Application Server Enterprise Edition provides the High Availability Database (HADB) for high availability storage of HTTP session and stateful session bean data. HADB is designed to support up to 99.999% service and data availability with load balancing, failover, and state recovery. Generally, you must configure and manage HADB independently of Application Server.
Keeping state management responsibilities separated from Application Server has significant benefits. Application Server instances spend their cycles performing as a scalable and high performance Java™ 2 Platform, Enterprise Edition (J2EE™ platform) containers delegating state replication to an external high availability state service. Due to this loosely coupled architecture, application server instances can be very 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.
For information on planning and setting up your application server installation for high availability with HADB, including determining hardware configuration, sizing, and topology, see Planning for Availability in Sun Java System Application Server Enterprise Edition 8.1 2005Q2 Deployment Planning Guide and Chapter 3, Selecting a Topology, in Sun Java System Application Server Enterprise Edition 8.1 2005Q2 Deployment Planning Guide.