Sticky Sessions and Session Replication

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I am evaluating the case of using sticky sessions with Session replication in tomcat. From my initial evaluation, I thought that if we enable Session replication, then session started in one tomcat node will be copied to all other tomcat nodes and thus we do not need sticky session to continue sessions and the request can be picked up by any node.

But it seems that session replication is in general used with sticky sessions, otherwise the session id needs to be changed whenever the request goes to some other node. ref: http://tomcat.apache.org/tomcat-6.0-doc/cluster-howto.html#Bind_session_after_crash_to_failover_node

Can anyone explain what is the real use of session replication if you have to enable sticky session? Because then you would be unnecessarily copying the session on each node, when the request with a given session id is always going to the same node. It could be beneficial in case of a node crashing, but then that does not happen frequently and using session replication only for that seems like an overkill.

session-replication session tomcat
3 Answers

Just to clarify configuration issues on JBoss 5.X in "all" base configuration with mod_jk. Setting sticky sessions in workers.properties file

 worker.list=loadbalancer

 ... nodes configuration omitted

 worker.loadbalancer.balance_workers=node1,node2
 worker.loadbalancer.sticky_session=True

does not prevent session replication. In order to switch off session replication on JBoss we need to set in $JBOSS_HOME\server\YOUR_NODE_NAME\deploy\cluster\jboss-cache-manager.sar\META-INF\jboss-cache-manager-jboss-beans.xml cacheMode parameter to LOCAL.

Usually in sticky session scenario we don't want session replication, since we do not want additional overhead connected with significant amount of I/O operations needed to replicate sessions.

In fact, if go with sticky sessions, we do not need to run JBoss in "all" configuration, we might use "default" or "standard" based configuration.

The only thing that has to be done is change in $JBOSS_HOME/server/YOUR_NODE_NAME/deploy/jbossweb.sar/server.xml:

<Engine name="jboss.web" defaultHost="localhost" jvmRoute="YOUR_NODE_NAME">
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