Work processes execute the individual dialog steps of ABAP application programs. They are components of ABAP application servers. The next two sections describe firstly the structure of a work process, and secondly the different types of work process in NetWeaver AS ABAP.
The following diagram shows the components of a work process:
In ABAP application programming, there is a difference between user interaction and processing logic. From a programming point of view, user interaction is controlled by screens. As well as the actual input mask, a screen also consists of flow logic, which controls a large part of the user interaction. NetWeaver AS ABAP contains a special language for programming screen flow logic. The screen processor executes the screen flow logic. Via the dispatcher, it takes over the responsibility for communication between the work process and the SAPgui, calls modules in the flow logic, and ensures that the field contents are transferred from the screen to the flow logic.
The actual processing logic of an application program is written in ABAP - SAP’s own programming language. The ABAP processor executes the processing logic of the application program, and communicates with the database interface. The screen processor tells the ABAP processor which module of the screen flow logic should be processed next. The following screen illustrates the interaction between the screen and the ABAP processors when an application program is running.
The database interface provides the following services:
· Establishing and terminating connections between the work process and the database.
· Access to database tables
· Access to Repository objects (ABAP programs, screens and so on)
· Access to catalog information (ABAP Dictionary)
· Controlling transactions (commit and rollback handling)
· Table buffer administration on the ABAP application server.
The following diagram shows the individual components of the database interface:
The diagram shows that there are two different ways of accessing databases: Open SQL and Native SQL.
Open SQL statements are a subset of Standard SQL that is fully integrated in ABAP. They allow you to access data irrespective of the database system that your installation is using. Open SQL consists of the Data Manipulation Language (DML) part of Standard SQL; in other words, it allows you to read (SELECT) and change (INSERT, UPDATE, DELETE) data. The tasks of the Data Definition Language (DDL) and Data Control Language (DCL) parts of Standard SQL are performed in NetWeaver AS ABAP by the ABAP Dictionary and the authorization system. These provide a unified range of functions, irrespective of database, and also contain functions beyond those offered by the various database systems.
Open SQL also goes beyond Standard SQL to provide statements that, in conjunction with other ABAP constructions, can simplify or speed up database access. It also allows you to buffer certain tables on the ABAP application server, saving excessive database access. In this case, the database interface is responsible for comparing the buffer with the database. Buffers are partly stored in the working memory of the current work process, and partly in the shared memory for all work processes on an ABAP application server. Where NetWeaver AS ABAP is distributed across more than one ABAP application server, the data in the various buffers is synchronized at set intervals by the buffer management. When buffering the database, you must remember that data in the buffer is not always up to date. For this reason, you should only use the buffer for data which does not often change.
Native SQL is only loosely integrated into ABAP, and allows access to all of the functions contained in the programming interface of the respective database system. In Native SQL, you can primarily use database-specific SQL statements. The Native SQL interface sends them as is to the database system where they are executed. You can use the full SQL language scope of the respective database which makes all programs using Native SQL specific for the database system installed. In addition, there is a small set of SAP-specific Native SQL statements which are handled in a special way by the Native SQL interface. ABAP applications contain as little Native SQL as possible. In fact, it is only used in a few components (for example, to create or change table definitions in the ABAP Dictionary).
The database-dependent layer in the diagram serves to hide the differences between database systems from the rest of the database interface. You choose the appropriate layer when you install NetWeaver AS ABAP. Thanks to the standardization of SQL, the differences in the syntax of statements are very slight. However, the semantics and behavior of the statements have not been fully standardized, and the differences in these areas can be greater. When you use Native SQL, the function of the database-dependent layer is minimal.
Before you start NetWeaver AS ABAP, you determine how many work processes each ABAP application server will have, and what their types will be. Since all work processes have the same structure (see preceding section), the type of work process does not determine the technical attrributes of the ABAP application server but the type of tasks to be performed on it. The dispatcher starts the work processes and only assigns them tasks that correspond to their type. This means that you can distribute work process types to optimize the use of the resources on your ABAP application servers.
The following diagram shows again the structure of an ABAP application server, but this time, includes the various possible work process types:
Dialog work processes deal with requests from an active user to execute dialog steps (see also Dialog Programming).
Update work processes execute database update requests. Update requests are part of an SAP LUW that bundle the database operations resulting from the dialog in a database LUW for processing in the background.
Background work processes process programs that can be executed without user interaction (background jobs).
The enqueue work process administers a lock table in the shared memory area. The lock table contains the logical database locks for NetWeaver AS ABAP and is an important part of the SAP LUW concept. In NW AS, you may only have one lock table. You may therefore also only have one ABAP application server with enqueue work processes. Normally, a single enqueue work process is sufficient to perform the required tasks.
The spool work process passes sequential datasets to a printer or to optical archiving. Each ABAP application server may contain only one spool work process.
The types of its work processes determine the services that an ABAP application server offers. The application server may, of course, have more than one function. For example, it may be both a dialog server and the enqueue server, if it has several dialog work processes and an enqueue work process.
You can use the system administration functions to switch a work process between dialog and background modes while the system is still running. This allows you, for example, to switch an SAP System between day and night operation, where you have more dialog than background work processes during the day, and the other way around during the night.