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From shra...@apache.org
Subject svn commit: r1621314 [1/2] - in /zookeeper/branches/branch-3.5: ./ docs/ src/docs/src/documentation/content/xdocs/
Date Fri, 29 Aug 2014 14:36:46 GMT
Author: shralex
Date: Fri Aug 29 14:36:46 2014
New Revision: 1621314

URL: http://svn.apache.org/r1621314
Log:
ZOOKEEPER-1660 Documentation for Dynamic Reconfiguration (Reed Wanderman-Milne via shralex)

Added:
    zookeeper/branches/branch-3.5/docs/zookeeperReconfig.html
    zookeeper/branches/branch-3.5/docs/zookeeperReconfig.pdf   (with props)
    zookeeper/branches/branch-3.5/src/docs/src/documentation/content/xdocs/zookeeperReconfig.xml
Modified:
    zookeeper/branches/branch-3.5/CHANGES.txt
    zookeeper/branches/branch-3.5/docs/index.html
    zookeeper/branches/branch-3.5/docs/index.pdf
    zookeeper/branches/branch-3.5/docs/zookeeperAdmin.html
    zookeeper/branches/branch-3.5/docs/zookeeperAdmin.pdf
    zookeeper/branches/branch-3.5/src/docs/src/documentation/content/xdocs/index.xml
    zookeeper/branches/branch-3.5/src/docs/src/documentation/content/xdocs/site.xml
    zookeeper/branches/branch-3.5/src/docs/src/documentation/content/xdocs/zookeeperAdmin.xml

Modified: zookeeper/branches/branch-3.5/CHANGES.txt
URL: http://svn.apache.org/viewvc/zookeeper/branches/branch-3.5/CHANGES.txt?rev=1621314&r1=1621313&r2=1621314&view=diff
==============================================================================
--- zookeeper/branches/branch-3.5/CHANGES.txt (original)
+++ zookeeper/branches/branch-3.5/CHANGES.txt Fri Aug 29 14:36:46 2014
@@ -5,7 +5,7 @@ NEW FEATURES:
 BUGFIXES:
 
 IMPROVEMENTS:
-
+  ZOOKEEPER-1660 Documentation for Dynamic Reconfiguration (Reed Wanderman-Milne via shralex)
 
 Release 3.5.0 - 8/4/2014
 

Modified: zookeeper/branches/branch-3.5/docs/index.html
URL: http://svn.apache.org/viewvc/zookeeper/branches/branch-3.5/docs/index.html?rev=1621314&r1=1621313&r2=1621314&view=diff
==============================================================================
--- zookeeper/branches/branch-3.5/docs/index.html (original)
+++ zookeeper/branches/branch-3.5/docs/index.html Fri Aug 29 14:36:46 2014
@@ -67,7 +67,7 @@
 <a class="unselected" href="https://cwiki.apache.org/confluence/display/ZOOKEEPER/">Wiki</a>
 </li>
 <li class="current">
-<a class="selected" href="index.html">ZooKeeper 3.5 Documentation</a>
+<a class="selected" href="index.html">ZooKeeper 3.4 Documentation</a>
 </li>
 </ul>
 <!--+
@@ -149,6 +149,9 @@ document.write("Last Published: " + docu
 <div class="menuitem">
 <a href="zookeeperObservers.html">Observers Guide</a>
 </div>
+<div class="menuitem">
+<a href="zookeeperReconfig.html">Dynamic Reconfiguration</a>
+</div>
 </div>
 <div onclick="SwitchMenu('menu_1.4', 'skin/')" id="menu_1.4Title" class="menutitle">Contributor</div>
 <div id="menu_1.4" class="menuitemgroup">
@@ -280,6 +283,9 @@ document.write("Last Published: " + docu
 <li>
 <a href="zookeeperObservers.html">Observers</a> - non-voting ensemble members that easily improve ZooKeeper's scalability</li>
       
+<li>
+<a href="zookeeperReconfig.html">Dynamic Reconfiguration</a> - a guide on how to use dynamic reconfiguration in ZooKeeper</li>
+      
 </ul>
       
 </li>

Modified: zookeeper/branches/branch-3.5/docs/index.pdf
URL: http://svn.apache.org/viewvc/zookeeper/branches/branch-3.5/docs/index.pdf?rev=1621314&r1=1621313&r2=1621314&view=diff
==============================================================================
Binary files - no diff available.

Modified: zookeeper/branches/branch-3.5/docs/zookeeperAdmin.html
URL: http://svn.apache.org/viewvc/zookeeper/branches/branch-3.5/docs/zookeeperAdmin.html?rev=1621314&r1=1621313&r2=1621314&view=diff
==============================================================================
--- zookeeper/branches/branch-3.5/docs/zookeeperAdmin.html (original)
+++ zookeeper/branches/branch-3.5/docs/zookeeperAdmin.html Fri Aug 29 14:36:46 2014
@@ -67,7 +67,7 @@
 <a class="unselected" href="https://cwiki.apache.org/confluence/display/ZOOKEEPER/">Wiki</a>
 </li>
 <li class="current">
-<a class="selected" href="index.html">ZooKeeper 3.5 Documentation</a>
+<a class="selected" href="index.html">ZooKeeper 3.4 Documentation</a>
 </li>
 </ul>
 <!--+
@@ -149,6 +149,9 @@ document.write("Last Published: " + docu
 <div class="menuitem">
 <a href="zookeeperObservers.html">Observers Guide</a>
 </div>
+<div class="menuitem">
+<a href="zookeeperReconfig.html">Dynamic Reconfiguration</a>
+</div>
 </div>
 <div onclick="SwitchMenu('menu_1.4', 'skin/')" id="menu_1.4Title" class="menutitle">Contributor</div>
 <div id="menu_1.4" class="menuitemgroup">
@@ -979,6 +982,18 @@ server.3=zoo3:2888:3888</pre>
       layouts are the same. If servers use different configuration files, care
       must be taken to ensure that the list of servers in all of the different
       configuration files match.</p>
+<div class="note">
+<div class="label">Note</div>
+<div class="content">
+        
+<p>In 3.5.0 and later, some of these parameters should be placed in
+         a dynamic configuration file. If they are placed in the static
+         configuration file, ZooKeeper will automatically move them over to the
+         dynamic configuration file. See <a href="zookeeperReconfig.html">
+         Dynamic Reconfiguration</a> for more information.</p>
+      
+</div>
+</div>
 <a name="sc_minimumConfiguration"></a>
 <h4>Minimum Configuration</h4>
 <p>Here are the minimum configuration keywords that must be defined

Modified: zookeeper/branches/branch-3.5/docs/zookeeperAdmin.pdf
URL: http://svn.apache.org/viewvc/zookeeper/branches/branch-3.5/docs/zookeeperAdmin.pdf?rev=1621314&r1=1621313&r2=1621314&view=diff
==============================================================================
Binary files - no diff available.

Added: zookeeper/branches/branch-3.5/docs/zookeeperReconfig.html
URL: http://svn.apache.org/viewvc/zookeeper/branches/branch-3.5/docs/zookeeperReconfig.html?rev=1621314&view=auto
==============================================================================
--- zookeeper/branches/branch-3.5/docs/zookeeperReconfig.html (added)
+++ zookeeper/branches/branch-3.5/docs/zookeeperReconfig.html Fri Aug 29 14:36:46 2014
@@ -0,0 +1,1098 @@
+<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
+<html>
+<head>
+<META http-equiv="Content-Type" content="text/html; charset=UTF-8">
+<meta content="Apache Forrest" name="Generator">
+<meta name="Forrest-version" content="0.9">
+<meta name="Forrest-skin-name" content="pelt">
+<title>ZooKeeper Dynamic Reconfiguration</title>
+<link type="text/css" href="skin/basic.css" rel="stylesheet">
+<link media="screen" type="text/css" href="skin/screen.css" rel="stylesheet">
+<link media="print" type="text/css" href="skin/print.css" rel="stylesheet">
+<link type="text/css" href="skin/profile.css" rel="stylesheet">
+<script src="skin/getBlank.js" language="javascript" type="text/javascript"></script><script src="skin/getMenu.js" language="javascript" type="text/javascript"></script><script src="skin/fontsize.js" language="javascript" type="text/javascript"></script>
+<link rel="shortcut icon" href="images/favicon.ico">
+</head>
+<body onload="init()">
+<script type="text/javascript">ndeSetTextSize();</script>
+<div id="top">
+<!--+
+    |breadtrail
+    +-->
+<div class="breadtrail">
+<a href="http://www.apache.org/">Apache</a> &gt; <a href="http://zookeeper.apache.org/">ZooKeeper</a> &gt; <a href="http://zookeeper.apache.org/">ZooKeeper</a><script src="skin/breadcrumbs.js" language="JavaScript" type="text/javascript"></script>
+</div>
+<!--+
+    |header
+    +-->
+<div class="header">
+<!--+
+    |start group logo
+    +-->
+<div class="grouplogo">
+<a href="http://hadoop.apache.org/"><img class="logoImage" alt="Hadoop" src="images/hadoop-logo.jpg" title="Apache Hadoop"></a>
+</div>
+<!--+
+    |end group logo
+    +-->
+<!--+
+    |start Project Logo
+    +-->
+<div class="projectlogo">
+<a href="http://zookeeper.apache.org/"><img class="logoImage" alt="ZooKeeper" src="images/zookeeper_small.gif" title="ZooKeeper: distributed coordination"></a>
+</div>
+<!--+
+    |end Project Logo
+    +-->
+<!--+
+    |start Search
+    +-->
+<div class="searchbox">
+<form action="http://www.google.com/search" method="get" class="roundtopsmall">
+<input value="zookeeper.apache.org" name="sitesearch" type="hidden"><input onFocus="getBlank (this, 'Search the site with google');" size="25" name="q" id="query" type="text" value="Search the site with google">&nbsp; 
+                    <input name="Search" value="Search" type="submit">
+</form>
+</div>
+<!--+
+    |end search
+    +-->
+<!--+
+    |start Tabs
+    +-->
+<ul id="tabs">
+<li>
+<a class="unselected" href="http://zookeeper.apache.org/">Project</a>
+</li>
+<li>
+<a class="unselected" href="https://cwiki.apache.org/confluence/display/ZOOKEEPER/">Wiki</a>
+</li>
+<li class="current">
+<a class="selected" href="index.html">ZooKeeper 3.4 Documentation</a>
+</li>
+</ul>
+<!--+
+    |end Tabs
+    +-->
+</div>
+</div>
+<div id="main">
+<div id="publishedStrip">
+<!--+
+    |start Subtabs
+    +-->
+<div id="level2tabs"></div>
+<!--+
+    |end Endtabs
+    +-->
+<script type="text/javascript"><!--
+document.write("Last Published: " + document.lastModified);
+//  --></script>
+</div>
+<!--+
+    |breadtrail
+    +-->
+<div class="breadtrail">
+
+             &nbsp;
+           </div>
+<!--+
+    |start Menu, mainarea
+    +-->
+<!--+
+    |start Menu
+    +-->
+<div id="menu">
+<div onclick="SwitchMenu('menu_1.1', 'skin/')" id="menu_1.1Title" class="menutitle">Overview</div>
+<div id="menu_1.1" class="menuitemgroup">
+<div class="menuitem">
+<a href="index.html">Welcome</a>
+</div>
+<div class="menuitem">
+<a href="zookeeperOver.html">Overview</a>
+</div>
+<div class="menuitem">
+<a href="zookeeperStarted.html">Getting Started</a>
+</div>
+<div class="menuitem">
+<a href="releasenotes.html">Release Notes</a>
+</div>
+</div>
+<div onclick="SwitchMenu('menu_1.2', 'skin/')" id="menu_1.2Title" class="menutitle">Developer</div>
+<div id="menu_1.2" class="menuitemgroup">
+<div class="menuitem">
+<a href="api/index.html">API Docs</a>
+</div>
+<div class="menuitem">
+<a href="zookeeperProgrammers.html">Programmer's Guide</a>
+</div>
+<div class="menuitem">
+<a href="javaExample.html">Java Example</a>
+</div>
+<div class="menuitem">
+<a href="zookeeperTutorial.html">Barrier and Queue Tutorial</a>
+</div>
+<div class="menuitem">
+<a href="recipes.html">Recipes</a>
+</div>
+</div>
+<div onclick="SwitchMenu('menu_selected_1.3', 'skin/')" id="menu_selected_1.3Title" class="menutitle" style="background-image: url('skin/images/chapter_open.gif');">Admin &amp; Ops</div>
+<div id="menu_selected_1.3" class="selectedmenuitemgroup" style="display: block;">
+<div class="menuitem">
+<a href="zookeeperAdmin.html">Administrator's Guide</a>
+</div>
+<div class="menuitem">
+<a href="zookeeperQuotas.html">Quota Guide</a>
+</div>
+<div class="menuitem">
+<a href="zookeeperJMX.html">JMX</a>
+</div>
+<div class="menuitem">
+<a href="zookeeperObservers.html">Observers Guide</a>
+</div>
+<div class="menupage">
+<div class="menupagetitle">Dynamic Reconfiguration</div>
+</div>
+</div>
+<div onclick="SwitchMenu('menu_1.4', 'skin/')" id="menu_1.4Title" class="menutitle">Contributor</div>
+<div id="menu_1.4" class="menuitemgroup">
+<div class="menuitem">
+<a href="zookeeperInternals.html">ZooKeeper Internals</a>
+</div>
+</div>
+<div onclick="SwitchMenu('menu_1.5', 'skin/')" id="menu_1.5Title" class="menutitle">Miscellaneous</div>
+<div id="menu_1.5" class="menuitemgroup">
+<div class="menuitem">
+<a href="https://cwiki.apache.org/confluence/display/ZOOKEEPER">Wiki</a>
+</div>
+<div class="menuitem">
+<a href="https://cwiki.apache.org/confluence/display/ZOOKEEPER/FAQ">FAQ</a>
+</div>
+<div class="menuitem">
+<a href="http://zookeeper.apache.org/mailing_lists.html">Mailing Lists</a>
+</div>
+</div>
+<div id="credit"></div>
+<div id="roundbottom">
+<img style="display: none" class="corner" height="15" width="15" alt="" src="skin/images/rc-b-l-15-1body-2menu-3menu.png"></div>
+<!--+
+  |alternative credits
+  +-->
+<div id="credit2"></div>
+</div>
+<!--+
+    |end Menu
+    +-->
+<!--+
+    |start content
+    +-->
+<div id="content">
+<div title="Portable Document Format" class="pdflink">
+<a class="dida" href="zookeeperReconfig.pdf"><img alt="PDF -icon" src="skin/images/pdfdoc.gif" class="skin"><br>
+        PDF</a>
+</div>
+<h1>ZooKeeper Dynamic Reconfiguration</h1>
+<div id="front-matter">
+<div id="minitoc-area">
+<ul class="minitoc">
+<li>
+<a href="#ch_reconfig_intro">Overview</a>
+</li>
+<li>
+<a href="#ch_reconfig_format">Changes to Configuration Format</a>
+<ul class="minitoc">
+<li>
+<a href="#sc_reconfig_clientport">Specifying the client port</a>
+</li>
+<li>
+<a href="#sc_reconfig_standaloneEnabled">The standaloneEnabled flag</a>
+</li>
+<li>
+<a href="#sc_reconfig_file">Dynamic configuration file</a>
+</li>
+<li>
+<a href="#sc_reconfig_backward">Backward compatibility</a>
+</li>
+</ul>
+</li>
+<li>
+<a href="#ch_reconfig_upgrade">Upgrading to 3.5.0</a>
+</li>
+<li>
+<a href="#ch_reconfig_dyn">Dynamic Reconfiguration of the ZooKeeper Ensemble</a>
+<ul class="minitoc">
+<li>
+<a href="#sc_reconfig_retrieving">Retrieving the current dynamic configuration</a>
+</li>
+<li>
+<a href="#sc_reconfig_modifying">Modifying the current dynamic configuration</a>
+<ul class="minitoc">
+<li>
+<a href="#sc_reconfig_general">General</a>
+</li>
+<li>
+<a href="#sc_reconfig_incremental">Incremental mode</a>
+</li>
+<li>
+<a href="#sc_reconfig_nonincremental">Non-incremental mode</a>
+</li>
+<li>
+<a href="#sc_reconfig_conditional">Conditional reconfig</a>
+</li>
+<li>
+<a href="#sc_reconfig_errors">Error conditions</a>
+</li>
+<li>
+<a href="#sc_reconfig_additional">Additional comments</a>
+</li>
+</ul>
+</li>
+</ul>
+</li>
+<li>
+<a href="#ch_reconfig_rebalancing">Rebalancing Client Connections</a>
+</li>
+</ul>
+</div>
+</div>
+  
+
+  
+  
+<a name="ch_reconfig_intro"></a>
+<h2 class="h3">Overview</h2>
+<div class="section">
+<p>Prior to the 3.5.0 release, the membership and all other configuration
+      parameters of Zookeeper were static - loaded during boot and immutable at
+      runtime. Operators resorted to ''rolling restarts'' - a manually intensive
+      and error-prone method of changing the configuration that has caused data
+      loss and inconsistency in production.</p>
+<p>Starting with 3.5.0, &ldquo;rolling restarts&rdquo; are no longer needed!
+      ZooKeeper comes with full support for automated configuration changes: the
+      set of Zookeeper servers, their roles (participant / observer), all ports,
+      and even the quorum system can be changed dynamically, without service
+      interruption and while maintaining data consistency. Reconfigurations are
+      performed immediately, just like other operations in ZooKeeper. Multiple
+      changes can be done using a single reconfiguration command. The dynamic
+      reconfiguration functionality does not limit operation concurrency, does
+      not require client operations to be stopped during reconfigurations, has a
+      very simple interface for administrators and no added complexity to other
+      client operations.</p>
+<p>New client-side features allow clients to find out about configuration
+      changes and to update the connection string (list of servers and their
+      client ports) stored in their ZooKeeper handle. A probabilistic algorithm
+      is used to rebalance clients across the new configuration servers while
+      keeping the extent of client migrations proportional to the change in
+      ensemble membership.</p>
+<p>This document provides the administrator manual for reconfiguration.
+      For a detailed description of the reconfiguration algorithms, performance
+      measurements, and more, please see our paper:</p>
+<dl>
+      
+<dt>
+<term>Shraer, A., Reed, B., Malkhi, D., Junqueira, F. Dynamic
+          Reconfiguration of Primary/Backup Clusters. In <em>USENIX Annual
+          Technical Conference (ATC) </em>(2012), 425-437</term>
+</dt>
+<dd>
+<p>Links: <a href="https://www.usenix.org/system/files/conference/atc12/atc12-final74.pdf">paper (pdf)</a>, <a href="https://www.usenix.org/sites/default/files/conference/protected-files/shraer_atc12_slides.pdf">slides (pdf)</a>, <a href="https://www.usenix.org/conference/atc12/technical-sessions/presentation/shraer">video</a>, <a href="http://www.slideshare.net/Hadoop_Summit/dynamic-reconfiguration-of-zookeeper">hadoop summit slides</a>
+</p>
+</dd>
+    
+</dl>
+</div>
+  
+<a name="ch_reconfig_format"></a>
+<h2 class="h3">Changes to Configuration Format</h2>
+<div class="section">
+<a name="sc_reconfig_clientport"></a>
+<h3 class="h4">Specifying the client port</h3>
+<p>A client port of a server is the port on which the server accepts
+        client connection requests. Starting with 3.5.0 the
+        <em>clientPort</em> and <em>clientPortAddress
+        </em> configuration parameters should no longer be used. Instead,
+        this information is now part of the server keyword specification, which
+        becomes as follows:</p>
+<p>
+<span class="codefrag computeroutput">server.&lt;positive id&gt; = &lt;address1&gt;:&lt;port1&gt;:&lt;port2&gt;[:role];[&lt;client port address&gt;:]&lt;client port&gt;</span>
+</p>
+<p>The client port specification is to the right of the semicolon. The
+        client port address is optional, and if not specified it defaults to
+        "0.0.0.0". As usual, role is also optional, it can be
+        <em>participant</em> or <em>observer</em>
+        (<em>participant</em> by default).</p>
+<p> Examples of legal server statements: </p>
+<ul>
+        
+<li>
+          
+<p>
+<span class="codefrag computeroutput">server.5 = 125.23.63.23:1234:1235;1236</span>
+</p>
+        
+</li>
+        
+<li>
+          
+<p>
+<span class="codefrag computeroutput">server.5 = 125.23.63.23:1234:1235:participant;1236</span>
+</p>
+        
+</li>
+        
+<li>
+          
+<p>
+<span class="codefrag computeroutput">server.5 = 125.23.63.23:1234:1235:observer;1236</span>
+</p>
+        
+</li>
+        
+<li>
+          
+<p>
+<span class="codefrag computeroutput">server.5 = 125.23.63.23:1234:1235;125.23.63.24:1236</span>
+</p>
+        
+</li>
+        
+<li>
+          
+<p>
+<span class="codefrag computeroutput">server.5 = 125.23.63.23:1234:1235:participant;125.23.63.23:1236</span>
+</p>
+        
+</li>
+      
+</ul>
+<a name="sc_reconfig_standaloneEnabled"></a>
+<h3 class="h4">The standaloneEnabled flag</h3>
+<p>Prior to 3.5.0, one could run ZooKeeper in Standalone mode or in a
+        Distributed mode. These are separate implementation stacks, and
+        switching between them during run time is not possible. By default (for
+        backward compatibility) <em>standaloneEnabled</em> is set to
+        <em>true</em>. The consequence of using this default is that
+        if started with a single server the ensemble will not be allowed to
+        grow, and if started with more than one server it will not be allowed to
+        shrink to contain fewer than two participants.</p>
+<p>Setting the flag to <em>false</em> instructs the system
+        to run the Distributed software stack even if there is only a single
+        participant in the ensemble. To achieve this the (static) configuration
+        file should contain:</p>
+<p>
+<span class="codefrag computeroutput">standaloneEnabled=false</span>
+</p>
+<p>With this setting it is possible to start a ZooKeeper ensemble
+        containing a single participant and to dynamically grow it by adding
+        more servers. Similarly, it is possible to shrink an ensemble so that
+        just a single participant remains, by removing servers.</p>
+<p>Since running the Distributed mode allows more flexibility, we
+        recommend setting the flag to <em>false</em>. We expect that
+        the legacy Standalone mode will be deprecated in the future.</p>
+<a name="sc_reconfig_file"></a>
+<h3 class="h4">Dynamic configuration file</h3>
+<p>Starting with 3.5.0 we're distinguishing between dynamic
+        configuration parameters, which can be changed during runtime, and
+        static configuration parameters, which are read from a configuration
+        file when a server boots and don't change during its execution. For now,
+        the following configuration keywords are considered part of the dynamic
+        configuration: <em>server</em>, <em>group</em>
+        and <em>weight</em>.</p>
+<p>Dynamic configuration parameters are stored in a separate file on
+        the server (which we call the dynamic configuration file). This file is
+        linked from the static config file using the new
+        <em>dynamicConfigFile</em> keyword.</p>
+<p>
+<strong>Example</strong>
+</p>
+<div class="note example">
+<div class="label">zoo_replicated1.cfg</div>
+<div class="content">
+        
+<title>zoo_replicated1.cfg</title>
+        
+<pre class="code">tickTime=2000
+dataDir=/zookeeper/data/zookeeper1
+initLimit=5
+syncLimit=2
+dynamicConfigFile=/zookeeper/conf/zoo_replicated1.cfg.dynamic</pre>
+      
+</div>
+</div>
+<div class="note example">
+<div class="label">zoo_replicated1.cfg.dynamic</div>
+<div class="content">
+        
+<title>zoo_replicated1.cfg.dynamic</title>
+        
+<pre class="code">server.1=125.23.63.23:2780:2783:participant;2791
+server.2=125.23.63.24:2781:2784:participant;2792
+server.3=125.23.63.25:2782:2785:participant;2793</pre>
+      
+</div>
+</div>
+<p>When the ensemble configuration changes, the static configuration
+        parameters remain the same. The dynamic parameters are pushed by
+        ZooKeeper and overwrite the dynamic configuration files on all servers.
+        Thus, the dynamic configuration files on the different servers are
+        usually identical (they can only differ momentarily when a
+        reconfiguration is in progress, or if a new configuration hasn't
+        propagated yet to some of the servers). Once created, the dynamic
+        configuration file should not be manually altered. Changed are only made
+        through the new reconfiguration commands outlined below. Note that
+        changing the config of an offline cluster could result in an
+        inconsistency with respect to configuration information stored in the
+        ZooKeeper log (and the special configuration znode, populated from the
+        log) and is therefore highly discouraged.</p>
+<p>
+<strong>Example 2</strong>
+</p>
+<p>Users may prefer to initially specify a single configuration file.
+        The following is thus also legal:</p>
+<div class="note example">
+<div class="label">zoo_replicated1.cfg</div>
+<div class="content">
+        
+<title>zoo_replicated1.cfg</title>
+        
+<pre class="code">tickTime=2000
+dataDir=/zookeeper/data/zookeeper1
+initLimit=5
+syncLimit=2
+clientPort=<strong>2791</strong>  // note that this line is now redundant and therefore not recommended
+server.1=125.23.63.23:2780:2783:participant;<strong>2791</strong>
+server.2=125.23.63.24:2781:2784:participant;2792
+server.3=125.23.63.25:2782:2785:participant;2793</pre>
+      
+</div>
+</div>
+<p>The configuration files on each server will be automatically split
+        into dynamic and static files, if they are not already in this format.
+        So the configuration file above will be automatically transformed into
+        the two files in Example 1. Note that the clientPort and
+        clientPortAddress lines (if specified) will be automatically removed
+        during this process, if they are redundant (as in the example above).
+        The original static configuration file is backed up (in a .bak
+        file).</p>
+<a name="sc_reconfig_backward"></a>
+<h3 class="h4">Backward compatibility</h3>
+<p>We still support the old configuration format. For example, the
+        following configuration file is acceptable (but not recommended):</p>
+<div class="note example">
+<div class="label">zoo_replicated1.cfg</div>
+<div class="content">
+        
+<title>zoo_replicated1.cfg</title>
+        
+<pre class="code">tickTime=2000
+dataDir=/zookeeper/data/zookeeper1
+initLimit=5
+syncLimit=2
+clientPort=2791
+server.1=125.23.63.23:2780:2783:participant
+server.2=125.23.63.24:2781:2784:participant
+server.3=125.23.63.25:2782:2785:participant</pre>
+      
+</div>
+</div>
+<p>During boot, a dynamic configuration file is created and contains
+        the dynamic part of the configuration as explained earlier. In this
+        case, however, the line "clientPort=2791" will remain in the static
+        configuration file of server 1 since it is not redundant -- it was not
+        specified as part of the "server.1=..." using the format explained in
+        the section <a href="#ch_reconfig_format">Changes to Configuration Format</a>. If a reconfiguration
+        is invoked that sets the client port of server 1, we remove
+        "clientPort=2791" from the static configuration file (the dynamic file
+        now contain this information as part of the specification of server
+        1).</p>
+</div>
+  
+<a name="ch_reconfig_upgrade"></a>
+<h2 class="h3">Upgrading to 3.5.0</h2>
+<div class="section">
+<p>Upgrading a running ZooKeeper ensemble to 3.5.0 should be done only
+      after upgrading your ensemble to the 3.4.6 release. Note that this is only
+      necessary for rolling upgrades (if you're fine with shutting down the
+      system completely, you don't have to go through 3.4.6). If you attempt a
+      rolling upgrade without going through 3.4.6 (for example from 3.4.5), you
+      may get the following error:</p>
+<pre class="code">2013-01-30 11:32:10,663 [myid:2] - INFO [localhost/127.0.0.1:2784:QuorumCnxManager$Listener@498] - Received connection request /127.0.0.1:60876
+2013-01-30 11:32:10,663 [myid:2] - WARN [localhost/127.0.0.1:2784:QuorumCnxManager@349] - Invalid server id: -65536</pre>
+<p>During a rolling upgrade, each server is taken down in turn and
+      rebooted with the new 3.5.0 binaries. Before starting the server with
+      3.5.0 binaries, we highly recommend updating the configuration file so
+      that all server statements "server.x=..." contain client ports (see the
+      section <a href="#sc_reconfig_clientport">Specifying the client port</a>). As explained earlier
+      you may leave the configuration in a single file, as well as leave the
+      clientPort/clientPortAddress statements (although if you specify client
+      ports in the new format, these statements are now redundant).</p>
+</div>
+  
+<a name="ch_reconfig_dyn"></a>
+<h2 class="h3">Dynamic Reconfiguration of the ZooKeeper Ensemble</h2>
+<div class="section">
+<p>The ZooKeeper Java and C API were extended with getConfig and reconfig
+      commands that facilitate reconfiguration. Both commands have a synchronous
+      (blocking) variant and an asynchronous one. We demonstrate these commands
+      here using the Java CLI, but note that you can similarly use the C CLI or
+      invoke the commands directly from a program just like any other ZooKeeper
+      command.</p>
+<a name="sc_reconfig_retrieving"></a>
+<h3 class="h4">Retrieving the current dynamic configuration</h3>
+<p>The dynamic configuration is stored in a special znode
+        ZooDefs.CONFIG_NODE = /zookeeper/config. The new
+        <span class="codefrag command">config</span> CLI command reads this znode (currently it is
+        simply a wrapper to <span class="codefrag command">get /zookeeper/config</span>). As with
+        normal reads, to retrieve the latest committed value you should do a
+        <span class="codefrag command">sync</span> first.</p>
+<pre class="code">[zk: 127.0.0.1:2791(CONNECTED) 3] config
+server.1=localhost:2780:2783:participant;localhost:2791
+server.2=localhost:2781:2784:participant;localhost:2792
+server.3=localhost:2782:2785:participant;localhost:2793
+<strong>version=400000003</strong>
+</pre>
+<p>Notice the last line of the output. This is the configuration
+        version. The version equals to the zxid of the reconfiguration command
+        which created this configuration. The version of the first established
+        configuration equals to the zxid of the NEWLEADER message sent by the
+        first successfully established leader. When a configuration is written
+        to a dynamic configuration file, the version automatically becomes part
+        of the filename and the static configuration file is updated with the
+        path to the new dynamic configuration file. Configuration files
+        corresponding to earlier versions are retained for backup
+        purposes.</p>
+<p>During boot time the version (if it exists) is extracted from the
+        filename. The version should never be altered manually by users or the
+        system administrator. It is used by the system to know which
+        configuration is most up-to-date. Manipulating it manually can result in
+        data loss and inconsistency.</p>
+<p>Just like a <span class="codefrag command">get</span> command, the
+        <span class="codefrag command">config</span> CLI command accepts the <span class="codefrag option">-w</span>
+        flag for setting a watch on the znode, and <span class="codefrag option">-s</span> flag for
+        displaying the Stats of the znode. It additionally accepts a new flag
+        <span class="codefrag option">-c</span> which outputs only the version and the client
+        connection string corresponding to the current configuration. For
+        example, for the configuration above we would get:</p>
+<pre class="code">[zk: 127.0.0.1:2791(CONNECTED) 17] config -c
+400000003 localhost:2791,localhost:2793,localhost:2792</pre>
+<p>Note that when using the API directly, this command is called
+        <span class="codefrag command">getConfig</span>.</p>
+<p>As any read command it returns the configuration known to the
+        follower to which your client is connected, which may be slightly
+        out-of-date. One can use the <span class="codefrag command">sync</span> command for
+        stronger guarantees. For example using the Java API:</p>
+<pre class="code">zk.sync(ZooDefs.CONFIG_NODE, void_callback, context);
+zk.getConfig(watcher, callback, context);</pre>
+<p>Note: in 3.5.0 it doesn't really matter which path is passed to the
+        <span class="codefrag command">sync() </span> command as all the server's state is brought
+        up to date with the leader (so one could use a different path instead of
+        ZooDefs.CONFIG_NODE). However, this may change in the future.</p>
+<a name="sc_reconfig_modifying"></a>
+<h3 class="h4">Modifying the current dynamic configuration</h3>
+<p>Modifying the configuration is done through the
+        <span class="codefrag command">reconfig</span> command. There are two modes of
+        reconfiguration: incremental and non-incremental (bulk). The
+        non-incremental simply specifies the new dynamic configuration of the
+        system. The incremental specifies changes to the current configuration.
+        The <span class="codefrag command">reconfig</span> command returns the new
+        configuration.</p>
+<p>A few examples are in: <span class="codefrag filename">ReconfigTest.java</span>,
+        <span class="codefrag filename">ReconfigRecoveryTest.java</span> and
+        <span class="codefrag filename">TestReconfigServer.cc</span>.</p>
+<a name="sc_reconfig_general"></a>
+<h4>General</h4>
+<p>
+<strong>Removing servers:</strong> Any server can
+          be removed, including the leader (although removing the leader will
+          result in a short unavailability, see Figures 6 and 8 in the <a href="https://www.usenix.org/conference/usenixfederatedconferencesweek/dynamic-recon%EF%AC%81guration-primarybackup-clusters">paper</a>). The server will not be shut-down automatically.
+          Instead, it becomes a "non-voting follower". This is somewhat similar
+          to an observer in that its votes don't count towards the Quorum of
+          votes necessary to commit operations. However, unlike a non-voting
+          follower, an observer doesn't actually see any operation proposals and
+          does not ACK them. Thus a non-voting follower has a more significant
+          negative effect on system throughput compared to an observer.
+          Non-voting follower mode should only be used as a temporary mode,
+          before shutting the server down, or adding it as a follower or as an
+          observer to the ensemble. We do not shut the server down automatically
+          for two main reasons. The first reason is that we do not want all the
+          clients connected to this server to be immediately disconnected,
+          causing a flood of connection requests to other servers. Instead, it
+          is better if each client decides when to migrate independently. The
+          second reason is that removing a server may sometimes (rarely) be
+          necessary in order to change it from "observer" to "participant" (this
+          is explained in the section <a href="#sc_reconfig_additional">Additional comments</a>).</p>
+<p>Note that the new configuration should have some minimal number of
+          participants in order to be considered legal. If the proposed change
+          would leave the cluster with less than 2 participants and standalone
+          mode is enabled (standaloneEnabled=true, see the section <a href="#sc_reconfig_standaloneEnabled">The standaloneEnabled flag</a>), the reconfig will not be
+          processed (BadArgumentsException). If standalone mode is disabled
+          (standaloneEnabled=false) then its legal to remain with 1 or more
+          participants.</p>
+<p>
+<strong>Adding servers:</strong> Before a
+          reconfiguration is invoked, the administrator must make sure that a
+          quorum (majority) of participants from the new configuration are
+          already connected and synced with the current leader. To achieve this
+          we need to connect a new joining server to the leader before it is
+          officially part of the ensemble. This is done by starting the joining
+          server using an initial list of servers which is technically not a
+          legal configuration of the system but (a) contains the joiner, and (b)
+          gives sufficient information to the joiner in order for it to find and
+          connect to the current leader. We list a few different options of
+          doing this safely.</p>
+<ol>
+          
+<li>
+            
+<p>Initial configuration of joiners is comprised of servers in
+              the last committed configuration and one or more joiners, where
+              <strong>joiners are listed as observers.</strong>
+              For example, if servers D and E are added at the same time to (A,
+              B, C) and server C is being removed, the initial configuration of
+              D could be (A, B, C, D) or (A, B, C, D, E), where D and E are
+              listed as observers. Similarly, the configuration of E could be
+              (A, B, C, E) or (A, B, C, D, E), where D and E are listed as
+              observers. <strong>Note that listing the joiners as
+              observers will not actually make them observers - it will only
+              prevent them from accidentally forming a quorum with other
+              joiners.</strong> Instead, they will contact the servers in the
+              current configuration and adopt the last committed configuration
+              (A, B, C), where the joiners are absent. Configuration files of
+              joiners are backed up and replaced automatically as this happens.
+              After connecting to the current leader, joiners become non-voting
+              followers until the system is reconfigured and they are added to
+              the ensemble (as participant or observer, as appropriate).</p>
+          
+</li>
+          
+<li>
+            
+<p>Initial configuration of each joiner is comprised of servers
+              in the last committed configuration + <strong>the
+              joiner itself, listed as a participant.</strong> For example, to
+              add a new server D to a configuration consisting of servers (A, B,
+              C), the administrator can start D using an initial configuration
+              file consisting of servers (A, B, C, D). If both D and E are added
+              at the same time to (A, B, C), the initial configuration of D
+              could be (A, B, C, D) and the configuration of E could be (A, B,
+              C, E). Similarly, if D is added and C is removed at the same time,
+              the initial configuration of D could be (A, B, C, D). Never list
+              more than one joiner as participant in the initial configuration
+              (see warning below).</p>
+          
+</li>
+          
+<li>
+            
+<p>Whether listing the joiner as an observer or as participant,
+              it is also fine not to list all the current configuration servers,
+              as long as the current leader is in the list. For example, when
+              adding D we could start D with a configuration file consisting of
+              just (A, D) if A is the current leader. however this is more
+              fragile since if A fails before D officially joins the ensemble, D
+              doesn&rsquo;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&rsquo;t
+            know that they are joining an existing ensemble; if multiple joiners
+            are listed as participants they may form an independent quorum
+            creating a split-brain situation such as processing operations
+            independently from your main ensemble. It is OK to list multiple
+            joiners as observers in an initial config.</p>
+        
+</div>
+</div>
+<p>Finally, note that once connected to the leader, a joiner adopts
+          the last committed configuration, in which it is absent (the initial
+          config of the joiner is backed up before being rewritten). If the
+          joiner restarts in this state, it will not be able to boot since it is
+          absent from its configuration file. In order to start it you&rsquo;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">&gt; 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">&gt; 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 &ndash; 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&lt;String&gt; leavingServers = new ArrayList&lt;String&gt;();
+leavingServers.add("1");
+leavingServers.add("2");
+byte[] config = zk.reconfig(null, leavingServers, null, -1, new Stat());</pre>
+<pre class="code">List&lt;String&gt; leavingServers = new ArrayList&lt;String&gt;();
+List&lt;String&gt; joiningServers = new ArrayList&lt;String&gt;();
+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&lt;String&gt;. See
+          src/java/main/org/apache/zookeeper/ZooKeeper.java.</p>
+<a name="sc_reconfig_nonincremental"></a>
+<h4>Non-incremental mode</h4>
+<p>The second mode of reconfiguration is non-incremental, whereby a
+          client gives a complete specification of the new dynamic system
+          configuration. The new configuration can either be given in place or
+          read from a file:</p>
+<p>
+<span class="codefrag userinput">&gt; 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">&gt; reconfig -members
+          server.1=125.23.63.23:2780:2783:participant;2791,server.2=125.23.63.24:2781:2784:participant;2792,server.3=125.23.63.25:2782:2785:participant;2793</span>
+</p>
+<p>The new configuration may use a different Quorum System. For
+          example, you may specify a Hierarchical Quorum System even if the
+          current ensemble uses a Majority Quorum System.</p>
+<p>Bulk mode - example using the Java API:</p>
+<pre class="code">ArrayList&lt;String&gt; newMembers = new ArrayList&lt;String&gt;();
+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&lt;String&gt;. See
+          src/java/main/org/apache/zookeeper/ZooKeeper.java.</p>
+<a name="sc_reconfig_conditional"></a>
+<h4>Conditional reconfig</h4>
+<p>Sometimes (especially in non-incremental mode) a new proposed
+          configuration depends on what the client "believes" to be the current
+          configuration, and should be applied only to that configuration.
+          Specifically, the <span class="codefrag command">reconfig</span> succeeds only if the
+          last configuration at the leader has the specified version.</p>
+<p>
+<span class="codefrag userinput">&gt; reconfig -file &lt;filename&gt; -v &lt;version&gt;</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 &amp;&amp;  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 &amp;&amp;  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 &gt; this.configVersion) {
+            hostList = config[1];
+            try {
+                // the following command is not blocking but may cause the client to close the socket and
+                // migrate to a different server. In practice its better to wait a short period of time, chosen
+                // randomly, so that different clients migrate at different times
+                zk.updateServerList(hostList);
+            } catch (IOException e) {
+                System.err.println("Error updating server list");
+                e.printStackTrace();
+            }
+            this.configVersion = version;
+} } }</pre>
+</div>
+
+<p align="right">
+<font size="-2"></font>
+</p>
+</div>
+<!--+
+    |end content
+    +-->
+<div class="clearboth">&nbsp;</div>
+</div>
+<div id="footer">
+<!--+
+    |start bottomstrip
+    +-->
+<div class="lastmodified">
+<script type="text/javascript"><!--
+document.write("Last Published: " + document.lastModified);
+//  --></script>
+</div>
+<div class="copyright">
+        Copyright &copy;
+         2008-2013 <a href="http://www.apache.org/licenses/">The Apache Software Foundation.</a>
+</div>
+<!--+
+    |end bottomstrip
+    +-->
+</div>
+</body>
+</html>

Added: zookeeper/branches/branch-3.5/docs/zookeeperReconfig.pdf
URL: http://svn.apache.org/viewvc/zookeeper/branches/branch-3.5/docs/zookeeperReconfig.pdf?rev=1621314&view=auto
==============================================================================
Binary file - no diff available.

Propchange: zookeeper/branches/branch-3.5/docs/zookeeperReconfig.pdf
------------------------------------------------------------------------------
    svn:mime-type = application/octet-stream

Modified: zookeeper/branches/branch-3.5/src/docs/src/documentation/content/xdocs/index.xml
URL: http://svn.apache.org/viewvc/zookeeper/branches/branch-3.5/src/docs/src/documentation/content/xdocs/index.xml?rev=1621314&r1=1621313&r2=1621314&view=diff
==============================================================================
--- zookeeper/branches/branch-3.5/src/docs/src/documentation/content/xdocs/index.xml (original)
+++ zookeeper/branches/branch-3.5/src/docs/src/documentation/content/xdocs/index.xml Fri Aug 29 14:36:46 2014
@@ -65,6 +65,7 @@
       <li><a href="zookeeperJMX.html">JMX</a> - how to enable JMX in ZooKeeper</li>
       <li><a href="zookeeperHierarchicalQuorums.html">Hierarchical quorums</a></li>
       <li><a href="zookeeperObservers.html">Observers</a> - non-voting ensemble members that easily improve ZooKeeper's scalability</li>
+      <li><a href="zookeeperReconfig.html">Dynamic Reconfiguration</a> - a guide on how to use dynamic reconfiguration in ZooKeeper</li>
       </ul>
       </li>
       

Modified: zookeeper/branches/branch-3.5/src/docs/src/documentation/content/xdocs/site.xml
URL: http://svn.apache.org/viewvc/zookeeper/branches/branch-3.5/src/docs/src/documentation/content/xdocs/site.xml?rev=1621314&r1=1621313&r2=1621314&view=diff
==============================================================================
--- zookeeper/branches/branch-3.5/src/docs/src/documentation/content/xdocs/site.xml (original)
+++ zookeeper/branches/branch-3.5/src/docs/src/documentation/content/xdocs/site.xml Fri Aug 29 14:36:46 2014
@@ -51,6 +51,7 @@ See http://forrest.apache.org/docs/linki
       <quota label="Quota Guide"            href="zookeeperQuotas.html" />
       <jmx   label="JMX"                    href="zookeeperJMX.html" />
       <observers label="Observers Guide" href="zookeeperObservers.html" />
+      <reconfig label="Dynamic Reconfiguration" href="zookeeperReconfig.html" />
   </docs>
   
   <docs label="Contributor">

Modified: zookeeper/branches/branch-3.5/src/docs/src/documentation/content/xdocs/zookeeperAdmin.xml
URL: http://svn.apache.org/viewvc/zookeeper/branches/branch-3.5/src/docs/src/documentation/content/xdocs/zookeeperAdmin.xml?rev=1621314&r1=1621313&r2=1621314&view=diff
==============================================================================
--- zookeeper/branches/branch-3.5/src/docs/src/documentation/content/xdocs/zookeeperAdmin.xml (original)
+++ zookeeper/branches/branch-3.5/src/docs/src/documentation/content/xdocs/zookeeperAdmin.xml Fri Aug 29 14:36:46 2014
@@ -590,6 +590,14 @@ server.3=zoo3:2888:3888</programlisting>
       must be taken to ensure that the list of servers in all of the different
       configuration files match.</para>
 
+      <note>
+        <para>In 3.5.0 and later, some of these parameters should be placed in
+         a dynamic configuration file. If they are placed in the static
+         configuration file, ZooKeeper will automatically move them over to the
+         dynamic configuration file. See <ulink url="zookeeperReconfig.html">
+         Dynamic Reconfiguration</ulink> for more information.</para>
+      </note>
+
       <section id="sc_minimumConfiguration">
         <title>Minimum Configuration</title>
 



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