Chapter 8 Creating J2EE Applications for BEA WebLogic
The WebLogic Server EJB container has clustering support for stateful session EJBs. Whereas in WebLogic Server 5.1 only the EJBHome is clustered for stateful session EJBs, the EJB container can also replicate the state of the EJB across clustered WebLogic Server instances.
Replication support for stateful session EJBs is transparent to clients of the EJB. When a stateful session EJB is deployed, WebLogic Server creates a cluster-aware EJBHome stub and a replica-aware EJBObject stub for the stateful session EJB. The EJBObject stub maintains a list of the primary WebLogic Server instances on which the EJB instance runs, as well as the name of a secondary WebLogic Server to use for replicating the bean's state.
Each time a client of the EJB commits a transaction that modifies the EJB's state, WebLogic Server replicates the bean's state to the secondary server instance. Replication of the bean's state occurs directly in memory, for best performance in a clustered environment.
Should the primary server instance fail, the client's next method invocation is automatically transferred to the EJB instance on the secondary server. The secondary server then becomes the primary WebLogic Server for the EJB instance, and a new secondary server is used to account for the possibility of additional failovers. Should the EJB's secondary server fail, WebLogic Server enlists a new secondary server instance from the cluster.
Clients of a stateful session EJB are therefore guaranteed to have quick access to the latest committed state of the EJB, except under the special circumstances described in Limitations of In-Memory Replication.
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