Managing Serviceguard Extension for SAP Version A.06.00 for Linux, December 2012

2 SAP cluster concepts
This chapter introduces the basic concepts used by the HP Serviceguard Extension for SAP for Linux
(HP SGeSAP/LX) and explains several naming conventions. The following sections provide
recommendations and examples for typical cluster layouts that can be implemented for SAP
environments.
SAP-specific cluster modules
HP SGeSAP extends HP Serviceguard's failover cluster capabilities to SAP application environments.
It is intended to be used in conjunction with the HP Serviceguard Linux product and the HP
Serviceguard Toolkit for NFS on Linux.
Serviceguard packages can be distinguished into legacy packages and module-based packages.
SGeSAP focuses on extending the module-based packaging by providing SAP-specific modules,
service monitors, cluster resources, cluster deployment and cluster verification tools as well as a
shared library that makes SAP’s startup framework cluster-aware.
There are four major Serviceguard modules delivered with SGeSAP. They allow quick configuration
of instance-failover clustering for all mandatory SAP Netweaver software services, that is mandatory
services categorize the software Single Points (SPOFs).
Most SAP applications rely on two central software services that define the major software Single
Point of Failure for SAP environments: the SAP Enqueue Service and the SAP Message Service.
These services are traditionally combined and run as part of a unique SAP Instance that is referred
to as JAVA System Central Service Instance (SCS) for SAP JAVA applications or ABAP System
Central Service Instance (ASCS) for SAP ABAP applications. If an SAP application has both JAVA
and ABAP components, it is possible to have both—an SCS and an ASCS instance—for one SAP
application. In this case, both instances are SPOFs that require clustering.
In pure ABAP environments, the term Central Instance (CI) is still in use for a software entity that
combines further SAP application services with these SPOFs in a single instance. As any other SAP
instance, a Central Instance has an Instance Name. Traditionally it is called DVEBMGS. Each letter
represents a service that is delivered by the instance. The "E" and the "M" stand for the Enqueue
and Message Service that were identified as SPOFs in the system. Other SAP services can potentially
be installed redundantly within additional Application Server instances, sometimes called Dialog
Instances.
As its naming convention may suggest, DVEBMGS, more services are available within the Central
Instance than just those that cause the SPOFs. An undesirable result is a Central Instance is a
complex software with a high resource demand. A Central Instance is complex software with a
high resource demand. Shutdown and startup of Central Instances is slower and more error-prone.
Starting with SAP Application Server 6.40, the SPOFs of the Central Instance was isolated in the
ABAP System Central Service Instance (ASCS). The installer for SAP Application Server allows
ASCS to install automatically. This installation procedure also creates a standard Dialog Instance
called DVEBMGS for compatibility reasons. This DVEBMGS instance provides no Enqueue Service
and no Message Service, and is not a Central Instance anymore.
A package that uses the sgesap/sapinstance module can be set up to cluster the SCS and or
ASCS (or Central Instance) of a single SAP application.
All instance types and use cases for SAP Netweaver web application server software are covered
by module sgesap/sapinstance. This module allows adding a set of SAP instances that belong
to the same Netweaver system to a module-based Serviceguard package. The package can
encapsulate the failover entity for a combination of ABAP-stack, JAVA-stack or double-stack instances.
NOTE: Split-stack installations require separate packages for each stack. In this case, a package
same_node dependency can be defined which ensures that split-stack packages can be handled
as a single entity.
SAP-specific cluster modules 7