Storing session state data enables the session state to be recovered after the failover of a server instance in a cluster. Recovering the session state enables the session to continue without loss of information. Enterprise Server provides the following types of high availability storage for HTTP session and stateful session bean data:
In-memory replication on other servers in the cluster
High availability database
In-memory replication on other servers provides lightweight storage of session state data without the need to obtain a separate database, such as HADB. This type of replication uses memory on other servers for high availability storage of HTTP session and stateful session bean data. Clustered server instances replicate session state in a ring topology. Each backup instance stores the replicated data in memory. Replication of session state data in memory on other servers enables sessions to be distributed.
The use of in-memory replication requires the Group Management Service (GMS) to be enabled. For more information about GMS, see Group Management Service.
If server instances in a cluster are located on different machines, ensure that the following prerequisites are met:
To ensure that GMS and in-memory replication function correctly, the machines must be on the same subnet.
To ensure that in-memory replication functions correctly, the system clocks on all machines in the cluster must be synchronized as closely as possible.
The HADB software is supplied with the Enterprise Server standalone distribution of Sun GlassFish Enterprise Server. For information about available distributions of Sun GlassFish Enterprise Server, see Distribution Types and Their Components in Sun GlassFish Enterprise Server 2.1 Installation Guide. HADB features are available only in the enterprise profile. For information about profiles, see Usage Profiles in Sun GlassFish Enterprise Server 2.1 Administration Guide.
GlassFish Communications Server 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 Enterprise Server.
Keeping state management responsibilities separated from GlassFish Communications Server has significant benefits. GlassFish Communications Server instances spend their cycles performing as a scalable and high performance application containers delegating state replication to an external high availability state service. Due to this loosely coupled architecture, GlassFish Communications 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 GlassFish Communications Server instance also performs replication, the performance of Java EE applications can suffer and can be subject to longer garbage collection pauses.
For information on planning and setting up your installation for high availability with HADB, including determining hardware configuration, sizing, and topology, see the Sun GlassFish Enterprise Server 2.1 Deployment Planning Guide.