Enterprise Java Community: Clustering JSR-168 Portlet Applications in Tomcat: "March 2006
(By John A. Lewis)
JSR-168 Portlet applications represent a special challenge when it comes to clustering within Tomcat (or any other servlet container, for that matter). In order to effectively cluster web applications, session data must be replicated or shared between the nodes in the cluster. Otherwise, the user experiences a complete loss of context during a node failover. While Tomcat has provided session replication for quite some time, it has not supported replication of session changes resulting from a cross-context call from one webapp to another."