Here’s several words which are related to my previous post Web AS Java Architecture :
Java Cluster consists of all Java components in SAP system like Java instance, Central Services and database. Minimal Cluster installation contains one Java instance ( Java dispatcher, server process, and SDM), one Central Services (Enqueue Server, Message Server) and database (Java Schema).
Java dispatcher has function like ABAP dispatcher which receives requests from client, it will then select free server process(es) to handle those requests. The dispatcher distributed new coming requests using ’round robin’ algorithm.
Server process runs a Java application. Every request from dispatcher will be processed here by system or application thread. The server processes are implemented as multi-threaded servers , therefore it can process multiple requests in parallel.
Central Services run on one host and form a separate Java instance. They consist os message service and enqueue services. Both message service and enqueue services has been described in my previous post. The Central Services is essentially required when a Java cluster is installed.