Managing HP Serviceguard Extension for SAP for Linux (May 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.
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.
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 startup framework cluster-aware.
SGeSAP has four major Serviceguard modules. The four modules are sgesap/sapinstance,
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 reduntant 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
SAP-specific cluster modules 7