Central Instance

*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.* * *APPLICATION SERVER: Application server or dialog instance is installed to reduce the load on CI (Central Instance). *

DB SERVER*

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* APP SERVER APP SERVER APP SERVER* * * *

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CI Resides* Here

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*