1.1.5. Distributed state replication services
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 two ways: the HASessionState MBean and the JBoss Cache framework.
The HASessionState MBean provides session replication and distributed cache services for EJB 2.x stateful session beans and HTTP load balancers in JBoss 3.x and 4.x. 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.
JBoss Cache is a fully featured distributed cache framework that can be used in any application server environment and standalone. It gradually replaces the HASessionState service. JBoss AS integrates JBoss Cache to provide cache services for HTTP sessions, EJB 3.0 session and entity beans, as well as Hibernate persistence objects. Each of these cache services is defined in a separate MBean. We will cover those MBeans when we discuss specific services in the next several sections.