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+<html><head><META http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><title>Apache Tomcat 6.0 (6.0.39) - Clustering/Session Replication HOW-TO</title><meta name="author" content="Filip Hanik"><meta name="author" content="Peter Rossbach"><style type="text/css" media="print">
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+ </style></head><body bgcolor="#ffffff" text="#000000" link="#525D76" alink="#525D76" vlink="#525D76"><table border="0" width="100%" cellspacing="0"><!--PAGE HEADER--><tr><td><!--PROJECT LOGO--><a href="http://tomcat.apache.org/"><img src="./images/tomcat.gif" align="right" alt="
+ The Apache Tomcat Servlet/JSP Container
+ " border="0"></a></td><td><h1><font face="arial,helvetica,sanserif">Apache Tomcat 6.0</font></h1><font face="arial,helvetica,sanserif">Version 6.0.39, Jan 27 2014</font></td><td><!--APACHE LOGO--><a href="http://www.apache.org/"><img src="./images/asf-logo.gif" align="right" alt="Apache Logo" border="0"></a></td></tr></table><table border="0" width="100%" cellspacing="4"><!--HEADER SEPARATOR--><tr><td colspan="2"><hr noshade="noshade" size="1"></td></tr><tr><!--LEFT SIDE NAVIGATION--><td width="20%" valign="top" nowrap="nowrap" class="noPrint"><p><strong>Links</strong></p><ul><li><a href="index.html">Docs Home</a></li><li><a href="http://wiki.apache.org/tomcat/FAQ">FAQ</a></li></ul><p><strong>User Guide</strong></p><ul><li><a href="introduction.html">1) Introduction</a></li><li><a href="setup.html">2) Setup</a></li><li><a href="appdev/index.html">3) First webapp</a></li><li><a href="deployer-howto.html">4) Deployer</a></li><li><a href="manager-howto.html">5) Manager</a></li><li><a href="realm-howto.html">6) Realms and AAA</a></li><li><a href="security-manager-howto.html">7) Security Manager</a></li><li><a href="jndi-resources-howto.html">8) JNDI Resources</a></li><li><a href="jndi-datasource-examples-howto.html">9) JDBC DataSources</a></li><li><a href="class-loader-howto.html">10) Classloading</a></li><li><a href="jasper-howto.html">11) JSPs</a></li><li><a href="ssl-howto.html">12) SSL</a></li><li><a href="ssi-howto.html">13) SSI</a></li><li><a href="cgi-howto.html">14) CGI</a></li><li><a href="proxy-howto.html">15) Proxy Support</a></li><li><a href="mbeans-descriptor-howto.html">16) MBean Descriptor</a></li><li><a href="default-servlet.html">17) Default Servlet</a></li><li><a href="cluster-howto.html">18) Clustering</a></li><li><a href="balancer-howto.html">19) Load Balancer</a></li><li><a href="connectors.html">20) Connectors</a></li><li><a href="monitoring.html">21) Monitoring and Management</a></li><li><a href="logging.html">22) Logging</a></li><li><a href="apr.html">23) APR/Native</a></li><li><a href="virtual-hosting-howto.html">24) Virtual Hosting</a></li><li><a href="aio.html">25) Advanced IO</a></li><li><a href="extras.html">26) Additional Components</a></li><li><a href="maven-jars.html">27) Mavenized</a></li></ul><p><strong>Reference</strong></p><ul><li><a href="RELEASE-NOTES.txt">Release Notes</a></li><li><a href="config/index.html">Configuration</a></li><li><a href="api/index.html">Javadocs</a></li><li><a href="http://tomcat.apache.org/connectors-doc/">JK 1.2 Documentation</a></li></ul><p><strong>Apache Tomcat Development</strong></p><ul><li><a href="building.html">Building</a></li><li><a href="changelog.html">Changelog</a></li><li><a href="http://wiki.apache.org/tomcat/TomcatVersions">Status</a></li><li><a href="developers.html">Developers</a></li><li><a href="architecture/index.html">Architecture</a></li><li><a href="funcspecs/index.html">Functional Specs.</a></li></ul></td><!--RIGHT SIDE MAIN BODY--><td width="80%" valign="top" align="left" id="mainBody"><h1>Apache Tomcat 6.0</h1><h2>Clustering/Session Replication HOW-TO</h2><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><font color="#ffffff" face="arial,helvetica.sanserif"><a name="Important Note"><!--()--></a><a name="Important_Note"><strong>Important Note</strong></a></font></td></tr><tr><td><blockquote>
+<p><b>You can also check the <a href="config/cluster.html">configuration reference documentation.</a></b>
+</p>
+</blockquote></td></tr></table><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><font color="#ffffff" face="arial,helvetica.sanserif"><a name="Table of Contents"><!--()--></a><a name="Table_of_Contents"><strong>Table of Contents</strong></a></font></td></tr><tr><td><blockquote>
+<ul><li><a href="#For_the_impatient">For the impatient</a></li><li><a href="#Cluster_Basics">Cluster Basics</a></li><li><a href="#Overview">Overview</a></li><li><a href="#Cluster_Information">Cluster Information</a></li><li><a href="#Bind_session_after_crash_to_failover_node">Bind session after crash to failover node</a></li><li><a href="#Configuration_Example">Configuration Example</a></li><li><a href="#Cluster_Architecture">Cluster Architecture</a></li><li><a href="#How_it_Works">How it Works</a></li><li><a href="#FAQ">FAQ</a></li></ul>
+</blockquote></td></tr></table><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><font color="#ffffff" face="arial,helvetica.sanserif"><a name="For the impatient"><!--()--></a><a name="For_the_impatient"><strong>For the impatient</strong></a></font></td></tr><tr><td><blockquote>
+ <p>
+ Simply add <div align="left"><table cellspacing="4" cellpadding="0" border="0"><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#ffffff" height="1"><pre><Cluster className="org.apache.catalina.ha.tcp.SimpleTcpCluster"/></pre></td><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr></table></div>
+ to your <code><Engine></code> or your <code><Host></code> element to enable clustering.
+ </p>
+ <p>
+ Using the above configuration will enable all-to-all session replication
+ using the <code>DeltaManager</code> to replicate session deltas. By all-to-all we mean that the session gets replicated to all the other
+ nodes in the cluster. This works great for smaller cluster but we don't recommend it for larger clusters(a lot of tomcat nodes).
+ Also when using the delta manager it will replicate to all nodes, even nodes that don't have the application deployed.<br>
+ To get around this problem, you'll want to use the BackupManager. This manager only replicates the session data to one backup
+ node, and only to nodes that have the application deployed. Downside of the BackupManager: not quite as battle tested as the delta manager.
+ <br>
+ Here are some of the important default values:<br>
+ 1. Multicast address is 228.0.0.4<br>
+ 2. Multicast port is 45564 (the port and the address together determine cluster membership.<br>
+ 3. The IP broadcasted is <code>java.net.InetAddress.getLocalHost().getHostAddress()</code> (make sure you don't broadcast 127.0.0.1, this is a common error)<br>
+ 4. The TCP port listening for replication messages is the first available server socket in range <code>4000-4100</code><br>
+ 5. Two listeners are configured <code>ClusterSessionListener</code> and <code>JvmRouteSessionIDBinderListener</code><br>
+ 6. Two interceptors are configured <code>TcpFailureDetector</code> and <code>MessageDispatch15Interceptor</code><br>
+ The following is the default cluster configuration:<br>
+ <div align="left"><table cellspacing="4" cellpadding="0" border="0"><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#ffffff" height="1"><pre>
+ <Cluster className="org.apache.catalina.ha.tcp.SimpleTcpCluster"
+ channelSendOptions="8">
+
+ <Manager className="org.apache.catalina.ha.session.DeltaManager"
+ expireSessionsOnShutdown="false"
+ notifyListenersOnReplication="true"/>
+
+ <Channel className="org.apache.catalina.tribes.group.GroupChannel">
+ <Membership className="org.apache.catalina.tribes.membership.McastService"
+ address="228.0.0.4"
+ port="45564"
+ frequency="500"
+ dropTime="3000"/>
+ <Receiver className="org.apache.catalina.tribes.transport.nio.NioReceiver"
+ address="auto"
+ port="4000"
+ autoBind="100"
+ selectorTimeout="5000"
+ maxThreads="6"/>
+
+ <Sender className="org.apache.catalina.tribes.transport.ReplicationTransmitter">
+ <Transport className="org.apache.catalina.tribes.transport.nio.PooledParallelSender"/>
+ </Sender>
+ <Interceptor className="org.apache.catalina.tribes.group.interceptors.TcpFailureDetector"/>
+ <Interceptor className="org.apache.catalina.tribes.group.interceptors.MessageDispatch15Interceptor"/>
+ </Channel>
+
+ <Valve className="org.apache.catalina.ha.tcp.ReplicationValve"
+ filter=""/>
+ <Valve className="org.apache.catalina.ha.session.JvmRouteBinderValve"/>
+
+ <Deployer className="org.apache.catalina.ha.deploy.FarmWarDeployer"
+ tempDir="/tmp/war-temp/"
+ deployDir="/tmp/war-deploy/"
+ watchDir="/tmp/war-listen/"
+ watchEnabled="false"/>
+
+ <ClusterListener className="org.apache.catalina.ha.session.JvmRouteSessionIDBinderListener"/>
+ <ClusterListener className="org.apache.catalina.ha.session.ClusterSessionListener"/>
+ </Cluster>
+ </pre></td><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr></table></div>
+ </p>
+ <p>Will cover this section in more detail later in this document.</p>
+</blockquote></td></tr></table><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><font color="#ffffff" face="arial,helvetica.sanserif"><a name="Cluster Basics"><!--()--></a><a name="Cluster_Basics"><strong>Cluster Basics</strong></a></font></td></tr><tr><td><blockquote>
+
+<p>To run session replication in your Tomcat 6.0 container, the following steps
+should be completed:</p>
+<ul>
+ <li>All your session attributes must implement <code>java.io.Serializable</code></li>
+ <li>Uncomment the <code>Cluster</code> element in server.xml</li>
+ <li>If you have defined custom cluster valves, make sure you have the <code>ReplicationValve</code> defined as well under the Cluster element in server.xml</li>
+ <li>If your Tomcat instances are running on the same machine, make sure the <code>tcpListenPort</code>
+ attribute is unique for each instance, in most cases Tomcat is smart enough to resolve this on it's own by autodetecting available ports in the range 4000-4100</li>
+ <li>Make sure your <code>web.xml</code> has the
+ <code><distributable/></code> element</li>
+ <li>If you are using mod_jk, make sure that jvmRoute attribute is set at your Engine <code><Engine name="Catalina" jvmRoute="node01" ></code>
+ and that the jvmRoute attribute value matches your worker name in workers.properties</li>
+ <li>Make sure that all nodes have the same time and sync with NTP service!</li>
+ <li>Make sure that your loadbalancer is configured for sticky session mode.</li>
+</ul>
+<p>Load balancing can be achieved through many techniques, as seen in the
+<a href="balancer-howto.html">Load Balancing</a> chapter.</p>
+<p>Note: Remember that your session state is tracked by a cookie, so your URL must look the same from the out
+ side otherwise, a new session will be created.</p>
+<p>Note: Clustering support currently requires the JDK version 1.5 or later.</p>
+<p>The Cluster module uses the Tomcat JULI logging framework, so you can configure logging
+ through the regular logging.properties file. To track messages, you can enable logging on the key:<code>org.apache.catalina.tribes.MESSAGES</code></p>
+</blockquote></td></tr></table><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><font color="#ffffff" face="arial,helvetica.sanserif"><a name="Overview"><strong>Overview</strong></a></font></td></tr><tr><td><blockquote>
+
+<p>To enable session replication in Tomcat, three different paths can be followed to achieve the exact same thing:</p>
+<ol>
+ <li>Using session persistence, and saving the session to a shared file system (PersistenceManager + FileStore)</li>
+ <li>Using session persistence, and saving the session to a shared database (PersistenceManager + JDBCStore)</li>
+ <li>Using in-memory-replication, using the SimpleTcpCluster that ships with Tomcat 6 (lib/catalina-tribes.jar + lib/catalina-ha.jar)</li>
+</ol>
+
+<p>In this release of session replication, Tomcat can perform an all-to-all replication of session state using the <code>DeltaManager</code> or
+ perform backup replication to only one node using the <code>BackupManager</code>.
+ The all-to-all replication is an algorithm that is only efficient when the clusters are small. For larger clusters, to use
+ a primary-secondary session replication where the session will only be stored at one backup server simply setup the BackupManager. <br>
+ Currently you can use the domain worker attribute (mod_jk > 1.2.8) to build cluster partitions
+ with the potential of having a more scaleable cluster solution with the DeltaManager(you'll need to configure the domain interceptor for this).
+ In order to keep the network traffic down in an all-to-all environment, you can split your cluster
+ into smaller groups. This can be easily achieved by using different multicast addresses for the different groups.
+ A very simple setup would look like this:
+ </p>
+
+<div align="left"><table cellspacing="4" cellpadding="0" border="0"><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#ffffff" height="1"><pre>
+ DNS Round Robin
+ |
+ Load Balancer
+ / \
+ Cluster1 Cluster2
+ / \ / \
+ Tomcat1 Tomcat2 Tomcat3 Tomcat4
+</pre></td><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr></table></div>
+
+<p>What is important to mention here, is that session replication is only the beginning of clustering.
+ Another popular concept used to implement clusters is farming, i.e., you deploy your apps only to one
+ server, and the cluster will distribute the deployments across the entire cluster.
+ This is all capabilities that can go into with the FarmWarDeployer (s. cluster example at <code>server.xml</code>)</p>
+<p>In the next section will go deeper into how session replication works and how to configure it.</p>
+
+</blockquote></td></tr></table><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><font color="#ffffff" face="arial,helvetica.sanserif"><a name="Cluster Information"><!--()--></a><a name="Cluster_Information"><strong>Cluster Information</strong></a></font></td></tr><tr><td><blockquote>
+<p>Membership is established using multicast heartbeats.
+ Hence, if you wish to subdivide your clusters, you can do this by
+ changing the multicast IP address or port in the <code><Membership></code> element.
+</p>
+<p>
+ The heartbeat contains the IP address of the Tomcat node and the TCP port that
+ Tomcat listens to for replication traffic. All data communication happens over TCP.
+</p>
+<p>
+ The <code>ReplicationValve</code> is used to find out when the request has been completed and initiate the
+ replication, if any. Data is only replicated if the session has changed (by calling setAttribute or removeAttribute
+ on the session).
+</p>
+<p>
+ One of the most important performance considerations is the synchronous versus asynchronous replication.
+ In a synchronous replication mode the request doesn't return until the replicated session has been
+ sent over the wire and reinstantiated on all the other cluster nodes.
+ Synchronous vs. asynchronous is configured using the <code>channelSendOptions</code>
+ flag and is an integer value. The default value for the <code>SimpleTcpCluster/DeltaManager</code> combo is
+ 8, which is asynchronous. You can read more on the <a href="tribes/introduction.html">send flag(overview)</a> or the
+ <a href="http://tomcat.apache.org/tomcat-6.0-doc/api/org/apache/catalina/tribes/Channel.html">send flag(javadoc)</a>.
+ During async replication, the request is returned before the data has been replicated. async replication yields shorter
+ request times, and synchronous replication guarantees the session to be replicated before the request returns.
+</p>
+</blockquote></td></tr></table><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><font color="#ffffff" face="arial,helvetica.sanserif"><a name="Bind session after crash to failover node"><!--()--></a><a name="Bind_session_after_crash_to_failover_node"><strong>Bind session after crash to failover node</strong></a></font></td></tr><tr><td><blockquote>
+<p>
+ If you are using mod_jk and not using sticky sessions or for some reasons sticky session don't
+ work, or you are simply failing over, the session id will need to be modified as it previously contained
+ the worker id of the previous tomcat (as defined by jvmRoute in the Engine element).
+ To solve this, we will use the JvmRouteBinderValve.
+</p>
+<p>
+ The JvmRouteBinderValve rewrites the session id to ensure that the next request will remain sticky
+ (and not fall back to go to random nodes since the worker is no longer available) after a fail over.
+ The valve rewrites the JSESSIONID value in the cookie with the same name.
+ Not having this valve in place, will make it harder to ensure stickyness in case of a failure for the mod_jk module.
+</p>
+<p>
+ By default, if no valves are configured, the JvmRouteBinderValve is added on.
+ The cluster message listener called JvmRouteSessionIDBinderListener is also defined by default and is used to actually rewrite the
+ session id on the other nodes in the cluster once a fail over has occurred.
+ Remember, if you are adding your own valves or cluster listeners in server.xml then the defaults are no longer valid,
+ make sure that you add in all the appropriate valves and listeners as defined by the default.
+</p>
+<p>
+ <b>Hint:</b><br>
+ With attribute <i>sessionIdAttribute</i> you can change the request attribute name that included the old session id.
+ Default attribute name is <i>org.apache.catalina.cluster.session.JvmRouteOrignalSessionID</i>.
+</p>
+<p>
+ <b>Trick:</b><br>
+ You can enable this mod_jk turnover mode via JMX before you drop a node to all backup nodes!
+ Set enable true on all JvmRouteBinderValve backups, disable worker at mod_jk
+ and then drop node and restart it! Then enable mod_jk Worker and disable JvmRouteBinderValves again.
+ This use case means that only requested session are migrated.
+</p>
+
+
+
+</blockquote></td></tr></table><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><font color="#ffffff" face="arial,helvetica.sanserif"><a name="Configuration Example"><!--()--></a><a name="Configuration_Example"><strong>Configuration Example</strong></a></font></td></tr><tr><td><blockquote>
+ <div align="left"><table cellspacing="4" cellpadding="0" border="0"><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#ffffff" height="1"><pre>
+ <Cluster className="org.apache.catalina.ha.tcp.SimpleTcpCluster"
+ channelSendOptions="6">
+
+ <Manager className="org.apache.catalina.ha.session.BackupManager"
+ expireSessionsOnShutdown="false"
+ notifyListenersOnReplication="true"
+ mapSendOptions="6"/>
+ <!--
+ <Manager className="org.apache.catalina.ha.session.DeltaManager"
+ expireSessionsOnShutdown="false"
+ notifyListenersOnReplication="true"/>
+ -->
+ <Channel className="org.apache.catalina.tribes.group.GroupChannel">
+ <Membership className="org.apache.catalina.tribes.membership.McastService"
+ address="228.0.0.4"
+ port="45564"
+ frequency="500"
+ dropTime="3000"/>
+ <Receiver className="org.apache.catalina.tribes.transport.nio.NioReceiver"
+ address="auto"
+ port="5000"
+ selectorTimeout="100"
+ maxThreads="6"/>
+
+ <Sender className="org.apache.catalina.tribes.transport.ReplicationTransmitter">
+ <Transport className="org.apache.catalina.tribes.transport.nio.PooledParallelSender"/>
+ </Sender>
+ <Interceptor className="org.apache.catalina.tribes.group.interceptors.TcpFailureDetector"/>
+ <Interceptor className="org.apache.catalina.tribes.group.interceptors.MessageDispatch15Interceptor"/>
+ <Interceptor className="org.apache.catalina.tribes.group.interceptors.ThroughputInterceptor"/>
+ </Channel>
+
+ <Valve className="org.apache.catalina.ha.tcp.ReplicationValve"
+ filter=".*\.gif;.*\.js;.*\.jpg;.*\.png;.*\.htm;.*\.html;.*\.css;.*\.txt;"/>
+
+ <Deployer className="org.apache.catalina.ha.deploy.FarmWarDeployer"
+ tempDir="/tmp/war-temp/"
+ deployDir="/tmp/war-deploy/"
+ watchDir="/tmp/war-listen/"
+ watchEnabled="false"/>
+
+ <ClusterListener className="org.apache.catalina.ha.session.ClusterSessionListener"/>
+ </Cluster>
+ </pre></td><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr></table></div>
+ <p>
+ Break it down!!
+ </p>
+ <div align="left"><table cellspacing="4" cellpadding="0" border="0"><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#ffffff" height="1"><pre>
+ <Cluster className="org.apache.catalina.ha.tcp.SimpleTcpCluster"
+ channelSendOptions="6">
+ </pre></td><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr></table></div>
+ <p>
+ The main element, inside this element all cluster details can be configured.
+ The <code>channelSendOptions</code> is the flag that is attached to each message sent by the
+ SimpleTcpCluster class or any objects that are invoking the SimpleTcpCluster.send method.
+ The description of the send flags is available at <a href="http://tomcat.apache.org/tomcat-6.0-doc/api/org/apache/catalina/tribes/Channel.html">
+ our javadoc site</a>
+ The <code>DeltaManager</code> sends information using the SimpleTcpCluster.send method, while the backup manager
+ sends it itself directly through the channel.
+ <br>For more info, Please visit the <a href="config/cluster.html">reference documentation</a>
+ </p>
+ <div align="left"><table cellspacing="4" cellpadding="0" border="0"><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#ffffff" height="1"><pre>
+ <Manager className="org.apache.catalina.ha.session.BackupManager"
+ expireSessionsOnShutdown="false"
+ notifyListenersOnReplication="true"
+ mapSendOptions="6"/>
+ <!--
+ <Manager className="org.apache.catalina.ha.session.DeltaManager"
+ expireSessionsOnShutdown="false"
+ notifyListenersOnReplication="true"/>
+ -->
+ </pre></td><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr></table></div>
+ <p>
+ This is a template for the manager configuration that will be used if no manager is defined in the <Context>
+ element. In Tomcat 5.x each webapp marked distributable had to use the same manager, this is no longer the case
+ since Tomcat 6 you can define a manager class for each webapp, so that you can mix managers in your cluster.
+ Obviously the managers on one node's application has to correspond with the same manager on the same application on the other node.
+ If no manager has been specified for the webapp, and the webapp is marked <distributable/> Tomcat will take this manager configuration
+ and create a manager instance cloning this configuration.
+ <br>For more info, Please visit the <a href="config/cluster-manager.html">reference documentation</a>
+ </p>
+ <div align="left"><table cellspacing="4" cellpadding="0" border="0"><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#ffffff" height="1"><pre>
+ <Channel className="org.apache.catalina.tribes.group.GroupChannel">
+ </pre></td><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr></table></div>
+ <p>
+ The channel element is <a href="tribes/introduction.html">Tribes</a>, the group communication framework
+ used inside Tomcat. This element encapsulates everything that has to do with communication and membership logic.
+ <br>For more info, Please visit the <a href="config/cluster-channel.html">reference documentation</a>
+ </p>
+ <div align="left"><table cellspacing="4" cellpadding="0" border="0"><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#ffffff" height="1"><pre>
+ <Membership className="org.apache.catalina.tribes.membership.McastService"
+ address="228.0.0.4"
+ port="45564"
+ frequency="500"
+ dropTime="3000"/>
+ </pre></td><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr></table></div>
+ <p>
+ Membership is done using multicasting. Please note that Tribes also supports static memberships using the
+ <code>StaticMembershipInterceptor</code> if you want to extend your membership to points beyond multicasting.
+ The address attribute is the multicast address used and the port is the multicast port. These two together
+ create the cluster separation. If you want a QA cluster and a production cluster, the easiest config is to
+ have the QA cluster be on a separate multicast address/port combination the the production cluster.<br>
+ The membership component broadcasts TCP adress/port of itselt to the other nodes so that communication between
+ nodes can be done over TCP. Please note that the address being broadcasted is the one of the
+ <code>Receiver.address</code> attribute.
+ <br>For more info, Please visit the <a href="config/cluster-membership.html">reference documentation</a>
+ </p>
+ <div align="left"><table cellspacing="4" cellpadding="0" border="0"><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#ffffff" height="1"><pre>
+ <Receiver className="org.apache.catalina.tribes.transport.nio.NioReceiver"
+ address="auto"
+ port="5000"
+ selectorTimeout="100"
+ maxThreads="6"/>
+ </pre></td><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr></table></div>
+ <p>
+ In tribes the logic of sending and receiving data has been broken into two functional components. The Receiver, as the name suggests
+ is responsible for receiving messages. Since the Tribes stack is thread less, (a popular improvement now adopted by other frameworks as well),
+ there is a thread pool in this component that has a maxThreads and minThreads setting.<br>
+ The address attribute is the host address that will be broadcasted by the membership component to the other nodes.
+ <br>For more info, Please visit the <a href="config/cluster-receiver.html">reference documentation</a>
+ </p>
+ <div align="left"><table cellspacing="4" cellpadding="0" border="0"><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#ffffff" height="1"><pre>
+
+ <Sender className="org.apache.catalina.tribes.transport.ReplicationTransmitter">
+ <Transport className="org.apache.catalina.tribes.transport.nio.PooledParallelSender"/>
+ </Sender>
+ </pre></td><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr></table></div>
+ <p>
+ The sender component, as the name indicates is responsible for sending messages to other nodes.
+ The sender has a shell component, the <code>ReplicationTransmitter</code> but the real stuff done is done in the
+ sub component, <code>Transport</code>.
+ Tribes support having a pool of senders, so that messages can be sent in parallel and if using the NIO sender,
+ you can send messages concurrently as well.<br>
+ Concurrently means one message to multiple senders at the same time and Parallel means multiple messages to multiple senders
+ at the same time.
+ <br>For more info, Please visit the <a href="config/cluster-sender.html">reference documentation</a>
+ </p>
+ <div align="left"><table cellspacing="4" cellpadding="0" border="0"><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#ffffff" height="1"><pre>
+ <Interceptor className="org.apache.catalina.tribes.group.interceptors.TcpFailureDetector"/>
+ <Interceptor className="org.apache.catalina.tribes.group.interceptors.MessageDispatch15Interceptor"/>
+ <Interceptor className="org.apache.catalina.tribes.group.interceptors.ThroughputInterceptor"/>
+ </Channel>
+ </pre></td><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr></table></div>
+ <p>
+ Tribes uses a stack to send messages through. Each element in the stack is called an interceptor, and works much like the valves do
+ in the Tomcat servlet container.
+ Using interceptors, logic can be broken into more managable pieces of code. The interceptors configured above are:<br>
+ TcpFailureDetector - verifies crashed members through TCP, if multicast packets get dropped, this interceptor protects against false positives,
+ ie the node marked as crashed even though it still is alive and running.<br>
+ MessageDispatch15Interceptor - dispatches messages to a thread (thread pool) to send message asynchrously.<br>
+ ThroughputInterceptor - prints out simple stats on message traffic.<br>
+ Please note that the order of interceptors is important. the way they are defined in server.xml is the way they are represented in the
+ channel stack. Think of it as a linked list, with the head being the first most interceptor and the tail the last.
+ <br>For more info, Please visit the <a href="config/cluster-interceptor.html">reference documentation</a>
+ </p>
+ <div align="left"><table cellspacing="4" cellpadding="0" border="0"><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#ffffff" height="1"><pre>
+ <Valve className="org.apache.catalina.ha.tcp.ReplicationValve"
+ filter=".*\.gif;.*\.js;.*\.jpg;.*\.png;.*\.htm;.*\.html;.*\.css;.*\.txt;"/>
+ </pre></td><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr></table></div>
+ <p>
+ The cluster uses valves to track requests to web applications, we've mentioned the ReplicationValve and the JvmRouteBinderValve above.
+ The <Cluster> element itself is not part of the pipeline in Tomcat, instead the cluster adds the valve to its parent container.
+ If the <Cluster> elements is configured in the <Engine> element, the valves get added to the engine and so on.
+ <br>For more info, Please visit the <a href="config/cluster-valve.html">reference documentation</a>
+ </p>
+ <div align="left"><table cellspacing="4" cellpadding="0" border="0"><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#ffffff" height="1"><pre>
+ <Deployer className="org.apache.catalina.ha.deploy.FarmWarDeployer"
+ tempDir="/tmp/war-temp/"
+ deployDir="/tmp/war-deploy/"
+ watchDir="/tmp/war-listen/"
+ watchEnabled="false"/>
+ </pre></td><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr></table></div>
+ <p>
+ The default tomcat cluster supports farmed deployment, ie, the cluster can deploy and undeploy applications on the other nodes.
+ The state of this component is currently in flux but will be addressed soon. There was a change in the deployment algorithm
+ between Tomcat 5.0 and 5.5 and at that point, the logic of this component changed to where the deploy dir has to match the
+ webapps directory.
+ <br>For more info, Please visit the <a href="config/cluster-deployer.html">reference documentation</a>
+ </p>
+ <div align="left"><table cellspacing="4" cellpadding="0" border="0"><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#ffffff" height="1"><pre>
+ <ClusterListener className="org.apache.catalina.ha.session.ClusterSessionListener"/>
+ </Cluster>
+ </pre></td><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr></table></div>
+ <p>
+ Since the SimpleTcpCluster itself is a sender and receiver of the Channel object, components can register themselves as listeners to
+ the SimpleTcpCluster. The listener above <code>ClusterSessionListener</code> listens for DeltaManager replication messages
+ and applies the deltas to the manager that in turn applies it to the session.
+ <br>For more info, Please visit the <a href="config/cluster-listener.html">reference documentation</a>
+ </p>
+
+</blockquote></td></tr></table><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><font color="#ffffff" face="arial,helvetica.sanserif"><a name="Cluster Architecture"><!--()--></a><a name="Cluster_Architecture"><strong>Cluster Architecture</strong></a></font></td></tr><tr><td><blockquote>
+
+<p><b>Component Levels:</b>
+<div align="left"><table cellspacing="4" cellpadding="0" border="0"><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#ffffff" height="1"><pre>
+ Server
+ |
+ Service
+ |
+ Engine
+ | \
+ | --- Cluster --*
+ |
+ Host
+ |
+ ------
+ / \
+ Cluster Context(1-N)
+ | \
+ | -- Manager
+ | \
+ | -- DeltaManager
+ | -- BackupManager
+ |
+ ---------------------------
+ | \
+ Channel \
+ ----------------------------- \
+ | \
+ Interceptor_1 .. \
+ | \
+ Interceptor_N \
+ ----------------------------- \
+ | | | \
+ Receiver Sender Membership \
+ -- Valve
+ | \
+ | -- ReplicationValve
+ | -- JvmRouteBinderValve
+ |
+ -- LifecycleListener
+ |
+ -- ClusterListener
+ | \
+ | -- ClusterSessionListener
+ | -- JvmRouteSessionIDBinderListener
+ |
+ -- Deployer
+ \
+ -- FarmWarDeployer
+
+
+</pre></td><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr></table></div>
+</p>
+
+</blockquote></td></tr></table><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><font color="#ffffff" face="arial,helvetica.sanserif"><a name="How it Works"><!--()--></a><a name="How_it_Works"><strong>How it Works</strong></a></font></td></tr><tr><td><blockquote>
+<p>To make it easy to understand how clustering works, We are gonna take you through a series of scenarios.
+ In the scenario we only plan to use two tomcat instances <code>TomcatA</code> and <code>TomcatB</code>.
+ We will cover the following sequence of events:</p>
+
+<ol>
+<li><code>TomcatA</code> starts up</li>
+<li><code>TomcatB</code> starts up (Wait that TomcatA start is complete)</li>
+<li><code>TomcatA</code> receives a request, a session <code>S1</code> is created.</li>
+<li><code>TomcatA</code> crashes</li>
+<li><code>TomcatB</code> receives a request for session <code>S1</code></li>
+<li><code>TomcatA</code> starts up</li>
+<li><code>TomcatA</code> receives a request, invalidate is called on the session (<code>S1</code>)</li>
+<li><code>TomcatB</code> receives a request, for a new session (<code>S2</code>)</li>
+<li><code>TomcatA</code> The session <code>S2</code> expires due to inactivity.</li>
+</ol>
+
+<p>Ok, now that we have a good sequence, we will take you through exactly what happens in the session repliction code</p>
+
+<ol>
+<li><b><code>TomcatA</code> starts up</b>
+ <p>
+ Tomcat starts up using the standard start up sequence. When the Host object is created, a cluster object is associated with it.
+ When the contexts are parsed, if the distributable element is in place in web.xml
+ Tomcat asks the Cluster class (in this case <code>SimpleTcpCluster</code>) to create a manager
+ for the replicated context. So with clustering enabled, distributable set in web.xml
+ Tomcat will create a <code>DeltaManager</code> for that context instead of a <code>StandardManager</code>.
+ The cluster class will start up a membership service (multicast) and a replication service (tcp unicast).
+ More on the architecture further down in this document.
+ </p><p></p>
+</li>
+<li><b><code>TomcatB</code> starts up</b>
+ <p>
+ When TomcatB starts up, it follows the same sequence as TomcatA did with one exception.
+ The cluster is started and will establish a membership (TomcatA,TomcatB).
+ TomcatB will now request the session state from a server that already exists in the cluster,
+ in this case TomcatA. TomcatA responds to the request, and before TomcatB starts listening
+ for HTTP requests, the state has been transferred from TomcatA to TomcatB.
+ In case TomcatA doesn't respond, TomcatB will time out after 60 seconds, and issue a log
+ entry. The session state gets transferred for each web application that has distributable in
+ its web.xml. Note: To use session replication efficiently, all your tomcat instances should be
+ configured the same.
+ </p><p></p>
+</li>
+<li><B><code>TomcatA</code> receives a request, a session <code>S1</code> is created.</B>
+ <p>
+ The request coming in to TomcatA is treated exactly the same way as without session replication.
+ The action happens when the request is completed, the <code>ReplicationValve</code> will intercept
+ the request before the response is returned to the user.
+ At this point it finds that the session has been modified, and it uses TCP to replicata the
+ session to TomcatB. Once the serialized data has been handed off to the operating systems TCP logic,
+ the request returns to the user, back through the valve pipeline.
+ For each request the entire session is replicated, this allows code that modifies attributes
+ in the session without calling setAttribute or removeAttribute to be replicated.
+ a useDirtyFlag configuration parameter can be used to optimize the number of times
+ a session is replicated.
+ </p><p></p>
+
+</li>
+<li><b><code>TomcatA</code> crashes</b>
+ <p>
+ When TomcatA crashes, TomcatB receives a notification that TomcatA has dropped out
+ of the cluster. TomcatB removes TomcatA from its membership list, and TomcatA will no longer
+ be notified of any changes that occurs in TomcatB.
+ The load balancer will redirect the requests from TomcatA to TomcatB and all the sessions
+ are current.
+ </p><p></p>
+</li>
+<li><b><code>TomcatB</code> receives a request for session <code>S1</code></b>
+ <p>Nothing exciting, TomcatB will process the request as any other request.
+ </p><p></p>
+</li>
+<li><b><code>TomcatA</code> starts up</b>
+ <p>Upon start up, before TomcatA starts taking new request and making itself
+ available to it will follow the start up sequence described above 1) 2).
+ It will join the cluster, contact TomcatB for the current state of all the sessions.
+ And once it receives the session state, it finishes loading and opens its HTTP/mod_jk ports.
+ So no requests will make it to TomcatA until it has received the session state from TomcatB.
+ </p><p></p>
+</li>
+<li><b><code>TomcatA</code> receives a request, invalidate is called on the session (<code>S1</code>)</b>
+ <p>The invalidate is call is intercepted, and the session is queued with invalidated sessions.
+ When the request is complete, instead of sending out the session that has changed, it sends out
+ an "expire" message to TomcatB and TomcatB will invalidate the session as well.
+ </p><p></p>
+
+</li>
+<li><b><code>TomcatB</code> receives a request, for a new session (<code>S2</code>)</b>
+ <p>Same scenario as in step 3)
+ </p><p></p>
+
+
+</li>
+<li><code>TomcatA</code> The session <code>S2</code> expires due to inactivity.
+ <p>The invalidate is call is intercepted the same was as when a session is invalidated by the user,
+ and the session is queued with invalidated sessions.
+ At this point, the invalidet session will not be replicated across until
+ another request comes through the system and checks the invalid queue.
+ </p><p></p>
+</li>
+</ol>
+
+<p>Phuuuhh! :)</p>
+
+<p><b>Membership</b>
+ Clustering membership is established using very simple multicast pings.
+ Each Tomcat instance will periodically send out a multicast ping,
+ in the ping message the instance will broad cast its IP and TCP listen port
+ for replication.
+ If an instance has not received such a ping within a given timeframe, the
+ member is considered dead. Very simple, and very effective!
+ Of course, you need to enable multicasting on your system.
+</p>
+
+<p><b>TCP Replication</b>
+ Once a multicast ping has been received, the member is added to the cluster
+ Upon the next replication request, the sending instance will use the host and
+ port info and establish a TCP socket. Using this socket it sends over the serialized data.
+ The reason I choose TCP sockets is because it has built in flow control and guaranteed delivery.
+ So I know, when I send some data, it will make it there :)
+</p>
+
+<p><b>Distributed locking and pages using frames</b>
+ Tomcat does not keep session instances in sync across the cluster.
+ The implementation of such logic would be to much overhead and cause all
+ kinds of problems. If your client accesses the same session
+ simultanously using multiple requests, then the last request
+ will override the other sessions in the cluster.
+</p>
+
+</blockquote></td></tr></table><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><font color="#ffffff" face="arial,helvetica.sanserif"><a name="FAQ"><strong>FAQ</strong></a></font></td></tr><tr><td><blockquote>
+<p>Please see <a href="http://wiki.apache.org/tomcat/FAQ/Clustering">the clustering section of the FAQ</a>.</p>
+</blockquote></td></tr></table></td></tr><!--FOOTER SEPARATOR--><tr><td colspan="2"><hr noshade="noshade" size="1"></td></tr><!--PAGE FOOTER--><tr><td colspan="2"><div align="center"><font color="#525D76" size="-1"><em>
+ Copyright © 1999-2014, Apache Software Foundation
+ </em></font></div></td></tr></table></body></html>
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