Hot deply new version of a webapp

[bmaso]

JBoss configuration:
* JBoss/Tomcat with a web app, clustered using ehcache (not JBoss caching -- but that's not important). 5 JBoss instances in the cluster.
* Apache mod_jk-based loadbalancing with sticky sessions

The problem:
How to deply a new version of the webapp *without bringing down JBoss*. We'd like to upgrade webapp so that
* New sessions get routed to the upgraded version of the webapp
* Existing sessions stay routed to older version of the webapp
* Clients are essentially unaware that upgrade is going on -- URLs to the webapp don't change, existing sessions continue without interruption, new sessions can be started, app never appears to come down from a client POV.

Any advise on which load balancer to use to achieve this? Particular brand of hardware or software? We think we got a handle on JBoss configuration and management, and management of the distributed caches, but getting we don't have any experience on involving the load balancer in the mix.

Thanks in advance for any advise, wisdom, revelations, or knowledge!

Best regards,
Brian Maso

[brian.stansberry]

When you say "without bringing down JBoss" are you saying you want to run 2 versions of the app concurrently in the same JBoss instance? Or are you talking about a rolling upgrade of a cluster?

If the latter, the solution involves the mod_jk and mod_cluster notion of "domains", which are essentially preference groups for failover. Have a look at the presentation Bela Ban did on this recently at JBoss World:

http://www.redhat.com/f/pdf/jbw/bban_320_large_clusters_in_jboss.pdf