*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*