Section B.2. Dispatcher, Work Processes, and Services

B 2 Dispatcher, Work Processes, and Services

The Basis run-time environment has one dispatcher as well as several work processes and services. The dispatcher shown in Figure B.3 gathers user requests and assigns them to work processes for processing. After the processing ends, the dispatcher sends responses back to users. The Dispatcher also manages the communications between SAP and external systems.

Figure B.3. DISPATCHER AND WORK PROCESSES

There are five types of work processes:

• Dialog
• Update
• Enqueue
• Background
• Spool

There are two types of services:

• Message
• Gateway

Each type of process or service has a special purpose.

B.2.1 Dialog Work Process

Dialog work processes take charge of the dialog steps involved in user sessions. If a screen is ready for processing after user input, for example, the dispatcher adds this job request to a queue. As soon as a dialog work process becomes available, the first job request waiting in the queue is submitted to it for processing.

An application server can have multiple dialog work processes.

B.2.2 Update Work Process

The update work process handles database update operations. An SAP transaction usually consists of several dialog steps. That is, it creates a log record for the dialog steps in an SAP transaction in the VBLOG table for the database update. An update work process later processes the log record at the end of the SAP transaction.

An application server can have multiple update work processes.

B.2.3 Enqueue Work Process

Each dialog step is a database LUW (logical unit of work). Because an update work process can execute more than one database LUW, the locking mechanism at database level is not sufficient in SAP.

An SAP LUW is an SAP transaction plus the database update. An enqueue work process manages this LUW. The update and enqueue work processes can run on different application servers.

A BW system has one and only one enqueue work process.

B.2.4 Background Work Process

The background work process executes ABAP/4 programs and reports at a scheduled time or in response to an event.

An application server can have multiple background work processes.

B.2.5 Spool Work Process

A spool work process generates a print-ready output stream for a variety of supported output devices and passes it to the host spool system. It relies on host spooling and printing services to transmit the print-ready output stream to a target output device. The output device can be a printer, fax, telex, or archiving device.

B.2.6 Message Service

The message service, or message handler, coordinates the internal message exchange between work processes in the same application server, and between dispatchers of different application servers. It performs load balancing among different application servers when users log on to BW.

A BW system has one and only one message service.

B.2.7 Gateway Service

The gateway service is for CPI-C (Common Programming Interface – Communications). It is used for the exchange of large amounts of data between application servers of the same SAP system or different SAP systems, and between SAP systems (via application servers) and non-SAP systems. BW BEx communicates with an application server using the gateway service.

A BW system has one and only one gateway service.

In BW, when a user logs on to a BW system through the BEx Browser, one gateway session is required to set up the communication link between the BEx Browser and the BW system. When the user fetches a workbook, four more gateway sessions are needed.