Each Oracle9iAS Instance has OPMN and DCM components, OC4J processes, and Oracle HTTP Server with mod_oc4j.
The client communicates with either of the instances through a network load balancer. The request goes to Oracle HTTP Server, and from there can go directly to OPMN or go through OC4J processes before going to OPMN. The request then goes to DCM, which communicates with the infrastructure.