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In a clustered server environment, distributed state management is a key service the cluster must
provide. For instance, in a stateful session bean application, the session state must be
synchronized among all bean instances across all nodes, so that the client application reaches the
same session state no matter which node serves the request. In an entity bean application, the bean
object sometimes needs to be cached across the cluster to reduce the database load. Currently, the state replication and distributed cache services in JBoss AS are provided via three ways: the HASessionState Mbean, the DistributedState MBean and the JBoss Cache framework.
The HASessionState MBean is a legacy service that provides session replication and distributed cache services for EJB 2.x stateful session beans. The MBean is defined in the all/deploy/cluster-service.xml file. We will show its configuration options in the EJB 2.x stateful session bean section later.
The DistributedState Mbean is a legacy service built on the HAPartition service. It is supported for backwards compatibility reasons, but new applications should not use it; they should use the much more sophisticated JBoss Cache instead.
As mentioned above JBoss Cache is used to provide cache services for HTTP sessions, EJB 3.0 session beans and EJB 3.0 entity beans. It is the primary distributed state management tool in JBoss AS, and is an excellent choice for any custom caching requirements your applications may have. We will cover JBoss Cache in more detail when we discuss specific services in the next several sections..