Java Cluster Architecture 
A Java cluster installation consists of:
· One or more Instances of the SAP Web AS Java
· The Central Services, which also create an instance
· One or several databases
The dispatchers and servers can be split up among different physical servers. The Central Services (message service and enqueue service) are installed on one host that meets possible requirements for high availability.
Minimum Cluster Installation
The following graphic shows the simplest cluster installation of the SAP Web AS Java. A server installed in this way can process only Java queries. The integrated scenario with J2EE and ABAP is described in the section SAP Web Application Server with ABAP and Java.
This Java cluster consists of:
· A (central) Java instance with a dispatcher, three server processes, and the Software Deployment Manager (SDM)
· The Central Services, which also create an instance
· The Database
A Java Instance consists of (with the exception of Central Services):
· a Java Dispatcher
· One or several server processes
Large Cluster Installation
The graphic below shows a larger Java cluster installation. It consists of four instances all of which have an instance number and each one can be started, stopped, and monitored separately.
Load Balancing
If you have a large Java cluster installation, the load is distributed from a load balancer onto the different Java dispatchers. This scenario with implementation of the SAP Web Dispatcher is described in the section SAP Web AS Load Balancing for J2EE Applications.
Further Information
You will find more detailed information on the cluster architecture in the following sections:
Java Startup and Control Framework
Load Balancing of the SAP Web AS for J2EE Applications
High Availability and Failover