Managing Serviceguard Extension for SAP on Linux (IA64 Integrity and x86_64), February 2008

Understanding Serviceguard Extension for SAP on Linux
The Replicated Enqueue
Chapter 124
The Replicated Enqueue
In case your environment has very high demands regarding guaranteed
uptime, it makes sense to active a Replicated Enqueue with
Serviceguard Extension for SAP on Linux. With this additional
mechanism it is possible to failover ABAP or JAVA System Central
Service Instances without impacting ongoing transactions on Dialog
Instances.
NOTE It doesn’t make sense to activate Enqueue Replication for systems that
have no Dialog Instances that run on nodes different from the primary
node of the System Central Service package.
Replicated Enqueue is only supported in 2-node configurations.
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 this cancels 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.