SAP High Availability Setup
How can I set up a high available system?
SAP supports the installation of High Available systems with several aspects. However, High Availability needs additional software, that is not delivered by SAP. In addition, it is important to analyze your system to be sure that no single points of failure are overseen, as it is possible to configure your system in that way or to write custom software that behaves that way. We recommend to engage experienced consultants on this analysis.
In general SAP systems are set up High Available through their technology components, which are the application servers. However, it is also responsibility of a running program to not introduce additional singel points of failure. At SAP this is ensured through extensive quality management, for custom development this should be carefully considered.
What is ASCS/SCS?
With SAP NetWeaver 04 Java, the Message Server and the Enqueue Server are separated from the Central Instance. These two services are grouped within the SAP Central Services Instance (SCS) as services. From NW04s the ABAP Central Services can be also separated from the Central Instance. Each stack, ABAP and Java, has its own Message Service and Enqueue Service. For ABAP systems the Central Services are referred to as ASCS, for Java systems the Central Services are referred to as SCS. The ASCS and the SCS are leveled as SPOF and require a High Availability Setup therefore. If the ASCS is integrated within the ABAP Central Instance (standard in NetWeaver 04) the Central Instance of the ABAP system needs a HA setup also.
What is the difference between HA for ABAP and HA for Java?
Within SAP NetWeaver 6.40 ABAP the Message Server and the Enqueue Server are integrated within the ABAP Central Instance (CI).
In SAP NetWeaver 6.40 Java the Message Server and the Enqueue Server are implemented as services within the SAP System Central Services Instance (SCS) and separated from the Central Instance (CI) this way.
With SAP NetWeaver 04s ABAP the Message Server and the Enqueue Server can be separated from the Central Instance (CI) to the ABAP SAP Central Services Instance (ASCS) in the ABAP stack also (which is recommended for HA setups due to the ASCS is a light component that can be switched over easily).
Is SAP HA also supported for heterogeneous system landscapes?
SAP does not support heterogeneous HA cluster environments at the moment officially. However, preparations for an official support of this type of setup are currently evaluated.
What is an "Enqueue Replication Server"?
The Enqueue Server contains the central locking table for the SAP cluster. Besides database locks it also consists of infrastructure locks of system wide objects. It is therefore necessary to secure the locking table in case of a Standalone Enqueue Server failure. The SAP Enqueue Replication Server provides a replication mechanism for the Enqueue Server by holding a copy of the locking table within its shared memory segment. After a failure of the Enqueue Server the locking table can be restored this way. Since SAP NW04 SP15/ NW04s SR1 an automated installation of the Enqueue replication server is available for Windows environments. UNIX/ Linux installations are handled by SAP hardware partners.
Note: you can only protect Stand-Alone Enqueue Servers with an Enqueue Replication Server. The standard Enqueue Server in an ABAP CI (Enqueue work process) cannot be protected by an Enqueue Replication Server.
Is an "Enqueue Replication Server" necessary for setting up an SAP HA system landscape?
Form Version NW04 SP15 the Replication Server is required for all SAP JAVA HA scenarios.
Is SAP NetWeaver AS Java available also if a central service or the database fails?
The SAP NetWeaver Application Server Java uses the database extensively. A loss of the database is very critical for the functionality of AS Java. However, some rudimentary functions and cached information are still available. After the database is available again the Engine is in a full operational state again. The picture below shows the approximated impact after a loss of the database and the central services. Please note that the diagram is only a rule of thumb and not an empiric study.
What is the difference between Enqueue Server and Standalone Enqueue Server?
Since NW04 Java the Enqueue Server and the Message Server for a J2EE Engine are standalone services hosted by the SAP Central Services instance (SCS). The difference between a Standalone Enqueue Server and Enqueue Service is therefore only formal: The term “serviceâ€