Central instance and SAP instance

[former_member199632]

hello,

Im confusing with Central Instance (CI) and SAP instance (Application Instance)

According to my understanding, i think SAP instance is running on all the Aplication servers. (if having 4 Application servers, in all those servers, SAP instance is running concurrently)

But Central instance is running on 1 of the Application servers only.

- How will the SAP decides on which Application should run the central instance.

Will that CI change automatically time to time.... or do we need to allocate 1 Application server as CI server, when doing the installation.

Is there any relationship between Message server and Central instance.

Does CI contains message services and enqueue service...

plz help me to reslove these issues

Rgds,

Edited by: zerandib on Apr 25, 2010 10:45 AM

[Former Member]

Hi,

When You install SAP system first it will install the Central instance(will have all the work processes) .

Centreal instance will have the message server. (It will do the load balancing)

Enque services are loking and uloking the database objects for consistancy this will also will be available tin CI only.

after that wat ever u install comes under application servers only. (this wont have message server will have only dialog wp) according to u r requiremnt u can configure this applicaiton server work processes(suppose if u want to use this only for backgroup jobs u can chage the work process to BGD min 2 work process are req for for any applicaiton servers).

Applicaion server can be use for load balancing . to reduce the load on the central instance. using SMLG trasaction.

thanks

[former_member184158]

*CENTRAL INSTANCE: The SAP Central Instance (CI) is a standalone SAP Basis

unit which provides services used by clients connected to the SAP system.

Among these services are the Message server and the Enqueue server, which

run only on the single SAP Central Instance. The Message server maintains a

list of all available resources in an SAP system, determines which instance

a user logs on to during a client connect, and handles all communication

between SAP instances. The Enqueue server is used by SAP to administer the

lock table in a distributed SAP system. If the CI

server hosting the Enqueue service fails, all SAP transaction locks that

have not yet been committed are lost. R/3 guarantees that no user can

perform a transaction while the Enqueue service is unavailable in order to

guarantee database consistency. Placing the Enqueue and Message services

together on the CI is recommended by SAP since the Message service must

always access the Enqueue service for inter-process communication. These

services provide critical SAP functions that, by existing only in the SAP

Central Instance, suffer from being a single point of failure in the SAP

environment. Obviously, the CI, which contains the Enqueue and Message

services, needs to be restarted as quickly as possible following a failure

so that normal operations can resume.*

Central Instance Has Message server and an enque work Process,

Application Server or dialog Server : Does not have message

server but have work process to reduce the Load on CI*

[Former Member]

it seems SAP  is recommending SAP Standalone Enqueue server for HA scenario, and direct TCP connectivity from App server.

Any clarifications?

Standalone Enqueue Server - The SAP Lock Concept (BC-CST-EQ) - SAP Library

[former_member119330]

"i think SAP instance is running on all the Aplication servers"

I think here you may mix the application server with SAP instance, in fact, they're referring to one entity. we call SAP instance also as application server.

relationship between msg server and CI:

msg server is the unique component within a SAP system, the whole SAP system only has one msg server, which locates in CI. msg server mainly acts as communicator and load balancer between all application servers.

how to find out CI:

a SAP system may have more than one instance(app. server), but it only has one central instance, to determine which instance is CI, just find out which instance has the component msg server. in a simple way, provided you don't use an external enqueue server, then from SM51, you'll see ENQ service in one of the intance, this one, is the so called CI.

on Windows platform, you may check it from SAPMMC, that one of the instance has the process 'msg server', on UNIX platform, you may use the command ps -ef|grep <sid>adm, which instance has the process like "ms.sapU6D_DVEBMGS06", this one is the CI.

hope it helps.

Rgds, Jane

[Former Member]

Hi

IF you are having multiple application servers and CI then initially when user loged in then all requests goes to the instance on which user is logged on ( string u have used in sap gui). IF u want system should to all load balancing automatically then you have to use the logon groups.

Regards

Bajrang Saini

[Former Member]

How will the SAP decides on which Application should run the central instance.
Will that CI change automatically time to time.... or do we need to allocate 1 Application server as CI server, when doing the installation.

Usualy in non HA installation's central instance are located on it's own host. But in some HA installation's central instance can start's on any available application server host (for example in our HA are this configured).

P.S.

Is there any relationship between Message server and Central instance.

Does CI contains message services and enqueue service...

Try to read http://help.sap.com/saphelp_nw70ehp2/helpdata/en/47/fe7aa040e23c8be10000000a42189c/frameset.htm

Regards.