这几天遇到一个案例，需要将某CI作为文件上传专用的Applcation Server，而不允许其他的数据加载Job在这个AS运行。

方法就是前篇所说的如何实现负载均衡。

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

SCN 的一些片段

Shaji Jacob

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.

Ibrahim Hatem

顺记：

What is SAP Instance & SID
  What is an Instance?
An SAP R/3 instance defines a group of resources such as
    memory
    work processes
    dispatcher
    gateway

usually for a single application or database server within an SAP R/3 client-server environment.

Basically there are three types of instances:-

    Dialog instance
    Central Instance
    Database Instance

1   SAP System= Dialog Instance + Central Instance + Database Instance.

For one SAP system all three instances share the same directory.

    Dialog Instance:- Dialog instance exists in the application layer.Its purpose is to maintain load on the server.Dialog instance exists on different host.If number of dialog instance increases hardware resources,dispatcher,workprocesses also increases so that more number of users can login at a time.

    Central Instance:- Central instance can also work as dialog instance.But the main thing is that it contains Enqueue and message servers. All dialog instances communicate with central instance before requesting  database with message server. When an instance is started, the dispatcher process attempts to establish a connection to the message server so that it can announce the services it provides (DIA, BTC, SPO, UPD, etc.).Lock table is managed in central instance by enqueue service.

    Database Instance:- As normal database instance accepts requests from central instance to fulfill the user's requests. As lock management system provided by enqueue server it will provide service to users.

What is  SID ?

Each R/3 installation (SAP system) of a database server and several app servers running the application logic is uniquely identified by a single SID (SAP System Identification), SAPSID — a three-character code such as C11, PRD, E56, etc.),

Logical System Names:-
When data is distributed between different systems, each system within a network has to be clearly identifiable. The "logical system" deals with this issue.

A logical system is an application system in which the applications work together on a common database. In SAP terms, the logical system is a client.

Since the logical system name is used to identify a system uniquely within the network, two systems cannot have the same name if they are connected to each other as BW systems or as source systems, or if there are plans to connect them in any way.

Example for production system logical system name might be:-

SID – PBG

SID Description - P=Production(type) , B=BW(component) , G=Germany.(plant name)

Logical System name-

PBGCLNT100.This form is easy to understand.

参考文献:

24092 Distribution of background jobs on application server.pdf
612838 Administration of job server groups.pdf