J2EE
Cluster Architecture 
A J2EE cluster installation consists of:
· One or several J2EE instances
· 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. A server installed in this way can process only J2EE queries. The integrated J2EE and ABAP scenario is described below.
This J2EE cluster consists of:
· A (central) J2EE instance with a dispatcher, three server processes, and the Software Deployment Manager (SDM)
· The Central Services, which also create an instance
· The Database
A J2EE Instance consists of (with the exception of Central Services):
· 0-n J2EE-dispatchers (usually one dispatcher is used for each instance)
· One or several server processes
Large Cluster Installation
The graphic below shows a larger J2EE cluster installation. It consists of four instances all of which has an instance number and each one can be started, stopped, and monitored separately.
Load Balancing
If you have a large J2EE cluster installation, the load is distributed from a load balancer onto the different J2EE 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:
J2EE Startup and Control Framework
Load Balancing of the SAP Web AS for J2EE Applications
High Availability and Failover