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 an article in the
uPortal Wiki site 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.
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