Hi All,

Here are the few basic steps that you need to follow in order to achieve the in-memory session replication between two or more Tomcat 6 instances.

This blog refers the Apache Tomcat documentation as found here: http://tomcat.apache.org/tomcat-6.0-doc/cluster-howto.html.
The Tomcat documentation provides more detailed explanation of the Clustering concepts as well as the definition of tags, attributes etc. used in server.xml file.

Step 1: Include the <distributable> tag in web.xml file i.e. you can simple write the following line in your deployment descriptor(i.e. web.xml):

<distributable />

Please read the following link to know more about this tag: http://wiki.metawerx.net/wiki/Web.xml.Distributable

Step 2: Add the following lines of XML in the server.xml file inside the <Engine> element/tag:

<Engine name="Catalina" defaultHost="localhost">
            <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>
	    	.
		.
		.
		.
	</Engine>

Please note that there might be some other elements such as <Host>, <Realm> etc. inside the <Engine> element. Also, for each tomcat instance the value defined for the ‘port’ attribute of the <Receiver> tag must be unique.

And, that’s all. You now have a basic session-replication in place. Your sessions are replicated amongst all the tomcat instances which are part of your cluster.

NOTE: Please make sure if you make any changes to the “context.xml” file, you have to delete the following xml file located in the <TOMCAT_HOME>/conf/Catalina/localhost/{yourAppName}.xml. If you don’t delete this file, then your changes will be ignored and the settings defined in this file will take effect.

Cheers!!!

Abhishek Tejpaul
abhishek@intelligrape.com
[Intelligrape Software Pvt. Ltd.]