SAP Netweaver Architecture

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An SAP system consists of SAP instances. An SAP instance is a group of processes that are started and stopped at the same time.

In SAP NetWeaver 7.0, there are the following instances:  
                • Central instance  
                • Central services instance  
                • Database instance  
                • Dialog instance

All instances, except the database instance, are identified by an instance number, which is a two-digit number between 00 and 97 that is unique on a host. Instances can reside on one host, or they can be distributed on several hosts.  
  
  
**Central Instance**

Every SAP system must have one central instanc e , which includes:  
    **• Usage type AS ABAP**  
             Dispatcher  
             Work processes (dialog, batch, spool, or update)  
             Gateway  
             Internet Communication Manager (ICM)  
             Internet Graphics Service (IGS)  
 **  • Usage type AS Java**  
             Java dispatcher  
             Server processes  
             Software Deployment Manager (SDM)  
             Internet Graphics Service (IGS)  
  
  When the central services are located on a separate instance, the configuration of the central instance is the same as the configuration of the dialog instance.  
   If the central services are not installed as a separate instance, the message server and enqueue server are part of the central instance.  
  
  
**Central Services Instance**

The central services instance – Java central services instance (SCS) or ABAP central services instance (ASCS) –), forms the basis of communication and synchronization for the Java or ABAP clusters.  
    The ASCS can only be installed for a high availability system with AS ABAP.  
A central services instance consists of the message server and the enqueue server:  
    **• Message server**  
           Only one message server can run on each AS Java or AS ABAP usage type. The message server handles the communication between the dialog instances and also supplies information to the SAP Web dispatcher about load balancing.

**• Enqueue server**  
    The enqueue server contains a lock table that handles logical database locks plus infrastructure locks set by Java server process. The enqueue server also synchronizes data in a Java cluster. In usage type AS ABAP, the enqueue server handles only locks on data objects.  
  
  
**Database Instance**  
     The database instance is a mandatory installation component for the installation of an SAP system. AS Java and AS ABAP have separate database schemes. When AS ABAP with AS Java (also known as Java Add-In) is installed, the AS ABAP and the AS Java database schemes are installed in the same database. It is not recommended to use separate databases for the AS Java and AS ABAP scheme.

The AS Java scheme, named SAPDB, holds the data stored by AS Java and all  
deployed application objects. The ABAP scheme is named SAP  
  
  
**Dialog Instance**  
    Dialog instances are optional and can be installed on separate hosts. Dialog instances include:  
        **• Usage type AS ABAP**

Dispatcher

Work Processes (dialog, batch, spool, or update)  
                  Gateway  
                  Internet Communication Manager (ICM)  
                  Internet Graphics Service (IGS)  
 **      • Usage type AS Java**  
                  Java dispatcher  
                  Server processes  
                  Internet Graphics Service (IGS)

When you add a dialog instance, it belongs to the software cluster. The central instance initiates an update of the dialog instance so the dialog instance has access to the deployed applications on that cluster (this also includes usage types such as EP or PI).  
A cluster always needs a load balancing solution, for example, SAP Web dispatcher.