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2.6 SAP components that support availability
This section describes the major components of SAP that support availability options.
2.6.1 Stand-alone enqueue server
The enqueue server is one of the stand-alone components of the SCS. It is critical for an SAP
system because it is not operational if the enqueue server is unavailable. An application
server instance connects directly to the enqueue server by using a virtual IP address (VIPA)
without involving the message server in the communication path.
The stand-alone enqueue server is now multi-threaded, allowing parallel processing and
replication. Limitations in terms of throughput are eliminated because the I/O processing for
the TCP/IP communications is now distributed over several threads.
With this setup, the performance of the enqueue server is improved and leads to a higher
throughput.
2.6.2 Enqueue replication server instance
As explained in “Enqueue replication server instance” on page 12, the enqueue replication
server is a backup for the enqueue server and holds a duplicate of the enqueue lock table. To
ensure continuous availability and transparent failover, the enqueue replication server is also
established as a stand-alone component.
The enqueue server is connected to the enqueue replication server and transmits replication data
to build a shadow enqueue table that is in shared memory. If an enqueue server failure occurs,
the enqueue replication server rebuilds the tables and data structures to allow a restart.
The enqueue replication server is not considered a member of the SCS because it runs on
another system. Therefore, if the enqueue replication server is unavailable, the SAP system
continues to run, although it no longer has a backup for the enqueue server.
2.6.3 Failover and recovery of SAP Central Services
If one system fails, a second system takes over the role of the first system. The following
procedures automatically start if a system failure occurs:
1. The IP address (VIPA) is taken over by the second system. The message server and the
enqueue server are restarted there. The enqueue table is rebuilt from its shadow table of
the enqueue replication server. Then, the application server reconnects to the enqueue
server and message server.
2. When the failover successfully finishes, the enqueue replication server is no longer needed
on the system and can be stopped. If another system is available or a new system becomes
available later, the enqueue replication server is started on that system to establish a new
shadow enqueue table. The failover is not apparent to the applications and the users.
2.6.4 SAP appliance solutions
The SAP NetWeaver Business Warehouse Accelerator, which is based on a blade center,
automatically fails over to a dedicated backup blade if loss of its original blade or an entire
BladeCenter occurs and continues the processing data there instead. This failover is
accomplished by using data replication in IBM General Parallel File System (GPFS).
Chapter 2. Architectural overview of SAP NetWeaver with DB2 for z/OS 19