Managing Serviceguard Extension for SAP, December 2007

Understanding Serviceguard Extension for SAP
Designing SGeSAP Cluster Scenarios
Chapter 1 17
Mutual Failover Scenarios Using the Two Package
Concept
SAP ABAP Engine based applications usually rely on two central
software services that define the software SPOFs: 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 called the
Central Instance (ci).
As any other SAP instance, the 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. Therefor, the most important package type of SGeSAP is the
Central Instance package (ci).
All other SAP services can be installed redundantly within additional
Application Server instances, sometimes also called Dialog Instances.
As its naming convention may suggest, DVEBMGS, there are more services
available within the Central Instance than just those that cause the
SPOFs. An undesirable result of this is, that a Central Instance is a
complex software with a high resource demand. Shutdown and startup of
Central Instances is slower and more error-prone than they could be.
Starting with SAP Application Server 6.40 it is possible to isolate the
SPOFs of the Central Instance in a separate Instance that is then called
the ABAP System Central Service Instance, in short ASCS. The installer
for SAP Application Server allows to install ASCS automatically. This
installation procedure will then also create a standard Dialog Instance
that is called DVEBMGS for compatibility reasons. This kind of DVEBMGS
instance provides no Enqueue Service and no Message Service and is not
a Central Instance.
NOTE An SGeSAP ci package contains either a full DVEBMGS instance or a
lightweight ASCS instance. In any case, the SGeSAP ci package
provides failover capabilities to SAP Enqueue Services and SAP Message
Services.