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data processing functions of a system and offer the corresponding services. Instances are
started, stopped, and monitored as one unit. Multiple instances (belonging to the same
system or to different systems) can be on one host. An instance can be uniquely identified by
the host name and a two-digit instance number.
In this section, each SAP NetWeaver AS instance is described.
Primary application server instance
Each SAP system must have at least one application server instance (central instance) for
either an Advanced Business Application Programming (ABAP) or a Java system. As shown
in Figure 2-2, this requirement can be realized by one application server instance that
contains central services (message and enqueue server) and the work processes that run the
user sessions. This type of setup variant is the classic SAP central instance.
For scalability and manageability reasons, especially in the context of high availability, a
different mapping might be more attractive. For example, to establish a separate instance for
messaging, an enqueue server (called a central services instance) and several dialog
instances serving the user sessions can be helpful. The number of dialog instances can be
adapted to the workload that will be processed.
SAP Central
Instance
SAP Central
Services Instance
SAP Dialog
Instance
Message
Server
Message
Server
Figure 2-2 Setup variants of SAP system instances
Enqueue
Server
Enqueue
Server
Work Work
Work
Work
Process
Process
Process
Work Work
Work
Work
Process
Process
Process
©SAP AG, 2011
Central services instance
SAP Central Services (SCS) instances contain components that are unique to the SAP
system (see 2.1, “SAP NetWeaver composition” on page 10). Because they run on only one
server at a time, these services must be independent and moveable, so that they can be
taken over by another server if a server fails. This takeover is not apparent to the SAP
applications and its users. The application resources are stored in the enqueue table, which is
explained more in 2.6.2, “Enqueue replication server instance” on page 19.
The traditional SAP central instance was redesigned into independent stand-alone
components that are grouped together and operate as SCS. Even though the components
can be started, stopped, and recovered independently, they share an instance directory and
an instance profile. If a component fails, the recovery is more efficient within this setup.
Chapter 2. Architectural overview of SAP NetWeaver with DB2 for z/OS 11