EJB Clustering Overview

Clustered EJBs behave in their own manner. However, only stateful session beans are clustered. To create an EJB cluster, you specify nodes that are to be involved in the cluster, configure each OC4J instance within the node with the same multicast address, username, and password, and deploy the EJB to one of these nodes.

Caution: EJB clustering only works over the ORMI protocol, not over the RMI/IIOP protocol.

The following characteristics apply to EJB clustering:

• Unlike HTTP clustering, EJBs involved in a cluster cannot be sub-grouped in an island. Instead, all EJBs within the cluster are in one group.

• Transactions cannot failover. There is no reinstating an interrupted transaction in another bean. Instead, the transaction rolls back and must start over.

• Load balancing occurs in a random fashion throughout all OC4J processes in the cluster for EJBs.

• The performance for clustering stateful session beans is dependent on the type of replication and load balancing options you choose.

Clustering for each of the session bean types are discussed in the following sections:

• Stateless Session Clustering

• Stateful Session Bean Clustering

• Combination of HTTP and EJB Clustering

  
  

  Note: For an overview of how clustering works, see the clustering chapter of the Oracle Application Server Containers for J2EE User's Guide.

  
  

Stateless Session Clustering

Stateless session beans do not have any state to be replicated among hosts in a cluster. Thus, no failover option is necessary. Load balancing is provided automatically with OPMN, which uses a random algorithm. Stateless session beans are not clustered; the load balancing happens in any environment where the OPMN components know about each other. You can configure the frequency of the load balancing from the client using the options described in "Load Balancing Options".

Stateful Session Bean Clustering

Load balancing is provided automatically with OPMN, which uses a random algorithm. Failover requires that the state of the bean is replicated, so that when the original bean terminates unexpectedly, the request can be forwarded to another OC4J process. You can configure the frequency of the load balancing from the client using the options described in "Load Balancing Options".

For failover, stateful session beans must replicate state among hosts. There are three options for stateful session bean replication, where each option defines the interval when the bean state is sent. All of the state is sent out to all other OC4J processes in the cluster, so it can have an impact on your performance. The fewer times the state is sent out, the better your performance. However, there is a trade-off between performance and the confidence that the bean state is replicated to cover for all areas of the bean instance failing. Thus, choose one of the following replication modes:

• JVM termination replication—The state of the stateful session bean is replicated to only one other host in the cluster (with the same multicast address, port) when the JVM is terminating. Since this uses JDK 1.3 shutdown hooks, you must use JVM version 1.3 or later. This is the most performant option, because the state is replicated only once. However, it is not very reliable for the following reasons:

  • Your state is not replicated if the host is terminated unexpectedly.

  • The state of the bean exists only on a single host at any time; you carry a higher risk that the state does not replicate and is lost.

• End of call replication—The state of the stateful session bean is replicated to all hosts in the cluster (with the same multicast address, port) at the end of each EJB method call. If the node loses power, then the state has already been replicated. This method is less performant than the JVM termination replication mode, because the state is sent out more often. However, the guarantee for reliance is higher.

See "Configure EJB Replication for Stateful Session Beans" for configuration and implementation details for each of these stateful session bean clustering options.

Combination of HTTP and EJB Clustering

If you have a servlet that invokes an EJB, you must configure both HTTP and EJB clustering. For HTTP clustering options, see the Clustering chapter of the Oracle Application Server Containers for J2EE User's Guide.