A highly availability cluster inSun Java System Application Server Enterprise Edition integrates a state replication service with the clusters and load balancer created earlier, enabling failover of HTTP sessions.
HttpSession objects and Stateful Session Bean state is stored in HADB, a high-availability database for storing session state. This horizontally scalable state management service can be managed independently of the application server tier. It was designed to support up to 99.999% service and data availability with load balancing, failover and state recovery capabilities.
Keeping state management responsibilities separated from Application Server has significant benefits. Application Server instances spend their cycles performing as a scalable and high performance JavaTM 2 Platform, Enterprise Edition (J2EETM platform) containers delegating state replication to an external high availability state service. Due to this loosely coupled architecture, application server instances can be easily added to or deleted from a cluster. The HADB state replication service can be independently scaled for optimum availability and performance. When an application server instance also performs replication, the performance of J2EE applications can suffer and can be subject to longer garbage collection pauses.
Because each HADB node requires 512 Mbytes of memory, you need 1 Gbyte of memory to run two HADB nodes on the same machine. If you have less memory, set up each node on a different machine. Running a two-node database on only one host is not recommended for deployment since it is not fault tolerant.