In older SAP releases (SAP_BASIS 750 and below) there was the possibility to configure an enqueue work process. It administers the lock table in the host’s working memory. The lock table contains the logical database locks of the ABAP run time environment of the SAP system. Only one enqueue work process is needed for an SAP system. In newer SAP releases (SAP_BASIS 700 and above) a central services instance can be used, then as an enqueue server, the enqueue is a process of the central services instance, not a dispatcher work process.
To summarize, the dispatcher of an AS ABAP manages different types of work processes: dialog, background, update, and spool work processes. These work processes take on different tasks when executing the business workflows in the SAP system.
The administrator of an SAP system can use profile parameters to configure the number of different work processes.
In addition to the work processes, the AS ABAP runtime system provides additional services (these are not work processes) for internal and external communication:
The ABAP Message Server (MS) handles communication between the distributed dispatchers within the AS ABAP, enabling scalability of several parallel application servers. It is configured exactly once for each SAP system and may be part of the central services instance.
The Enqueue Server (ES) manages the lock table with the logical database locks of the ABAP runtime environment of the SAP system. It exists once for each SAP system and is part of the central services instance.
The RFC gateway reader (GW) enables communication between SAP systems or between SAP systems and external application systems. There is one per dispatcher.
The Internet Communication Manager (ICM) enables the communication with the SAP system using web protocols such as HTTP, HTTPS, and SMTP. The ICM receives requests from the client and forwards them to the SAP system for processing. It can also direct HTTP requests from an SAP system to a Web server and send the response back to the SAP system. There is one ICM process per dispatcher.
Note
The usage of a central service instance is possible since AS ABAP 7.00. The usage of a central service instance is mandatory since AS ABAP 7.50 in case more than one application server is installed, and it is mandatory as of AS ABAP 7.51 in any case. When newly installing an SAP system, a central service instance is installed in any case.Note
To display an overview of the application servers of an AS ABAP installation, call transaction SM51.
Call transaction SM50 to obtain an overview of the work processes of the SAP system. The initial screen of this transaction only displays the work processes that are configured on the instance on which your logon session is running.
The AS ABAP Message Server provides the AS ABAP with a central message service for internal communication (for example, for starting updates, requesting and removing locks, triggering background requests). The message server also provides information on which application servers of the system are currently available.
The ABAP dispatchers of the individual application servers communicate via the message server, which is installed exactly once per SAP system.
When you log on to the AS ABAP using the SAP GUI for Windows or the SAP GUI for Java using logon groups, the message server performs a load distribution of users to the available instances. This load distribution, which takes place during the logon procedure, is also known as logon load balancing. After the load balancing by the message server, the SAP GUI communicates directly with the dispatcher. The user remains logged on to this application server until log off.
Note
To display an overview of users who are logged on to the same application server you are, call transaction SM04.
To see which application server you are logged on to, choose System → Status....
As of AS ABAP 7.40, you can also see all the users logged on system-wide in transaction SM04.
If you are accessing the AB ABAP via Web protocols such as HTTP using the browser, the Internet Communication Manager (ICM) receives the request. This forwards the request to the dispatcher of its application server.
Communication from other SAP systems via Remote Function Call (RFC) is accepted by the Gateway Reader (GW).
The following figure shows the architecture of an AS ABAP based SAP system:
The Primary Application Server (PAS) is the application server that was first installed for the SAP system. If an AS ABAP-based SAP system does not have a central service instance, then the PAS also includes the enqueue work process and the message server.
The PAS is located at file system level at /usr/sap/<SID>/D<instance_number>.
Note
In older releases, the PAS was located at /usr/sap/<SID>/DVEBMGS<instance_number>.You cannot log on to the central services instance as a dialog user. The central services instance is not called an application server.
The central services instance is located at file system level at /usr/sap/<SID>/ASCS<instance_number>.
All other application servers of an SAP system are called Additional Application Servers (AAS).
An AAS is located at file system level at /usr/sap/<SID>/D<instance_number>.
If the PAS and database (and the central services instance, if any) are installed on the same host, this is called a central system.
The following changes to architecture and terminology were introduced with AS ABAP 7.10:
Optionally, an integrated SAP Web Dispatcher and an integrated RFC GW can be installed as part of the central services instance.
The application server that is installed first is often referred to as the Primary Application Server, or PAS for short. Further application servers for a system are referred to as Additional Application Servers, or AAS for short.
