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.
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
Basic Cluster Test
Measure HttpSession Replication Size
First link ("Design and Setup...") is broken. Target is moved?
First link ("Design and Setup...") is broken. Target is moved?