Single Point of Failure (SPOF)

In a distributed or standard SAP installation the SAP Central Services, the database server, and the NFS server are single points of failures (SPOFs).

To minimize the impact of SPOF services outages, it is necessary to setup redundancy. Run one or more standby servers to which each of the SPOF services can be failed over and restarted independently. Each SPOF service must be associated with its own virtual host name, which is started where the service runs. Clients reconnect to the same host name independently of where the SPOF service runs.

The following SAP components are available for a distributed SAP system.

Figure 1. Components of a distributed SAP system

(A)SCS node

The (A)SCS node consists of the stand-alone components Enqueue Server (ES) and Message Server (MS) that operate as SAP Central Services instance. Depending on the SAP solution the (A)SCS node contains the ABAP, Java, or both components.

An SAP Instance Agent is running for each instance.

SAP uses the following terms and abbreviations:

ASCS: SAP Central Services for ABAP instances.
SCS: SAP Central Services for Java™ instances.
(A)SCS: SAP Central Services for ABAP or Java instances.

Primary AS node: The Primary AS node consists of the Primary Application Server (PAS) instance that is running the SAP Services Dialog, Update, Batch, Gateway, and Spool. An Instance Agent accompanies the Primary Application Server.
Note: In a standard non-distributed SAP system all the main instances (SCS, PAS, and database) run on the same node.

Additional Application Server node: Additional AS nodes are optional. They host the Additional Application Server (AAS) instances, which were called Dialog Instance (DI) in releases prior then SAP kernel 7.1. You can have one or more Additional Application Servers. Again an Instance Agent accompanies each Additional Application Server.
Web Dispatcher and SAProuter node: The optional Web Dispatcher and SAProuter nodes run the SAP Web Dispatcher (WD) and SAProuter, which are used as proxies to access the other SAP instances.
An Instance Agent is running for the Web Dispatcher.
Host Agents: One SAP Host Agent runs on each node that hosts SAP-provided components.
Database node: The database node holds the database instance. The database product can be IBM DB2 or another SAP supported database.
NFS node: The NFS node runs the NFS server. It can also be a NAS device, which exports the NFS file systems.
SAP clients: The SAP clients connect directly to the Application Servers or to an optional Web Dispatcher.

For more information about the SAP components, see High availability impact.