The central instance is a concept that is unique to SAP. The central instance is a combination of hardware and software. It contains a physical server (the application server) and numerous software components, including a message server, a database gateway (a pre-established connection between SAP and Oracle--or another database), and various update, enqueue, dialog, and spool facility software. In most generic SAP architectures, there are numerous application servers but only a single central instance. However, in addition to managing the SAP interfaces, the central instance can also serve as an application server.
A dialog instance is a software program that is running the SAP kernel and it is the job of the dialog instance to execute the ABAP programs and manage the requests for data and services. While there is generally a one-to-one mapping between an application server and a dialog instance, it is possible to have more than one dialog instance on an application server.
Central Services
Definition
The Central Services run on one physical server and are one Java instance.
They comprise the message service and the enqueue services.
Use
The Central Services form the basis of communication and synchronization for the Java cluster.
The Message Service keeps a list of the dispatchers and server processes of the Java cluster. It represents the infrastructure for data exchange (small datasets only) between the participating nodes. The message service also supplies information to the SAP Web Dispatcher about Load Balancing Between Many Java Instances.
The Enqueue Service manages logical database locks, which are set by the executed application program in a server process. The enqueue service also synchronizes data across the cluster.
The section Deployment of a New Application describes how the central components handle the deployment of a new application.
Integration
Central Services are always required when a Java cluster is installed. They are started on a server with their own system number and the system ID (SID) of the whole system.
When Central Services are running, further Java instances are started with the program JControl (see Java Startup and Control Framework).
Edited by: Shaji Jacob on Apr 7, 2008 7:09 AM
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