Does uPortal support clustering?

No, uPortal does not support full container-implemented application clustering. However, uPortal does support load balancing and synchronization across load balanced nodes of some state, such as modifications to entities and groups. These features allow uPortal deployers to realize many of the advantages of container-implemented clustering using load balancing technology.

With some constraints in the way you use uPortal, uPortal in theory supports container-managed replication of user sessions. IChannels will not replicate correctly, but JSR-168 portlets should. To the extent that your uPortal usage is entirely using JSR-168 portlets, your user sessions should be usefully replicable across Tomcat-clustered uPortal nodes.

To enable replication you also need to add the <distributable> element to uPortal's web.xml and to the web.xml of each portlet being used.

This replication support has had only limited testing. Eric Dalquist of the University of Wisconsin-Madison writes:

we added the option but have never actually put it into use since load balanced nodes are good enough. uPortal has been rock-solid stable for us and since we never need to restart other than during our maintenance window we really don't have a need for replication.

An answer by Eric Dalquist on the uportal-user@ email list served as a significant source for this KBA.