The figure shows four nodes.

Node 1 and node 2 run Oracle HTTP Server (which includes mod_oc4j). These nodes are fronted by a load balancer.

Node 3 and node 4 run OC4J. The figure shows "App 1" in each OC4J instance.

Oracle HTTP Servers on nodes 1 and 2 can direct requests to OC4J in either node (node 3 or node 4).

All four Oracle Application Server instances belong to the same OracleAS Cluster. This enables Oracle HTTP Server to direct requests to any OC4J instance in the cluster, and also enables OC4J to notify Oracle HTTP Server about applications that are deployed or undeployed. On the Oracle HTTP Server instances, you set load balancing directives for mod_oc4j to use. You set these directives in the mod_oc4j.conf file.

Application-level clustering enables state information to be replicated. "App 1" on nodes 3 and 4 are clustered at the application-level. You specify the replication settings in the application's XML files.