/system/files/conference/atc12/atc12-final74.pdf">paper (pdf)</a>, <a href="https://www.usenix.org/sites/default/files/conference/protected-files/shraer_atc12_slides.pdf">slides (pdf)</a>, <a href="https://www.usenix.org/conference/atc12/technical-sessions/presentation/shraer">video</a>, <a href="http://www.slideshare.net/Hadoop_Summit/dynamic-reconfiguration-of-zookeeper">hadoop summit slides</a>
-</p>
-</dd>
-
-</dl>
-<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>
-</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">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);</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>
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http://git-wip-us.apache.org/repos/asf/zookeeper/blob/e5fc1228/docs/zookeeperReconfig.pdf
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