Managing Serviceguard Extension for SAP Version B.05.10, December 2012

Follow-and-Push Clusters with Replicated Enqueue
In case an environment has very high demands regarding guaranteed uptime, it makes sense to
activate a Replicated Enqueue with SGeSAP. With this additional mechanism, it is possible to
failover ABAP and/or JAVA System Central Service Instances without impacting ongoing transactions
on Dialog Instances.
NOTE: It only makes sense to activate Enqueue Replication for systems that have Dialog Instances
that run on nodes different from the primary node of the System Central Service package.
Each SAP Enqueue Service maintains a table of exclusive locks that can temporarily be granted
exclusively to an ongoing transaction. This mechanism avoids inconsistencies that could be caused
by parallel transactions that access the same data in the database simultaneously. In case of a
failure of the Enqueue Service, the table with all locks that have been granted gets lost. After
package failover and restart of the Enqueue Service, all Dialog Instances need to get notified that
the lock table content got lost. As a reaction they cancel ongoing transactions that still hold granted
locks. These transactions need to be restarted.
Enqueue Replication provides a concept that prevents the impact of a failure of the Enqueue Service
on the Dialog Instances. Transactions no longer need to be restarted. The Enqueue Server has the
ability to create a copy of its memory content to a Replicated Enqueue Service that needs to be
running as part of a Enqueue Replication Service Instance (ERS) on a remote host. This is a realtime
copy mechanism and ensures that the replicated memory accurately reflects the status of the Enqueue
Server at all times.
There might be two ERS instances for a single SAP system, replicating SCS and ASCS locks
separately.
Figure 3 Replicated Enqueue Clustering for ABAP and JAVA Instances
Enqueue Services also come as an integral part of each ABAP DVEBMGS Central Instance. This
integrated version of the Enqueue Service is not able to utilize replication features. The DVEBMGS
Instance needs to be split up in a standard Dialog Instance and a ABAP System Central Service
Instance (ASCS).
Follow-and-Push Clusters with Replicated Enqueue 13