Mod cluster is completely optional. you can use it or not.
it is completely up to you, if you have existing clustering solution you could just use that.
Correct, mod_cluster is optional as it is only the load-balancer integration piece. You can completely remove the extension and subsystem.
The problem you are running into seems to be that your current load-balancer is not using sticky sessions, please make sure to enable it in your balancer.
The IT department is resisting sticky sessions as they are pushing for session replication.
It seems that if sticky sessions were in play, it wouldn't be necessary to cluster JBoss at all since a user session would be completely handled by one node...unless I'm missing something fundamental here.
Nah, sticky sessions mean that LB would direct sessions to the node that created them unless that node goes down
in this case any other node in cluster would serve it.
Thing is that session replication takes some time and if you configure it that you have SYNC session replication against all nodes in cluster.
every request would take extra time to complete, as it would wait until session state is replicated before returning response to LB.
If you go with sticky sessions you can have async replication and nodes will only take over sessions if original node fails.
in short it will work in both cases, but using sticky sessions will be much faster