Installation guide

1.5 SAP Overview
In an SAP NetWeaver environment, these single points of failure (SPOF) must be considered:
 Database
 SAP Central Services Instance (SCS/ASCS)
 SAP System Mount Directory (/sapmnt/<SID>)
The SAP example system in the above illustration is a double stack with Enqueue
Replication, both for ASCS and SCS. Although they are not SPOF, the Enqueue Replication
Servers (ERS) are controlled by the cluster software. To create a working Enqueue
Replication, it is important that the ERS does not run on the same cluster node as the
(A)SCS. This is because the original enqueue table lies within the same shared memory
segment as the replicated table.
When the (A)SCS fails and a failover is triggered by the cluster software, (A)SCS must start
on the node where the ERS is running. When (A)SCS comes up, it shuts down the ERS
instance without cleaning its shared memory and attaches itself to the shared memory
segment where the ERS had stored the replicated enqueue table. Now the replicated
enqueue table has become the new original enqueue table.
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