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.]