JBoss provides excellent failover capability for the HttpSession.  This page aggregates some useful docs you should read if you are going set this up and use it.

Setup

• Design and Setup of JBoss HttpSession Replication

• JBoss Docs for HttpSession Replication

• mod_jk setup

• Single SignOn

• Two Clustered JBoss Instances On the Same Box

• Two Clusters On the Same Network

• A High Performance Session Replication Config for 4.0.x
  

• Using JBoss Serialization for HttpSession replication

• HttpSession Passivation

• Buddy Replication

Debug and Analysis

• Basic Cluster Test

• Measure HttpSession Replication Size