Switchover Units

Definition

SAP recommends grouping the system-critical SPOFs in the SAP NetWeaver ASinto the following switchover units:

?     CI – central instance of SAP NetWeaver AS ABAP, with enqueue and message service

?     SCS ­­– central services (SCS) of SAP NetWeaver AS Java instance, with enqueue and message service

?     DB – database management system (DBMS)

?     NFS – network file system

We recommend you to place both the enqueue and message server on the CI because the enqueue service is always accessed through the message service for inter-process communication. Distributing the message and enqueue services across different host machines normally lowers performance because of the network overhead.

We consider DB and NFS services to be separate service packages. Depending on how many host machines are available in the switchover cluster, there are several ways to distribute them. In this section, we include NFS in the switchover scheme. For more information, contact your hardware and switchover product vendors.

Structure

The table below shows the different ways you can distribute services across two or three hosts in a switchover cluster for ABAP and Java:

Distribution of System-Critical Services for SAP NetWeaver AS(ABAP)

Cluster Node 1

Cluster Node 2

Cluster Node 3

Use

DB / CI / NFS

Central system

DB 

CI / NFS

Mutual takeover, NFS on CI node

DB / NFS

CI  

Mutual takeover, NFS on DB node

DB

CI

NFS

Separation of all services, NFS on dedicated server (UNIX only)

Distribution of System-Critical Services for SAP NetWeaver AS(Java)

Cluster Node 1

Cluster Node 2

Cluster Node 3

Use

DB / SCS / NFS

Central system

DB

SCS / NFS

Mutual takeover, NFS on SCS node

DB / NFS

SCS

Mutual takeover, NFS on DB node

DB

SCS

NFS

Separation of all services, NFS on dedicated server (UNIX only)

Integration

When choosing an appropriate configuration for a switchover cluster, consider the following:

·        Separation of the CI and DB

It is often better to separate the DB and CI, locating them on two different host machines of the switchover cluster. Separating the DB service from the CI is a general option that is available with SAP’s three-tiered architecture. We recommend it when your SAP system runs in a multi-host environment with a heavy database workload. The dedicated DB host is freed of both the CI workload and the dialog workload of additional application servers.

 

If the DB host does not include the CI or another AS instance, you need to take special precautions if you want all available functions in the Computer Center Management (CCMS) to work. The saposcol process must run on the DB host.

For more information, see SAP Note 20624.

·        NFS Service

In a SAP NetWeaver ASsystem, several directories have to be NFS mounted. In most cases either the database node or the central instance node act as the NFS server, but this depends on your database. For example, if you are using an Informix database, the NFS server has to be located on the DB node. In this case one NFS belongs to the database.

From the perspective of SAP NetWeaver AS, NFS is considered to be part of the operating system (OS) underlying SAP NetWeaver AS. Contact your vender of OS and switchover software for help with setting up NFS correctly for your database.

·        Failure and Switchover of SAP NetWeaver AS Units

The most critical units of the SAP NetWeaver AS are the CI and the DB. Failure of a machine hosting one of these has a severe impact on the availability of the SAP NetWeaver ASsystem. The impact differs, depending on the unit that is affected:

0     The most important consequence of CI or SCS host failure is the loss of the enqueue service. The locks held by the SAP NetWeaver ASsystem are lost and the enqueue service has to be restarted, unless you are using replicated enqueue. The message service is also disabled. Communication between the different application servers in the distributed system also fails or is impeded.

Ў        Loss of the database host implies that transactions accessing the database are held up as long as the DB is unavailable. If the DB reconnect feature is enabled, work processes on another unaffected application server can reconnect to the database system that is started up after switchover.