SAP BASIS Administration Tutorials: Architecture of an SAP System

Topic Overview

In this Topic, you will learn about the structure and architecture of an SAP system, and how to use the terms system and instance correctly.

Elements of an SAP System

An SAP system consists of the components shown in the below: One database and one or more instances. The instance that, together with the database, forms a functional SAP system is also known as the “central instance”. There should be a central instance configured in every SAP system. A “central system” exists if the system contains only a single instance, and this runs together with “its” database on one host.

It is certainly possible to install two instances of a system or even of different systems on one server. Before you configure two systems (and their databases) on one server, you should examine the extent to which the chosen hardware is capable of handling the anticipated load. Other aspects to be considered are situations such as upgrades or restore scenarios (for example, one system is to be recreated from a backup whilst the other system should continue to be used without interference).
Within a company, no SAP System IDs (SIDs) should be assigned more than once. You can only exchange data between two systems with the same SID in aneffective way by going to a considerable amount of effort (renaming the systems).

What Is an Instance of an SAP System?

An instance of an SAP system is an administrative unit in which components of an SAP system that provide one or more services are combined. The services offered for an instance can be started or stopped together. Instances of an SAP system can be installed in three forms.

1. ABAP-based instances

2. Java-based instances

3. ABAP/Java instances

NOTE : These three variants cannot all be installed at the same time in one SAP system. If one instance is only Java-based, the same must apply for all the other instances. Other combinations are possible (all instances ABAP-based; some instances only ABAP-based and some instances ABAP/Java-based; all instances ABAP/Java-based).

ABAP-Based Instances

First, we will take a look at pure ABAP instances.The (ABAP) dispatcher is the determining process of an ABAP instance. This

process starts other processes that belong to the instance, such as the gateway (not shown in the below), the Internet Communication Manager (ICM), and a

configured number of work processes.

You configure an ABAP instance using an instance profile. The ABAP instance has shared (main) memory areas and its own directory structure in the file system.

An (ABAP) instance only ever has one (ABAP) dispatcher. The start of an instance always begins with the start of the associated dispatcher. An instance requires a minimum of two dialog work processes. Otherwise it is not possible to start it. You can configure several dispatchers on a host, however, these must have different instance numbers. The default for the instance number of a dispatcher is 00; that is, port 3200 receives communication for this dispatcher. Where two instances exist on a host, they are normally assigned the port numbers 3200 and 3201 etc. You can determine these instance numbers yourself to a certain extent when you install an instance. An instance number cannot be assigned more than once on a host. If several instances are installed on a shared host, these instances use their own, separate, (main) memory areas, and each instance has its own directory structure in the file system.

An (ABAP or ABAP+Java-based) SAP system can have several instances. One single instance is distinguished from all other instances, namely the (ABAP) central instance. It includes an additional process - the ABAP Message Server - which can only be configured once across the whole system. The ABAP Message Server is started before the dispatcher of the central instance. Furthermore, the central instance is the only instance that offers one or more enqueue-type work processes.

An instance is also called an application server in the software-oriented view of the client-server model. The application server provides the runtime environment

for the business applications of SAP systems.

Java-Based Instances

The (Java) dispatcher is the central process of a Java instance. This process distributes the incoming requests to the available server processes.

A Java instance only ever has one Java dispatcher. An instance requires a minimum of one server process. You can configure several dispatchers on a host,however, they must all have different instance numbers.
A (Java-based) SAP system can have several instances. One single instanceis distinguished from all other instances, namely the (Java) central instance. It includes an additional process - the Software Deployment Manager (SDM) -which can only be configured once across the whole system. There is also a Java Central Services (CS) instance. This CS instance offers the Java Message Server and the Java Enqueue Server. In a classic scenario, the Java central instance and the CS instance are located on a shared host.
You can install other Java instances on the same host as the central instance or on other separate hosts.

ABAP+Java-Based Instances

ABAP+Java-based instances offer ABAP and Java-based processes. This results in the following definition of a central instance: The central instance of an ABAP+Java-based SAP system offers all the processes of an ABAP central instance and a Java central instance.

Note : If you restart the central instance of an ABAP/+Java-based system, for example, the restart affects the following processes. Remember the definition that an instance is distinguished by the fact that all its processes can be started and stopped together.

All Java server processes
The Java dispatcher
The Software Deployment Manager (SDM), if applicable
The Internet Communication Manager (ICM)
The gateway
All ABAP work processes
The ABAP dispatcher
The ABAP Message Server

The Java Central Services instance does not belong to the ABAP+Java central instance. It is started and stopped separately.

An (ABAP+Java-based) SAP system always provides one ABAP+Java-based central instance. Other instances can be ABAP-based or also ABAP+Java-based.

Appendix: ABAP Central Services (ASCS)

As of SAP NetWeaver 7.0, for high availability (ABAP-based) SAP systems (such as SAP systems on Windows clusters) you have to set up a separate instance for central services on the ABAP side of an SAP system, namely the ABAP Central Services (ASCS). The ASCS instance enables you to separate the ABAP Message Server and the ABAP Enqueue Service (not implemented as a work process in this case) from the ABAP central instance. In the cluster you will then have the ASCS

instance and the database, as well as any number of equal ABAP instances outside the cluster. This means that there is no “central instance” (in the classic sense) in your system any more. You already know a similar concept from Java-based SAP systems in which the Java Message Server and the Java Enqueue Server are also stored in the Java Central Services instance. On the Java side you can still talk of a central instance since the Software Deployment Manager is the determining

criterion here (not to be confused with the Java Central Services instance).