HTTP session replication is used to replicate the state associated with your web clients on other nodes of a cluster. Thus, in the event one of your node crashes, another node in the cluster will be able to recover. Two distinct functions must be performed:
Session state replication
Load-balance of incoming invocations
State replication is directly handled by JBoss. When you run JBoss in the all configuration, session state replication is enabled by default. Just deploy your web application and its session state is already replicated across all JBoss instances in the cluster.
However, Load-balancing is a different story, it is not handled by JBoss itself and requires additional software. As a very common scenario, we will demonstrate how to setup Apache and mod_jk. This activity could be either performed by specialized hardware switches or routers (Cisco LoadDirector for example) or any other dedicated software though.
A load-balancer tracks the HTTP requests and, depending on the session to which is linked the request, it dispatches the request to the appropriate node. This is called a load-balancer with sticky-sessions: once a session is created on a node, every future request will also be processed by the same node. Using a load-balancer that supports sticky-sessions without replicating the sessions allows you to scale very well without the cost of session state replication: each query will always be handled by the same node. But in the case a node dies, the state of all client sessions hosted by this node are lost (the shopping carts, for example) and the clients will most probably need to login on another node and restart with a new session. In many situations, it is acceptable not to replicate HTTP sessions because all critical state is stored in the database. In other situations, loosing a client session is not acceptable and, in this case, session state replication is the price one has to pay.
Apache is a well-known web server which can be extended by plugging modules. One of these modules, mod_jk (and the newest mod_jk2) has been specifically designed to allow forward requests from Apache to a Servlet container. Furthermore, it is also able to load-balance HTTP calls to a set of Servlet containers while maintaining sticky sessions, and this is what is actually interesting for us.