Availability is typically measured in the form of system down time. The single most important mechanism to achieve high availability (HA) is through redundancy and replication. For example, by running multiple server instances on different hardware, we are eliminating a single point of failure. However, in many cases, hardware redundancy alone is not sufficient, especially when the application maintains a conversation state, or session, with the client. In this case, in order to deliver high availability for the application, we must be able to replicate the session data so that in the event of a server instance failure, the session state information maintained in that server instance is not lost, and a different server instance can restore the session from the replicated session data store.
For session replication, GlassFish supports two options, using the in-memory replication, and using the High Availability Database (HADB) for more reliable state persistence...