User's Guide

dependencies. The information is for example used to delay SAP instance package startups while the database
is starting in a separate package, but not yet ready to accept connections.
A cluster can be configured in a way that two nodes back up each other. The principle layout is depicted
in figure 1-1. This picture as well as the following drawings are meant to illustrate basic principles in a clear
and simple fashion. They omit other aspects and the level of detail that would be required for a reasonable
and complete high availability configuration.
Figure 1-1 Two-Package Failover with Mutual Backup Scenario
It is a best practice to base the package naming on the SAP instance naming conventions whenever possible.
Each package name should also include the SAP System Identifier (SID) of the system to which the package
belongs. If similar packages of the same type get added later, they have a distinct namespace because they
have a different SID.
Example: A simple mutual failover scenario for an ABAP application defines two packages, called dbSID
and ascsSID (or ciSID for old SAP releases).
Robust Failover Using the One Package Concept
In a one-package configuration, the database, NFS and SAP SPOFs run on the same node at all times and
are configured in one SGeSAP package. Other nodes in the Serviceguard cluster function as failover nodes
for the primary node on which the system runs during normal operation.
NOTE: Module-based SGeSAP packages cannot be combined with a legacy based NFS toolkit to create
a single package.
It is not required to maintain an expensive idle standby. SGeSAP allows to utilize the secondary node(s)
with different instances during normal operation. A common setup installs one or more non-mission critical
SAP Systems on the failover nodes, typically SAP Consolidation, Quality Assurance or Development Systems.
They can gracefully be shutdown by SGeSAP during failover to free up the computing resources for the
critical production system. For modular packages, the sgesap/sapextinstance module can be added
to the package to allow specifying this kind of behavior.
Development environments tend to be less stable than production systems. This should be taken into
consideration before mixing these use-cases in a single cluster. A feasible alternative would be to install
Dialog Instances of the production system on the failover node.
14 Designing SGeSAP Cluster Scenarios