7.5.3 Process affinity
Regardless of the workload management server selection policy, if an EJB is available in the same cluster member as the client, all requests coming from that client are directed to the EJB in the same JVM process. This is called process affinity, because all requests are in-process requests. The advantage of process affinity is that there is no need for serialization for method calls. Parameters can be passed by value without any serialization costs, since all method calls are performed within the same Java Virtual Machine, in the same memory space.
To take advantage of process affinity, the client can only be a servlet or an EJB. In the case of a servlet, process affinity is only possible if the Web container running the servlet is in the same appserver as the EJB container. In the case of an EJB (a stateless session bean acting as a facade, for instance, as recommended by EJB development best practices), process affinity occurs when the called EJB is in the same EJB container as the calling EJB. With IBM WAS ND V6, you can only have one EJB container per appserver.
Process affinity overwrites the Prefer local policy. See 7.5.2, Prefer local for more information about this policy.