Managing HP Serviceguard Extension for SAP for Linux, December 2013

2 SAP cluster concepts
This chapter introduces the basic concepts used by SGeSAP for Linux. It also includes
recommendations and typical cluster layouts that can be implemented for SAP environments.
2.1 SAP-specific cluster modules
HP SGeSAP extends Serviceguard 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.
SGeSAP provides SAP-specific modules, service monitors, cluster resources, cluster deployment,
and cluster verification tools as well as a shared library that makes SAP startup framework
cluster-aware.
SGeSAP extends Serviceguard with five new modules. The five modules are
sgesap/sapinstance, sgesap/hdbinstance, sgesap/mdminstance,
sgesap/livecache, and sgesap/dbinstance. These modules allow quick configuration of
instance-failover and clustering of all mandatory SAP Netweaver software services. The mandatory
services are categorized as Single Points of Failures (SPOFs) software.
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 Advanced Business
Application Programming (ABAP) System Central Service instance (ASCS) for SAP ABAP
applications. You can also configure both JAVA (SCS) and ABAP (ASCS) components in one SAP
application. In this case, both instances are SPOFs that require clustering.
In ABAP environments, the term Central Instance (CI) refers to a software entity that combines
additional SAP application services with the SPOFs instance. As any other SAP instance, a CI 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. By
default, there are five other software services that are part of the CI.
An undesirable result is that a CI is complex software with a high resource demand.. Shutdown
and startup of CI is slower and more error-prone due to presence of redundant non-critical services.
Starting with SAP Application Server 6.40, a CI installation creates two instances instead of one,
an ASCS instance and a DVEBMGS instance. The SPOFs of the Central Instance are isolated into
the first instance. The second instance still named DVEBMGS for compatibility reasons but unlike
the name suggests, it includes 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 of a set of SAP instances that
belong to the same Netweaver system into 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.
Instance-type specific handling is provided by the module for SAP ABAP Central Service Instance,
SAP JAVA Central Service Instance, SAP ABAP Application Server Instances, SAP JAVA Application
2.1 SAP-specific cluster modules 9