Clustering JSR-168 Portlet Applications in Tomcat

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.

Portlets that are deployed as separate webapps from the portal webapp 
must be accessed by cross-context calls from the portal. This 
cross-context access creates a number of issues: servlet filters are not 
applied, session sharing between servlets and portlets is difficult, and 
session replication in Tomcat did not work.

We've recently worked with the Tomcat developers to get Cross-Context 
Session Replication built into Tomcat 5.5 and we've demonstrated that 
portlet session data can now be properly replicated. I'd like to give a 
special thanks to Peter Rossbach from the Tomcat team for working on 
this with us and for getting this done so quickly. The changes for 
Tomcat are currently in the latest development codebase and will be 
included in version 5.5.16 and later.

To document how all this is done, I've published a Wiki article that 
describes how to use Tomcat 5.5, mod_jk 1.2, Apache 2.0, and Pluto 1.0.1 
to construct a functioning Tomcat cluster that will properly replicate 
portlet session information:

http://www.ja-sig.org/wiki/display/PLT/Clustering+JSR-168+Portlet+Applications+in+Tomcat

John A. Lewis
Unicon, Inc.