HADB is a horizontally-scalable database that can be run and managed independently of the application server tier. It is designed to support up to 99.999% service and data availability with load balancing, failover, and state recovery capabilities.
Application Server uses HADB to store HTTP and stateful session bean (SFSB) session data. Without a session persistence mechanism, the HTTP or SFSB session state data is lost when a web or EJB container fails over.
Keeping state management separate 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, you can easily add application server instances to and remove instances from a cluster. You can independently scale HADB state replication service for optimum availability and performance.