You are viewing a plain text version of this content. The canonical link for it is here.
Posted to commits@zookeeper.apache.org by an...@apache.org on 2018/11/09 16:41:39 UTC
[02/18] zookeeper git commit: ZOOKEEPER-3155: Remove Forrest XMLs and their build process from the …
http://git-wip-us.apache.org/repos/asf/zookeeper/blob/ab59048a/zookeeper-docs/src/documentation/content/xdocs/zookeeperQuotas.xml
----------------------------------------------------------------------
diff --git a/zookeeper-docs/src/documentation/content/xdocs/zookeeperQuotas.xml b/zookeeper-docs/src/documentation/content/xdocs/zookeeperQuotas.xml
deleted file mode 100644
index 7668e6a..0000000
--- a/zookeeper-docs/src/documentation/content/xdocs/zookeeperQuotas.xml
+++ /dev/null
@@ -1,71 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
- <!--
- Copyright 2002-2004 The Apache Software Foundation Licensed under the
- Apache License, Version 2.0 (the "License"); you may not use this file
- except in compliance with the License. You may obtain a copy of the
- License at http://www.apache.org/licenses/LICENSE-2.0 Unless required
- by applicable law or agreed to in writing, software distributed under
- the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
- CONDITIONS OF ANY KIND, either express or implied. See the License for
- the specific language governing permissions and limitations under the
- License.
- -->
- <!DOCTYPE article PUBLIC "-//OASIS//DTD Simplified DocBook XML V1.0//EN"
- "http://www.oasis-open.org/docbook/xml/simple/1.0/sdocbook.dtd">
-<article id="bk_Quota">
- <title>ZooKeeper Quota's Guide</title>
- <subtitle>A Guide to Deployment and Administration</subtitle>
- <articleinfo>
- <legalnotice>
- <para>
- Licensed under the Apache License, Version 2.0 (the "License"); you
- may not use this file except in compliance with the License. You may
- obtain a copy of the License at
- <ulink url="http://www.apache.org/licenses/LICENSE-2.0">http://www.apache.org/licenses/LICENSE-2.0
- </ulink>
- .
- </para>
- <para>Unless required by applicable law or agreed to in
- writing, software distributed under the License is distributed on an
- "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
- express or implied. See the License for the specific language
- governing permissions and limitations under the License.</para>
- </legalnotice>
- <abstract>
- <para>This document contains information about deploying,
- administering and mantaining ZooKeeper. It also discusses best
- practices and common problems.</para>
- </abstract>
- </articleinfo>
- <section id="zookeeper_quotas">
- <title>Quotas</title>
- <para> ZooKeeper has both namespace and bytes quotas. You can use the ZooKeeperMain class to setup quotas.
- ZooKeeper prints <emphasis>WARN</emphasis> messages if users exceed the quota assigned to them. The messages
- are printed in the log of the ZooKeeper.
- </para>
- <para><computeroutput>$ bin/zkCli.sh -server host:port</computeroutput></para>
- <para> The above command gives you a command line option of using quotas.</para>
- <section>
- <title>Setting Quotas</title>
- <para>You can use
- <emphasis>setquota</emphasis> to set a quota on a ZooKeeper node. It has an option of setting quota with
- -n (for namespace)
- and -b (for bytes). </para>
- <para> 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.
- </para>
- </section>
- <section>
- <title>Listing Quotas</title>
- <para> You can use
- <emphasis>listquota</emphasis> to list a quota on a ZooKeeper node.
- </para>
- </section>
- <section>
- <title> Deleting Quotas</title>
- <para> You can use
- <emphasis>delquota</emphasis> to delete quota on a ZooKeeper node.
- </para>
- </section>
- </section>
- </article>
http://git-wip-us.apache.org/repos/asf/zookeeper/blob/ab59048a/zookeeper-docs/src/documentation/content/xdocs/zookeeperReconfig.xml
----------------------------------------------------------------------
diff --git a/zookeeper-docs/src/documentation/content/xdocs/zookeeperReconfig.xml b/zookeeper-docs/src/documentation/content/xdocs/zookeeperReconfig.xml
deleted file mode 100644
index ae22951..0000000
--- a/zookeeper-docs/src/documentation/content/xdocs/zookeeperReconfig.xml
+++ /dev/null
@@ -1,883 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<!--
- Copyright 2002-2004 The Apache Software Foundation
-
- Licensed under the Apache License, Version 2.0 (the "License");
- you may not use this file except in compliance with the License.
- You may obtain a copy of the License at
-
- http://www.apache.org/licenses/LICENSE-2.0
-
- Unless required by applicable law or agreed to in writing, software
- distributed under the License is distributed on an "AS IS" BASIS,
- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- See the License for the specific language governing permissions and
- limitations under the License.
--->
-<!DOCTYPE article PUBLIC "-//OASIS//DTD Simplified DocBook XML V1.0//EN"
-"http://www.oasis-open.org/docbook/xml/simple/1.0/sdocbook.dtd">
-<article id="ar_reconfig">
- <title>ZooKeeper Dynamic Reconfiguration</title>
-
- <articleinfo>
- <legalnotice>
- <para>Licensed under the Apache License, Version 2.0 (the "License");
- you may not use this file except in compliance with the License. You may
- obtain a copy of the License at <ulink
- url="http://www.apache.org/licenses/LICENSE-2.0">http://www.apache.org/licenses/LICENSE-2.0</ulink>.</para>
-
- <para>Unless required by applicable law or agreed to in writing,
- software distributed under the License is distributed on an "AS IS"
- BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
- implied. See the License for the specific language governing permissions
- and limitations under the License.</para>
- </legalnotice>
-
- <abstract>
- <para>This document contains information about Dynamic Reconfiguration in
- ZooKeeper.</para>
- </abstract>
- </articleinfo>
- <section id="ch_reconfig_intro">
- <title>Overview</title>
- <para>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.</para>
- <para>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.</para>
- <para>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.</para>
- <para>This document provides the administrator manual for reconfiguration.
- For a detailed description of the reconfiguration algorithms, performance
- measurements, and more, please see our paper:</para>
- <variablelist>
- <varlistentry>
- <term>Shraer, A., Reed, B., Malkhi, D., Junqueira, F. Dynamic
- Reconfiguration of Primary/Backup Clusters. In <emphasis>USENIX Annual
- Technical Conference (ATC) </emphasis>(2012), 425-437</term>
- <listitem>
- <para>Links: <ulink
- url="https://www.usenix.org/system/files/conference/atc12/atc12-final74.pdf"
- >paper (pdf)</ulink>, <ulink
- url="https://www.usenix.org/sites/default/files/conference/protected-files/shraer_atc12_slides.pdf"
- >slides (pdf)</ulink>, <ulink
- url="https://www.usenix.org/conference/atc12/technical-sessions/presentation/shraer"
- >video</ulink>, <ulink
- url="http://www.slideshare.net/Hadoop_Summit/dynamic-reconfiguration-of-zookeeper"
- >hadoop summit slides</ulink></para>
- </listitem>
- </varlistentry>
- </variablelist>
- <para><emphasis role="bold">Note:</emphasis> Starting with 3.5.3, the dynamic reconfiguration
- feature is disabled by default, and has to be explicitly turned on via
- <ulink url="zookeeperAdmin.html#sc_advancedConfiguration">
- reconfigEnabled </ulink> configuration option.
- </para>
- </section>
- <section id="ch_reconfig_format">
- <title>Changes to Configuration Format</title>
- <section id="sc_reconfig_clientport">
- <title>Specifying the client port</title>
- <para>A client port of a server is the port on which the server accepts
- client connection requests. Starting with 3.5.0 the
- <emphasis>clientPort</emphasis> and <emphasis>clientPortAddress
- </emphasis> configuration parameters should no longer be used. Instead,
- this information is now part of the server keyword specification, which
- becomes as follows:</para>
- <para><computeroutput><![CDATA[server.<positive id> = <address1>:<port1>:<port2>[:role];[<client port address>:]<client port>]]></computeroutput></para>
- <para>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
- <emphasis>participant</emphasis> or <emphasis>observer</emphasis>
- (<emphasis>participant</emphasis> by default).</para>
- <para> Examples of legal server statements: </para>
- <itemizedlist>
- <listitem>
- <para><computeroutput>server.5 = 125.23.63.23:1234:1235;1236</computeroutput></para>
- </listitem>
- <listitem>
- <para><computeroutput>server.5 = 125.23.63.23:1234:1235:participant;1236</computeroutput></para>
- </listitem>
- <listitem>
- <para><computeroutput>server.5 = 125.23.63.23:1234:1235:observer;1236</computeroutput></para>
- </listitem>
- <listitem>
- <para><computeroutput>server.5 = 125.23.63.23:1234:1235;125.23.63.24:1236</computeroutput></para>
- </listitem>
- <listitem>
- <para><computeroutput>server.5 = 125.23.63.23:1234:1235:participant;125.23.63.23:1236</computeroutput></para>
- </listitem>
- </itemizedlist>
- </section>
- <section id="sc_reconfig_standaloneEnabled">
- <title>The <emphasis>standaloneEnabled</emphasis> flag</title>
- <para>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) <emphasis>standaloneEnabled</emphasis> is set to
- <emphasis>true</emphasis>. 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.</para>
- <para>Setting the flag to <emphasis>false</emphasis> 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:</para>
- <para><computeroutput>standaloneEnabled=false</computeroutput></para>
- <para>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.</para>
- <para>Since running the Distributed mode allows more flexibility, we
- recommend setting the flag to <emphasis>false</emphasis>. We expect that
- the legacy Standalone mode will be deprecated in the future.</para>
- </section>
- <section id="sc_reconfig_reconfigEnabled">
- <title>The <emphasis>reconfigEnabled</emphasis> flag</title>
- <para>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 <ulink url="zookeeperAdmin.html#sc_advancedConfiguration">
- reconfigEnabled </ulink> 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 <emphasis role="bold">reconfigEnabled</emphasis> is set to
- <emphasis role="bold">true</emphasis>.
- </para>
- <para>To set the option to true, the configuration file (zoo.cfg) should contain:</para>
- <para><computeroutput>reconfigEnabled=true</computeroutput></para>
- </section>
- <section id="sc_reconfig_file">
- <title>Dynamic configuration file</title>
- <para>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: <emphasis>server</emphasis>, <emphasis>group</emphasis>
- and <emphasis>weight</emphasis>.</para>
- <para>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
- <emphasis>dynamicConfigFile</emphasis> keyword.</para>
- <para><emphasis role="bold">Example</emphasis></para>
- <example>
- <title>zoo_replicated1.cfg</title>
- <programlisting>tickTime=2000
-dataDir=/zookeeper/data/zookeeper1
-initLimit=5
-syncLimit=2
-dynamicConfigFile=/zookeeper/conf/zoo_replicated1.cfg.dynamic</programlisting>
- </example>
- <example>
- <title>zoo_replicated1.cfg.dynamic</title>
- <programlisting>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</programlisting>
- </example>
- <para>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.</para>
- <para><emphasis role="bold">Example 2</emphasis></para>
- <para>Users may prefer to initially specify a single configuration file.
- The following is thus also legal:</para>
- <example>
- <title>zoo_replicated1.cfg</title>
- <programlisting>tickTime=2000
-dataDir=/zookeeper/data/zookeeper1
-initLimit=5
-syncLimit=2
-clientPort=<emphasis role="bold">2791</emphasis> // note that this line is now redundant and therefore not recommended
-server.1=125.23.63.23:2780:2783:participant;<emphasis role="bold">2791</emphasis>
-server.2=125.23.63.24:2781:2784:participant;2792
-server.3=125.23.63.25:2782:2785:participant;2793</programlisting>
- </example>
- <para>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).</para>
- </section>
- <section id="sc_reconfig_backward">
- <title>Backward compatibility</title>
- <para>We still support the old configuration format. For example, the
- following configuration file is acceptable (but not recommended):</para>
- <example>
- <title>zoo_replicated1.cfg</title>
- <programlisting>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</programlisting>
- </example>
- <para>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 <xref linkend="ch_reconfig_format"/>. 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).</para>
- </section>
- </section>
- <section id="ch_reconfig_upgrade">
- <title>Upgrading to 3.5.0</title>
- <para>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:</para>
- <programlisting>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</programlisting>
- <para>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 <xref linkend="sc_reconfig_clientport"/>). 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).</para>
- </section>
-
- <section id="ch_reconfig_dyn">
- <title>Dynamic Reconfiguration of the ZooKeeper Ensemble</title>
- <para>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.</para>
-
- <section id="ch_reconfig_api">
- <title>API</title>
- <para>There are two sets of APIs for both Java and C client.
- </para>
- <variablelist>
- <varlistentry>
- <term><emphasis role="bold">Reconfiguration API</emphasis></term>
-
- <listitem>
- <para>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 <xref linkend="sc_reconfig_access_control"/> for more information.).
- </para>
-
- <para>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.
- </para>
- </listitem>
- </varlistentry>
-
- <varlistentry>
- <term><emphasis role="bold">Get Configuration API</emphasis></term>
- <listitem>
- <para>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.</para>
- </listitem>
- </varlistentry>
- </variablelist>
- </section>
-
- <section id="sc_reconfig_access_control">
- <title>Security</title>
- <para>Prior to <emphasis role="bold">3.5.3</emphasis>, 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.
- </para>
- <para>To address this security concern, we introduced access control over reconfig
- starting from <emphasis role="bold">3.5.3</emphasis> 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.
- </para>
- <para>
- 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 <ulink url="zookeeperAdmin.html#sc_authOptions">"skipACL"</ulink> to "yes" which will
- skip ACL check and allow any user to reconfigure cluster.
- </para>
- <para>
- 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.
- </para>
- <variablelist>
- <varlistentry>
- <term><emphasis role="bold">Access Control</emphasis></term>
-
- <listitem>
- <para>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.
- </para>
-
- <para>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.
- </para>
-
- <para>A few examples of how to setup ACLs and use reconfiguration API with authentication can be found in
- ReconfigExceptionTest.java and TestReconfigServer.cc.</para>
- </listitem>
- </varlistentry>
-
- <varlistentry>
- <term><emphasis role="bold">Authentication</emphasis></term>
-
- <listitem>
- <para>Authentication of users is orthogonal to the access control and is delegated to
- existing authentication mechanism supported by ZooKeeper's pluggable authentication schemes.
- See <ulink
- url="https://cwiki.apache.org/confluence/display/ZOOKEEPER/Zookeeper+and+SASL"
- >ZooKeeper and SASL</ulink> for more details on this topic.
- </para>
- </listitem>
- </varlistentry>
-
- <varlistentry>
- <term><emphasis role="bold">Disable ACL check</emphasis></term>
- <listitem>
- <para>
- ZooKeeper supports <ulink
- url="zookeeperAdmin.html#sc_authOptions">"skipACL"</ulink> 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.
- </para>
- </listitem>
- </varlistentry>
- </variablelist>
- </section>
-
- <section id="sc_reconfig_retrieving">
- <title>Retrieving the current dynamic configuration</title>
- <para>The dynamic configuration is stored in a special znode
- ZooDefs.CONFIG_NODE = /zookeeper/config. The new
- <command>config</command> CLI command reads this znode (currently it is
- simply a wrapper to <command>get /zookeeper/config</command>). As with
- normal reads, to retrieve the latest committed value you should do a
- <command>sync</command> first.</para>
- <programlisting>[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
-<emphasis role="bold">version=400000003</emphasis></programlisting>
- <para>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.</para>
- <para>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.</para>
- <para>Just like a <command>get</command> command, the
- <command>config</command> CLI command accepts the <option>-w</option>
- flag for setting a watch on the znode, and <option>-s</option> flag for
- displaying the Stats of the znode. It additionally accepts a new flag
- <option>-c</option> which outputs only the version and the client
- connection string corresponding to the current configuration. For
- example, for the configuration above we would get:</para>
- <programlisting>[zk: 127.0.0.1:2791(CONNECTED) 17] config -c
-400000003 localhost:2791,localhost:2793,localhost:2792</programlisting>
- <para>Note that when using the API directly, this command is called
- <command>getConfig</command>.</para>
- <para>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 <command>sync</command> command for
- stronger guarantees. For example using the Java API:</para>
- <programlisting>zk.sync(ZooDefs.CONFIG_NODE, void_callback, context);
-zk.getConfig(watcher, callback, context);</programlisting>
- <para>Note: in 3.5.0 it doesn't really matter which path is passed to the
- <command>sync() </command> 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.</para>
- </section>
- <section id="sc_reconfig_modifying">
- <title>Modifying the current dynamic configuration</title>
- <para>Modifying the configuration is done through the
- <command>reconfig</command> 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 <command>reconfig</command> command returns the new
- configuration.</para>
- <para>A few examples are in: <filename>ReconfigTest.java</filename>,
- <filename>ReconfigRecoveryTest.java</filename> and
- <filename>TestReconfigServer.cc</filename>.</para>
- <section id="sc_reconfig_general">
- <title>General</title>
- <para><emphasis role="bold">Removing servers:</emphasis> 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 <ulink
- url="https://www.usenix.org/conference/usenixfederatedconferencesweek/dynamic-recon%EF%AC%81guration-primarybackup-clusters"
- >paper</ulink>). 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 <xref linkend="sc_reconfig_additional"
- />).</para>
- <para>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 <xref
- linkend="sc_reconfig_standaloneEnabled"/>), the reconfig will not be
- processed (BadArgumentsException). If standalone mode is disabled
- (standaloneEnabled=false) then its legal to remain with 1 or more
- participants.</para>
- <para><emphasis role="bold">Adding servers:</emphasis> 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.</para>
- <orderedlist>
- <listitem>
- <para>Initial configuration of joiners is comprised of servers in
- the last committed configuration and one or more joiners, where
- <emphasis role="bold">joiners are listed as observers.</emphasis>
- 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. <emphasis role="bold">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.</emphasis> 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).</para>
- </listitem>
- <listitem>
- <para>Initial configuration of each joiner is comprised of servers
- in the last committed configuration + <emphasis role="bold">the
- joiner itself, listed as a participant.</emphasis> 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).</para>
- </listitem>
- <listitem>
- <para>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.</para>
- </listitem>
- </orderedlist>
- <note>
- <title>Warning</title>
- <para>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.</para>
- </note>
- <para>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.</para>
- <para>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.</para>
- <para><emphasis role="bold">Modifying server parameters:</emphasis> 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
- <xref linkend="sc_reconfig_incremental"/> and an exception to this
- rule in the section <xref linkend="sc_reconfig_additional"/>.</para>
- <para>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.</para>
- <para><emphasis role="bold">Performance Impact:</emphasis> 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 <ulink
- url="https://www.usenix.org/conference/usenixfederatedconferencesweek/dynamic-recon%EF%AC%81guration-primarybackup-clusters"
- >paper</ulink>.</para>
- <para>The most significant disruption will happen when a leader change
- is caused, in one of the following cases:</para>
- <orderedlist>
- <listitem>
- <para>Leader is removed from the ensemble.</para>
- </listitem>
- <listitem>
- <para>Leader's role is changed from participant to observer.</para>
- </listitem>
- <listitem>
- <para>The port used by the leader to send transactions to others
- (quorum port) is modified.</para>
- </listitem>
- </orderedlist>
- <para>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 <ulink
- url="https://www.usenix.org/conference/usenixfederatedconferencesweek/dynamic-recon%EF%AC%81guration-primarybackup-clusters"
- >paper</ulink>).</para>
- <para>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.</para>
- <para><emphasis role="bold">Progress guarantees:</emphasis> 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
- (<command>set-data</command>, or some other quorum operation, but not
- a <command>sync</command>) 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).</para>
- </section>
- <section id="sc_reconfig_incremental">
- <title>Incremental mode</title>
- <para>The incremental mode allows adding and removing servers to the
- current configuration. Multiple changes are allowed. For
- example:</para>
- <para><userinput>> reconfig -remove 3 -add
- server.5=125.23.63.23:1234:1235;1236</userinput></para>
- <para>Both the add and the remove options get a list of comma separated
- arguments (no spaces):</para>
- <para><userinput>> reconfig -remove 3,4 -add
- server.5=localhost:2111:2112;2113,6=localhost:2114:2115:observer;2116</userinput></para>
- <para>The format of the server statement is exactly the same as
- described in the section <xref linkend="sc_reconfig_clientport"/> 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.</para>
- <para>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).</para>
- <para>Incremental mode - examples using the Java API:</para>
- <programlisting><![CDATA[List<String> leavingServers = new ArrayList<String>();
-leavingServers.add("1");
-leavingServers.add("2");
-byte[] config = zk.reconfig(null, leavingServers, null, -1, new Stat());]]></programlisting>
-
- <programlisting><![CDATA[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);]]></programlisting>
- <para>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.</para>
- </section>
- <section id="sc_reconfig_nonincremental">
- <title>Non-incremental mode</title>
- <para>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:</para>
- <para><userinput>> reconfig -file newconfig.cfg
- </userinput>//newconfig.cfg is a dynamic config file, see <xref
- linkend="sc_reconfig_file"/></para>
- <para><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</userinput></para>
- <para>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.</para>
- <para>Bulk mode - example using the Java API:</para>
- <programlisting><![CDATA[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);]]></programlisting>
- <para>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.</para>
- </section>
- <section id="sc_reconfig_conditional">
- <title>Conditional reconfig</title>
- <para>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 <command>reconfig</command> succeeds only if the
- last configuration at the leader has the specified version.</para>
- <para><userinput><![CDATA[> reconfig -file <filename> -v <version>]]></userinput></para>
- <para>In the previously listed Java examples, instead of -1 one could
- specify a configuration version to condition the
- reconfiguration.</para>
- </section>
- <section id="sc_reconfig_errors">
- <title>Error conditions</title>
- <para>In addition to normal ZooKeeper error conditions, a
- reconfiguration may fail for the following reasons:</para>
- <orderedlist>
- <listitem>
- <para>another reconfig is currently in progress
- (ReconfigInProgress)</para>
- </listitem>
- <listitem>
- <para>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)</para>
- </listitem>
- <listitem>
- <para>no quorum of the new configuration was connected and
- up-to-date with the leader when the reconfiguration processing
- began (NewConfigNoQuorum)</para>
- </listitem>
- <listitem>
- <para><userinput>-v x</userinput> was specified, but the version
- <userinput>y</userinput> of the latest configuration is not
- <userinput>x</userinput> (BadVersionException)</para>
- </listitem>
- <listitem>
- <para>an incremental reconfiguration was requested but the last
- configuration at the leader uses a Quorum System which is
- different from the Majority system (BadArgumentsException)</para>
- </listitem>
- <listitem>
- <para>syntax error (BadArgumentsException)</para>
- </listitem>
- <listitem>
- <para>I/O exception when reading the configuration from a file
- (BadArgumentsException)</para>
- </listitem>
- </orderedlist>
- <para>Most of these are illustrated by test-cases in
- <filename>ReconfigFailureCases.java</filename>.</para>
- </section>
- <section id="sc_reconfig_additional">
- <title>Additional comments</title>
- <para><emphasis role="bold">Liveness:</emphasis> 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 <command>config</command> 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
- <command>reconfig</command> 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 <xref
- linkend="sc_reconfig_conditional"/>. In this way it may avoid blindly
- overwriting the configuration of C1 if C1's configuration reached the
- leader first.</para>
- <para>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.</para>
- <para><emphasis role="bold">Changing an observer into a
- follower:</emphasis> 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.</para>
- </section>
- </section>
- </section>
- <section id="ch_reconfig_rebalancing">
- <title>Rebalancing Client Connections</title>
- <para>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 <ulink
- url="https://www.usenix.org/conference/usenixfederatedconferencesweek/dynamic-recon%EF%AC%81guration-primarybackup-clusters"
- >paper</ulink>.</para>
- <para>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 <command>getConfig</command> API command). When
- the watch is triggered, the client should read the new configuration by
- invoking <command>sync</command> and <command>getConfig</command> and if
- the configuration is indeed new invoke the
- <command>updateServerList</command> 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
- <command>updateServerList</command>.</para>
- <para>A few examples can be found in:
- <filename>StaticHostProviderTest.java</filename> and
- <filename>TestReconfig.cc</filename></para>
- <para>Example (this is not a recipe, but a simplified example just to
- explain the general idea):</para>
- <programlisting><![CDATA[
-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;
-} } }]]></programlisting>
- </section>
-</article>
http://git-wip-us.apache.org/repos/asf/zookeeper/blob/ab59048a/zookeeper-docs/src/documentation/content/xdocs/zookeeperStarted.xml
----------------------------------------------------------------------
diff --git a/zookeeper-docs/src/documentation/content/xdocs/zookeeperStarted.xml b/zookeeper-docs/src/documentation/content/xdocs/zookeeperStarted.xml
deleted file mode 100644
index e5cd777..0000000
--- a/zookeeper-docs/src/documentation/content/xdocs/zookeeperStarted.xml
+++ /dev/null
@@ -1,419 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<!--
- Copyright 2002-2004 The Apache Software Foundation
-
- Licensed under the Apache License, Version 2.0 (the "License");
- you may not use this file except in compliance with the License.
- You may obtain a copy of the License at
-
- http://www.apache.org/licenses/LICENSE-2.0
-
- Unless required by applicable law or agreed to in writing, software
- distributed under the License is distributed on an "AS IS" BASIS,
- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- See the License for the specific language governing permissions and
- limitations under the License.
--->
-
-<!DOCTYPE article PUBLIC "-//OASIS//DTD Simplified DocBook XML V1.0//EN"
-"http://www.oasis-open.org/docbook/xml/simple/1.0/sdocbook.dtd">
-<article id="bk_GettStartedGuide">
- <title>ZooKeeper Getting Started Guide</title>
-
- <articleinfo>
- <legalnotice>
- <para>Licensed under the Apache License, Version 2.0 (the "License");
- you may not use this file except in compliance with the License. You may
- obtain a copy of the License at <ulink
- url="http://www.apache.org/licenses/LICENSE-2.0">http://www.apache.org/licenses/LICENSE-2.0</ulink>.</para>
-
- <para>Unless required by applicable law or agreed to in writing,
- software distributed under the License is distributed on an "AS IS"
- BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
- implied. See the License for the specific language governing permissions
- and limitations under the License.</para>
- </legalnotice>
-
- <abstract>
- <para>This guide contains detailed information about creating
- distributed applications that use ZooKeeper. It discusses the basic
- operations ZooKeeper supports, and how these can be used to build
- higher-level abstractions. It contains solutions to common tasks, a
- troubleshooting guide, and links to other information.</para>
- </abstract>
- </articleinfo>
-
- <section id="ch_GettingStarted">
- <title>Getting Started: Coordinating Distributed Applications with
- ZooKeeper</title>
-
- <para>This document contains information to get you started quickly with
- ZooKeeper. It is aimed primarily at developers hoping to try it out, and
- contains simple installation instructions for a single ZooKeeper server, a
- few commands to verify that it is running, and a simple programming
- example. Finally, as a convenience, there are a few sections regarding
- more complicated installations, for example running replicated
- deployments, and optimizing the transaction log. However for the complete
- instructions for commercial deployments, please refer to the <ulink
- url="zookeeperAdmin.html">ZooKeeper
- Administrator's Guide</ulink>.</para>
-
- <section id="sc_Prerequisites">
- <title>Pre-requisites</title>
-
- <para>See <ulink url="zookeeperAdmin.html#sc_systemReq">
- System Requirements</ulink> in the Admin guide.</para>
- </section>
-
- <section id="sc_Download">
- <title>Download</title>
-
- <para>To get a ZooKeeper distribution, download a recent
- <ulink url="http://zookeeper.apache.org/releases.html">
- stable</ulink> release from one of the Apache Download
- Mirrors.</para>
- </section>
-
- <section id="sc_InstallingSingleMode">
- <title>Standalone Operation</title>
-
- <para>Setting up a ZooKeeper server in standalone mode is
- straightforward. The server is contained in a single JAR file,
- so installation consists of creating a configuration.</para>
-
- <para>Once you've downloaded a stable ZooKeeper release unpack
- it and cd to the root</para>
-
- <para>To start ZooKeeper you need a configuration file. Here is a sample,
- create it in <emphasis role="bold">conf/zoo.cfg</emphasis>:</para>
-
-<programlisting>
-tickTime=2000
-dataDir=/var/lib/zookeeper
-clientPort=2181
-</programlisting>
-
- <para>This file can be called anything, but for the sake of this
- discussion call
- it <emphasis role="bold">conf/zoo.cfg</emphasis>. Change the
- value of <emphasis role="bold">dataDir</emphasis> to specify an
- existing (empty to start with) directory. Here are the meanings
- for each of the fields:</para>
-
- <variablelist>
- <varlistentry>
- <term><emphasis role="bold">tickTime</emphasis></term>
-
- <listitem>
- <para>the basic time unit in milliseconds used by ZooKeeper. It is
- used to do heartbeats and the minimum session timeout will be
- twice the tickTime.</para>
- </listitem>
- </varlistentry>
- </variablelist>
-
- <variablelist>
- <varlistentry>
- <term><emphasis role="bold">dataDir</emphasis></term>
-
- <listitem>
- <para>the location to store the in-memory database snapshots and,
- unless specified otherwise, the transaction log of updates to the
- database.</para>
- </listitem>
- </varlistentry>
-
- <varlistentry>
- <term><emphasis role="bold">clientPort</emphasis></term>
-
- <listitem>
- <para>the port to listen for client connections</para>
- </listitem>
- </varlistentry>
- </variablelist>
-
- <para>Now that you created the configuration file, you can start
- ZooKeeper:</para>
-
- <programlisting>bin/zkServer.sh start</programlisting>
-
- <para>ZooKeeper logs messages using log4j -- more detail
- available in the
- <ulink url="zookeeperProgrammers.html#Logging">Logging</ulink>
- section of the Programmer's Guide. You will see log messages
- coming to the console (default) and/or a log file depending on
- the log4j configuration.</para>
-
- <para>The steps outlined here run ZooKeeper in standalone mode. There is
- no replication, so if ZooKeeper process fails, the service will go down.
- This is fine for most development situations, but to run ZooKeeper in
- replicated mode, please see <ulink
- url="#sc_RunningReplicatedZooKeeper">Running Replicated
- ZooKeeper</ulink>.</para>
- </section>
-
- <section id="sc_FileManagement">
- <title>Managing ZooKeeper Storage</title>
- <para>For long running production systems ZooKeeper storage must
- be managed externally (dataDir and logs). See the section on
- <ulink
- url="zookeeperAdmin.html#sc_maintenance">maintenance</ulink> for
- more details.</para>
- </section>
-
- <section id="sc_ConnectingToZooKeeper">
- <title>Connecting to ZooKeeper</title>
-
- <programlisting>$ bin/zkCli.sh -server 127.0.0.1:2181</programlisting>
-
- <para>This lets you perform simple, file-like operations.</para>
-
- <para>Once you have connected, you should see something like:
- </para>
- <programlisting>
-<![CDATA[
-Connecting to localhost:2181
-log4j:WARN No appenders could be found for logger (org.apache.zookeeper.ZooKeeper).
-log4j:WARN Please initialize the log4j system properly.
-Welcome to ZooKeeper!
-JLine support is enabled
-[zkshell: 0]
-]]> </programlisting>
- <para>
- From the shell, type <command>help</command> to get a listing of commands that can be executed from the client, as in:
- </para>
- <programlisting>
-<![CDATA[
-[zkshell: 0] help
-ZooKeeper host:port cmd args
- get path [watch]
- ls path [watch]
- set path data [version]
- delquota [-n|-b] path
- quit
- printwatches on|off
- create path data acl
- stat path [watch]
- listquota path
- history
- setAcl path acl
- getAcl path
- sync path
- redo cmdno
- addauth scheme auth
- delete path [version]
- deleteall path
- setquota -n|-b val path
-
-]]> </programlisting>
- <para>From here, you can try a few simple commands to get a feel for this simple command line interface. First, start by issuing the list command, as
- in <command>ls</command>, yielding:
- </para>
- <programlisting>
-<![CDATA[
-[zkshell: 8] ls /
-[zookeeper]
-]]> </programlisting>
- <para>Next, create a new znode by running <command>create /zk_test my_data</command>. This creates a new znode and associates the string "my_data" with the node.
- You should see:</para>
- <programlisting>
-<![CDATA[
-[zkshell: 9] create /zk_test my_data
-Created /zk_test
-]]> </programlisting>
- <para> Issue another <command>ls /</command> command to see what the directory looks like:
- </para>
- <programlisting>
-<![CDATA[
-[zkshell: 11] ls /
-[zookeeper, zk_test]
-
-]]> </programlisting><para>
- Notice that the zk_test directory has now been created.
- </para>
- <para>Next, verify that the data was associated with the znode by running the <command>get</command> command, as in:
- </para>
- <programlisting>
-<![CDATA[
-[zkshell: 12] get /zk_test
-my_data
-cZxid = 5
-ctime = Fri Jun 05 13:57:06 PDT 2009
-mZxid = 5
-mtime = Fri Jun 05 13:57:06 PDT 2009
-pZxid = 5
-cversion = 0
-dataVersion = 0
-aclVersion = 0
-ephemeralOwner = 0
-dataLength = 7
-numChildren = 0
-]]> </programlisting>
- <para>We can change the data associated with zk_test by issuing the <command>set</command> command, as in:
- </para>
- <programlisting>
-<![CDATA[
-[zkshell: 14] set /zk_test junk
-cZxid = 5
-ctime = Fri Jun 05 13:57:06 PDT 2009
-mZxid = 6
-mtime = Fri Jun 05 14:01:52 PDT 2009
-pZxid = 5
-cversion = 0
-dataVersion = 1
-aclVersion = 0
-ephemeralOwner = 0
-dataLength = 4
-numChildren = 0
-[zkshell: 15] get /zk_test
-junk
-cZxid = 5
-ctime = Fri Jun 05 13:57:06 PDT 2009
-mZxid = 6
-mtime = Fri Jun 05 14:01:52 PDT 2009
-pZxid = 5
-cversion = 0
-dataVersion = 1
-aclVersion = 0
-ephemeralOwner = 0
-dataLength = 4
-numChildren = 0
-]]> </programlisting>
- <para>
- (Notice we did a <command>get</command> after setting the data and it did, indeed, change.</para>
- <para>Finally, let's <command>delete</command> the node by issuing:
- </para>
- <programlisting>
-<![CDATA[
-[zkshell: 16] delete /zk_test
-[zkshell: 17] ls /
-[zookeeper]
-[zkshell: 18]
-]]></programlisting>
- <para>That's it for now. To explore more, continue with the rest of this document and see the <ulink url="zookeeperProgrammers.html">Programmer's Guide</ulink>. </para>
- </section>
-
- <section id="sc_ProgrammingToZooKeeper">
- <title>Programming to ZooKeeper</title>
-
- <para>ZooKeeper has a Java bindings and C bindings. They are
- functionally equivalent. The C bindings exist in two variants: single
- threaded and multi-threaded. These differ only in how the messaging loop
- is done. For more information, see the <ulink
- url="zookeeperProgrammers.html#ch_programStructureWithExample">Programming
- Examples in the ZooKeeper Programmer's Guide</ulink> for
- sample code using of the different APIs.</para>
- </section>
-
- <section id="sc_RunningReplicatedZooKeeper">
- <title>Running Replicated ZooKeeper</title>
-
- <para>Running ZooKeeper in standalone mode is convenient for evaluation,
- some development, and testing. But in production, you should run
- ZooKeeper in replicated mode. A replicated group of servers in the same
- application is called a <emphasis>quorum</emphasis>, and in replicated
- mode, all servers in the quorum have copies of the same configuration
- file.</para>
- <note>
- <para>
- For replicated mode, a minimum of three servers are required,
- and it is strongly recommended that you have an odd number of
- servers. If you only have two servers, then you are in a
- situation where if one of them fails, there are not enough
- machines to form a majority quorum. Two servers is inherently
- <emphasis role="bold">less</emphasis>
- stable than a single server, because there are two single
- points of failure.
- </para>
- </note>
- <para>
- The required
- <emphasis role="bold">conf/zoo.cfg</emphasis>
- file for replicated mode is similar to the one used in standalone
- mode, but with a few differences. Here is an example:
- </para>
-
-<programlisting>
-tickTime=2000
-dataDir=/var/lib/zookeeper
-clientPort=2181
-initLimit=5
-syncLimit=2
-server.1=zoo1:2888:3888
-server.2=zoo2:2888:3888
-server.3=zoo3:2888:3888
-</programlisting>
-
- <para>The new entry, <emphasis role="bold">initLimit</emphasis> is
- timeouts ZooKeeper uses to limit the length of time the ZooKeeper
- servers in quorum have to connect to a leader. The entry <emphasis
- role="bold">syncLimit</emphasis> limits how far out of date a server can
- be from a leader.</para>
-
- <para>With both of these timeouts, you specify the unit of time using
- <emphasis role="bold">tickTime</emphasis>. In this example, the timeout
- for initLimit is 5 ticks at 2000 milleseconds a tick, or 10
- seconds.</para>
-
- <para>The entries of the form <emphasis>server.X</emphasis> list the
- servers that make up the ZooKeeper service. When the server starts up,
- it knows which server it is by looking for the file
- <emphasis>myid</emphasis> in the data directory. That file has the
- contains the server number, in ASCII.</para>
-
- <para>Finally, note the two port numbers after each server
- name: " 2888" and "3888". Peers use the former port to connect
- to other peers. Such a connection is necessary so that peers
- can communicate, for example, to agree upon the order of
- updates. More specifically, a ZooKeeper server uses this port
- to connect followers to the leader. When a new leader arises, a
- follower opens a TCP connection to the leader using this
- port. Because the default leader election also uses TCP, we
- currently require another port for leader election. This is the
- second port in the server entry.
- </para>
-
- <note>
- <para>If you want to test multiple servers on a single
- machine, specify the servername
- as <emphasis>localhost</emphasis> with unique quorum &
- leader election ports (i.e. 2888:3888, 2889:3889, 2890:3890 in
- the example above) for each server.X in that server's config
- file. Of course separate <emphasis>dataDir</emphasis>s and
- distinct <emphasis>clientPort</emphasis>s are also necessary
- (in the above replicated example, running on a
- single <emphasis>localhost</emphasis>, you would still have
- three config files).</para>
- <para>Please be aware that setting up multiple servers on a single
- machine will not create any redundancy. If something were to
- happen which caused the machine to die, all of the zookeeper
- servers would be offline. Full redundancy requires that each
- server have its own machine. It must be a completely separate
- physical server. Multiple virtual machines on the same physical
- host are still vulnerable to the complete failure of that host.</para>
- </note>
- </section>
-
- <section>
- <title>Other Optimizations</title>
-
- <para>There are a couple of other configuration parameters that can
- greatly increase performance:</para>
-
- <itemizedlist>
- <listitem>
- <para>To get low latencies on updates it is important to
- have a dedicated transaction log directory. By default
- transaction logs are put in the same directory as the data
- snapshots and <emphasis>myid</emphasis> file. The dataLogDir
- parameters indicates a different directory to use for the
- transaction logs.</para>
- </listitem>
-
- <listitem>
- <para><emphasis>[tbd: what is the other config param?]</emphasis></para>
- </listitem>
- </itemizedlist>
- </section>
- </section>
-</article>