不过除了这个，重新温习了一下Central Instance， 感觉有必要记录一下。
关键字：MESSAGE SERVER / ENQUEUE SERVER
The central instance is a concept that is unique to SAP. The central instance is a combination of hardware and software. It contains a physical server (the application server) and numerous software components, including a message server, a database gateway (a pre-established connection between SAP and Oracle--or another database), and various update, enqueue, dialog, and spool facility software. In most generic SAP architectures, there are numerous application servers but only a single central instance. However, in addition to managing the SAP interfaces, the central instance can also serve as an application server.
A dialog instance is a software program that is running the SAP kernel and it is the job of the dialog instance to execute the ABAP programs and manage the requests for data and services. While there is generally a one-to-one mapping between an application server and a dialog instance, it is possible to have more than one dialog instance on an application server.
*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*
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