The R/3 Basis System: Overview
The R/3 Basis system is the platform for all other applications
(financial accounting, logistics, human resources management) in the R/3
System.
This documentation explains just what the Basis system is, and how it ties in with the R/3 System as a whole. It starts by introducing the Basis system in general. The second part concentrates on one central component - the application server. Finally, it will explain about work processes, which are components of the application server.
Position of the Basis System Within the R/3 System
The following sections describe three different views of the R/3 System, which show the role of the Basis system. Logical View The following illustration represents a logical view of the R/3 System.
The difference between the logical view and a hardware- or software-based view is that not all of the above components can be assigned to a particular hardware or software unit. The above diagram shows how the R/3 Basis system forms a central platform within the R/3 System. Below are listed the tasks of the three logical components of the R/3 Basis system. Kernel and Basis Services The kernel and basis services component is a runtime environment for all R/3 applications that is hardware-, operating system- and database-specific. The runtime environment is written principally in C and C++. However, some parts are also written in ABAP. The tasks of the kernel and basis services component are as follows:
This documentation explains just what the Basis system is, and how it ties in with the R/3 System as a whole. It starts by introducing the Basis system in general. The second part concentrates on one central component - the application server. Finally, it will explain about work processes, which are components of the application server.
Position of the Basis System Within the R/3 System
The following sections describe three different views of the R/3 System, which show the role of the Basis system. Logical View The following illustration represents a logical view of the R/3 System.
The difference between the logical view and a hardware- or software-based view is that not all of the above components can be assigned to a particular hardware or software unit. The above diagram shows how the R/3 Basis system forms a central platform within the R/3 System. Below are listed the tasks of the three logical components of the R/3 Basis system. Kernel and Basis Services The kernel and basis services component is a runtime environment for all R/3 applications that is hardware-, operating system- and database-specific. The runtime environment is written principally in C and C++. However, some parts are also written in ABAP. The tasks of the kernel and basis services component are as follows:
- Running applications
All R/3 applications run on software processors (virtual machines) within this component. - User and process administration
An R/3 System is a multi-user environment, and each user can run several independent applications. In short, this component is responsible for the tasks that usually belong to an operating system. Users log onto the R/3 System and run applications within it. In this way, they do not come into contact with the actual operating system of the host. The R/3 System is the only user of the host operating system. - Database access
Each R/3 System is linked to a database system, consisting of a database management system (DBMS) and the database itself. The applications do not communicate directly with the database. Instead, they use Basis services. - Communication
R/3 applications can communicate with other R/3 Systems and with non-SAP systems. It is also possible to access R/3 applications from external systems using a BAPI interface. The services required for communication are all part of the kernel and basis services component. - System Monitoring and Administration
The component contains programs that allow you to monitor and control the R/3 System while it is running, and to change its runtime parameters.
ABAP Workbench
The ABAP Workbench component is a fully-fledged development environment
for applications in the ABAP language. With it, you can create, edit,
test, and organize application developments. It is fully integrated in
the R/3 Basis system and, like other R/3 applications, is itself written
in ABAP.
Presentation Components
The presentation components are responsible for the interaction between the R/3 System and the user, and for desktop component integration (such as word processing and spreadsheets).
Software-oriented View
The following illustration represents a software-oriented view of the
R/3 System. The software-oriented view describes the various software
components that make up the R/3 System. In the software-oriented view,
all of the SAPgui components and application servers in the R/3 System
make up the R/3 Basis system.
The R/3 Basis system is a multi-tier client/server system. The individual software components are arranged in tiers and function, depending on their position, as a client for the components below them or a server for the components above them. The classic configuration of an R/3 System contains the following software layers: Database Layer The database layer consists of a central database system containing all of the data in the R/3 System. The database system has two components - the database management system (DBMS), and the databse itself. SAP does not manufacture its own database. Instead, the R/3 System supports the following database systems from other suppliers: ADABAS D, DB2/400 (on AS/400), DB2/Common Server, DB2/MVS,INFORMIX, Microsoft SQL Server, ORACLE, and ORACLE Parallel Server.
The database does not only contain the master data and transaction data from your business applications, all data for the entire R/3 System is stored there. For example, the database contains the control and Customizing data that determine how your R/3 System runs. It also contains the program code for your applications. Applications consist of program code, screen definitions, menus, function modules, and various other components. These are stored in a special section of the database called the R/3 Repository, and are accordingly called Repository objects. You work with them in the ABAP Workbench. Application Layer The application layer consists of one or more application servers and a message server. Each application server contains a set of services used to run the R/3 System. Theoretically, you only need one application server to run an R/3 System. In practice, the services are distributed across more than one application server. This means that not all application servers will provide the full range of services. The message server is responsible for communication between the application servers. It passes requests from one application server to another within the system. It also contains information about application server groups and the current load balancing within them. It uses this information to choose an appropriate server when a user logs onto the system. Presentation Layer The presentation layer contains the software components that make up the SAPgui (graphical user interface). This layer is the interface between the R/3 System and its users. The R/3 System uses the SAPgui to provide an intuitive graphical user interface for entering and displaying data. The presentation layer sends the user’s input to the application server, and receives data for display from it. While a SAPgui component is running, it remains linked to a user’s terminal session in the R/3 System.
This software-oriented view can be expanded to include further layers, such as an Intenet Transaction Server (ITS). Software-oriented and Hardware-oritented View The software-oriented view has nothing to do with the hardware configuration of the system. There are many different hardware configuration possibilities for both layers and components. When distributing the layers, for example, you can have all layers on a single host, or, at the other extreme, you could have at least one host for each layer. When dealing with components, the distribution of the database components depends on the database sytsem you are using. The application layer and presentation layer components can be distributed across any number of hosts. It is also possible to install more than one application server on a single host. A common configuration is to run the database system and a single application server (containing special database services) on one host, and to run each further application server on its own host. The presentation layer components usually run on the desktop computers of the users. Advantages of the Multi-tier Architecture The distribution of the R/3 software over three layers means that the system load is also distributed. This leads to better system performance.
Since the database system contains all of the data for the entire R/3 System, it is subject to a very heavy load when the sytsem is running. It is therefore a good idea not to run application programs on the same host. The architecture of the R/3 System, in which the application layer and database layer are separate, allows you to install them on separate hosts and let them communicate using the network.
It also makes sense to separate program execution from the tasks of processing user input and formatting data output. This is made possible by separating the presentation layer and the application layer. SAPgui and the application servers are designed so that the minimum amount of data has to be transported between the two layers. This means that the presentation layer components can even be used on hosts that have slow connections to application servers a long way away.
The system is highly scalable, due to the fact that the software components of an R/3 System can be distributed in almost any configuration across various hosts. This is particularly valuable in the application layer, where you can easily adapt your R/3 System to meet increasing demand by installing further application servers. Consequences for Application Programming The fact that the application and presentation layers are separate carries an important consequence for application programmers. When you run an application program that requires user interaction, control of the program is continually passed backwards and forwards between the layers. When a screen is ready for user input, the presentation layer is active, and the application server is inactive with regard to that particular program, but free for other tasks. Once the user has entered data on the screen, program control passes back to the application layer. Now, the presentation layer is inactive. The SAPgui is still visible to the user during this time, and it is still displaying the screen, but it cannot accept user input The SAPgui does not become active again until the application program has called a new screen and sent it to the presentation server.
As a consequence, the program logic in an application program that occurs between two screens is known as a dialog step.
User-oriented View The following illustration represents a user-oriented view of the R/3 System:
For the user, the visible components of the R/3 System are those that appear as a window on the screen. The windows are generated by the presentation layer of the R/3 System, and form a part of the R/3 Basis system.
Before the user logs onto the R/3 System, he or she must start a utility called SAP Logon, which is installed at the front end. In SAP Logon, the user chooses one of the available R/3 Systems. The program then connects to the message server of that system and obtains the address of a suitable (most lightly-used) application server. It then starts a SAPgui, connected to that application server. The SAP Logon program is then no longer required for this connection.
SAPgui starts the logon screen. Once the user has successfully logged on, it displays the initial screen of the R/3 System in an R/3 window on the screen. Within SAPgui, the R/3 window is represented as a session. After logging on, the user can open up to five further sessions (R/3 windows) within the single SAPgui. These behave almost like independent SAPguis. The different sessions allow you to run different applications in parallel, independently of one another.
Within a session, the user can run applications that themselves call further windows (such as dialog boxes and graphic windows). These windows are not independent - they belong to the session from which they were called. These windows can be either modal (the original window is not ready for input) or amodal (both windows are ready for input).
The user can open other SAPguis, using SAP Logon, to log onto the same system or another R/3 System. The individual SAPguis and corresponding R/3 terminal sessions are totally independent. This means that you can have SAPguis representing the presentation layers of several R/3 Systems open on your desktop computer.
Application Servers
R/3 programs run on application servers. They are an important component of the R/3 System. The following sections describe application servers in more detail. Structure of an Application Server The application layer of an R/3 System is made up of the application servers and the message server. Application programs in an R/3 System are run on application servers. The application servers communicate with the presentation components, the database, and also with each other, using the message server.
The following diagram shows the structure of an application server:
The individual components are: Work Processes An application server contains work processes, which are components that can run an application. Each work process is linked to a memory area containing the context of the application being run. The context contains the current data for the application program. This needs to be available in each dialog step. Further information about the different types of work process is contained later on in this documentation. Dispatcher Each application server contains a dispatcher. The dispatcher is the link between the work processes and the users logged onto the application server. Its task is to receive requests for dialog steps from the SAPgui and direct them to a free work process. In the same way, it directs screen output resulting from the dialog step back to the appropriate user. Gateway Each application server contains a gateway. This is the interface for the R/3 communication protocols (RFC, CPI/C). It can communicate with other application servers in the same R/3 System, with other R/3 Systems, with R/2 Systems, or with non-SAP systems.
The application server structure as described here aids the performance and scalability of the entire R/3 System. The fixed number of work processes and dispatching of dialog steps leads to optimal memory use, since it means that certain components and the memory areas of a work process are application-independent and reusable. The fact that the individual work processes work independently makes them suitable for a multi-procecssor architecuture. The methods used in the dispatcher to distribute tasks to work processes are discussed more closely in the section Dispatching Dialog Steps. Shared Memory All of the work processes on an application server use a common main memory area called shared memory to save contexts or to buffer constant data locally.
The resources that all work processes use (such as programs and table contents) are contained in shared memory. Memory management in the R/3 System ensres that the work processes always address the correct context, that is the data relevant to the current state of the program that is running. A mapping process projects the required context for a dialog step from shared memory into the address of the relevant work process. This reduces the actual copying to a minimum.
Local buffering of data in the shared memory of the application server reduces the number of database reads required. This reduces access times for application programs considerably. For optimal use of the buffer, you can concentrate individual applications (financial accounting, logistics, human resources) into separate application server groups. Database Connection When you start up an R/3 System, each application server registers its work proceses with the database layer, and receives a single dedicated channel for each. While the system is running, each work process is a user (client) of the database system (server). You cannot change the work process registration while the system is running. Neither can you reassign a database channel from one work process to another. For this reason, a work process can only make database changes within a single database logical unit of work (LUW). A database LUW is an inseparable sequence of database operations. This has important consequences for the programming model explained below. Dispatching Dialog Steps The number of users logged onto an application server is often many times greater than the number of available work processes. Furthermore, it is not restricted by the R/3 system architecture. Furthermore, each user can run several applications at once. The dispatcher has the important task of distributing all dialog steps among the work processes on the application server.
The following diagram is an example of how this might happen:
The R/3 Basis system is a multi-tier client/server system. The individual software components are arranged in tiers and function, depending on their position, as a client for the components below them or a server for the components above them. The classic configuration of an R/3 System contains the following software layers: Database Layer The database layer consists of a central database system containing all of the data in the R/3 System. The database system has two components - the database management system (DBMS), and the databse itself. SAP does not manufacture its own database. Instead, the R/3 System supports the following database systems from other suppliers: ADABAS D, DB2/400 (on AS/400), DB2/Common Server, DB2/MVS,INFORMIX, Microsoft SQL Server, ORACLE, and ORACLE Parallel Server.
The database does not only contain the master data and transaction data from your business applications, all data for the entire R/3 System is stored there. For example, the database contains the control and Customizing data that determine how your R/3 System runs. It also contains the program code for your applications. Applications consist of program code, screen definitions, menus, function modules, and various other components. These are stored in a special section of the database called the R/3 Repository, and are accordingly called Repository objects. You work with them in the ABAP Workbench. Application Layer The application layer consists of one or more application servers and a message server. Each application server contains a set of services used to run the R/3 System. Theoretically, you only need one application server to run an R/3 System. In practice, the services are distributed across more than one application server. This means that not all application servers will provide the full range of services. The message server is responsible for communication between the application servers. It passes requests from one application server to another within the system. It also contains information about application server groups and the current load balancing within them. It uses this information to choose an appropriate server when a user logs onto the system. Presentation Layer The presentation layer contains the software components that make up the SAPgui (graphical user interface). This layer is the interface between the R/3 System and its users. The R/3 System uses the SAPgui to provide an intuitive graphical user interface for entering and displaying data. The presentation layer sends the user’s input to the application server, and receives data for display from it. While a SAPgui component is running, it remains linked to a user’s terminal session in the R/3 System.
This software-oriented view can be expanded to include further layers, such as an Intenet Transaction Server (ITS). Software-oriented and Hardware-oritented View The software-oriented view has nothing to do with the hardware configuration of the system. There are many different hardware configuration possibilities for both layers and components. When distributing the layers, for example, you can have all layers on a single host, or, at the other extreme, you could have at least one host for each layer. When dealing with components, the distribution of the database components depends on the database sytsem you are using. The application layer and presentation layer components can be distributed across any number of hosts. It is also possible to install more than one application server on a single host. A common configuration is to run the database system and a single application server (containing special database services) on one host, and to run each further application server on its own host. The presentation layer components usually run on the desktop computers of the users. Advantages of the Multi-tier Architecture The distribution of the R/3 software over three layers means that the system load is also distributed. This leads to better system performance.
Since the database system contains all of the data for the entire R/3 System, it is subject to a very heavy load when the sytsem is running. It is therefore a good idea not to run application programs on the same host. The architecture of the R/3 System, in which the application layer and database layer are separate, allows you to install them on separate hosts and let them communicate using the network.
It also makes sense to separate program execution from the tasks of processing user input and formatting data output. This is made possible by separating the presentation layer and the application layer. SAPgui and the application servers are designed so that the minimum amount of data has to be transported between the two layers. This means that the presentation layer components can even be used on hosts that have slow connections to application servers a long way away.
The system is highly scalable, due to the fact that the software components of an R/3 System can be distributed in almost any configuration across various hosts. This is particularly valuable in the application layer, where you can easily adapt your R/3 System to meet increasing demand by installing further application servers. Consequences for Application Programming The fact that the application and presentation layers are separate carries an important consequence for application programmers. When you run an application program that requires user interaction, control of the program is continually passed backwards and forwards between the layers. When a screen is ready for user input, the presentation layer is active, and the application server is inactive with regard to that particular program, but free for other tasks. Once the user has entered data on the screen, program control passes back to the application layer. Now, the presentation layer is inactive. The SAPgui is still visible to the user during this time, and it is still displaying the screen, but it cannot accept user input The SAPgui does not become active again until the application program has called a new screen and sent it to the presentation server.
As a consequence, the program logic in an application program that occurs between two screens is known as a dialog step.
User-oriented View The following illustration represents a user-oriented view of the R/3 System:
For the user, the visible components of the R/3 System are those that appear as a window on the screen. The windows are generated by the presentation layer of the R/3 System, and form a part of the R/3 Basis system.
Before the user logs onto the R/3 System, he or she must start a utility called SAP Logon, which is installed at the front end. In SAP Logon, the user chooses one of the available R/3 Systems. The program then connects to the message server of that system and obtains the address of a suitable (most lightly-used) application server. It then starts a SAPgui, connected to that application server. The SAP Logon program is then no longer required for this connection.
SAPgui starts the logon screen. Once the user has successfully logged on, it displays the initial screen of the R/3 System in an R/3 window on the screen. Within SAPgui, the R/3 window is represented as a session. After logging on, the user can open up to five further sessions (R/3 windows) within the single SAPgui. These behave almost like independent SAPguis. The different sessions allow you to run different applications in parallel, independently of one another.
Within a session, the user can run applications that themselves call further windows (such as dialog boxes and graphic windows). These windows are not independent - they belong to the session from which they were called. These windows can be either modal (the original window is not ready for input) or amodal (both windows are ready for input).
The user can open other SAPguis, using SAP Logon, to log onto the same system or another R/3 System. The individual SAPguis and corresponding R/3 terminal sessions are totally independent. This means that you can have SAPguis representing the presentation layers of several R/3 Systems open on your desktop computer.
Application Servers
R/3 programs run on application servers. They are an important component of the R/3 System. The following sections describe application servers in more detail. Structure of an Application Server The application layer of an R/3 System is made up of the application servers and the message server. Application programs in an R/3 System are run on application servers. The application servers communicate with the presentation components, the database, and also with each other, using the message server.
The following diagram shows the structure of an application server:
The individual components are: Work Processes An application server contains work processes, which are components that can run an application. Each work process is linked to a memory area containing the context of the application being run. The context contains the current data for the application program. This needs to be available in each dialog step. Further information about the different types of work process is contained later on in this documentation. Dispatcher Each application server contains a dispatcher. The dispatcher is the link between the work processes and the users logged onto the application server. Its task is to receive requests for dialog steps from the SAPgui and direct them to a free work process. In the same way, it directs screen output resulting from the dialog step back to the appropriate user. Gateway Each application server contains a gateway. This is the interface for the R/3 communication protocols (RFC, CPI/C). It can communicate with other application servers in the same R/3 System, with other R/3 Systems, with R/2 Systems, or with non-SAP systems.
The application server structure as described here aids the performance and scalability of the entire R/3 System. The fixed number of work processes and dispatching of dialog steps leads to optimal memory use, since it means that certain components and the memory areas of a work process are application-independent and reusable. The fact that the individual work processes work independently makes them suitable for a multi-procecssor architecuture. The methods used in the dispatcher to distribute tasks to work processes are discussed more closely in the section Dispatching Dialog Steps. Shared Memory All of the work processes on an application server use a common main memory area called shared memory to save contexts or to buffer constant data locally.
The resources that all work processes use (such as programs and table contents) are contained in shared memory. Memory management in the R/3 System ensres that the work processes always address the correct context, that is the data relevant to the current state of the program that is running. A mapping process projects the required context for a dialog step from shared memory into the address of the relevant work process. This reduces the actual copying to a minimum.
Local buffering of data in the shared memory of the application server reduces the number of database reads required. This reduces access times for application programs considerably. For optimal use of the buffer, you can concentrate individual applications (financial accounting, logistics, human resources) into separate application server groups. Database Connection When you start up an R/3 System, each application server registers its work proceses with the database layer, and receives a single dedicated channel for each. While the system is running, each work process is a user (client) of the database system (server). You cannot change the work process registration while the system is running. Neither can you reassign a database channel from one work process to another. For this reason, a work process can only make database changes within a single database logical unit of work (LUW). A database LUW is an inseparable sequence of database operations. This has important consequences for the programming model explained below. Dispatching Dialog Steps The number of users logged onto an application server is often many times greater than the number of available work processes. Furthermore, it is not restricted by the R/3 system architecture. Furthermore, each user can run several applications at once. The dispatcher has the important task of distributing all dialog steps among the work processes on the application server.
The following diagram is an example of how this might happen:
- The dispatcher receives the request to execute a dialog step from user 1 and directs it to work process 1, which happens to be free. The work process addresses the context of the application program (in shared memory) and executes the dialog step. It then becomes free again.
- The dispatcher receives the request to execute a dialog step from user 2 and directs it to work process 1, which is now free again. The work process executes the dialog step as in step 1.
- While work process 1 is still working, the dispatcher receives a further request from user 1 and directs it to work process 2, which is free.
- After work processes 1 and 2 have finished processing their dialog steps, the dispatcher receives another request from user 1 and directs it to work process 1, which is free again.
- While work process 1 is still working, the dispatcher receives a further request from user 2 and directs it to work process 2, which is free.
From this example, we can see that:
- A dialog step from a program is assigned to a single work process for execution.
- The individual dialog steps of a program can be executed on different work processes, and the program context must be addressed for each new work process.
- A work process can execute dialog steps of different programs from different users.
The example does not show that the dispatcher tries to distribute the
requests to the work processes such that the same work process is used
as often as possible for the successive dialog steps in an application.
This is useful, since it saves the program context having to be
addressed each time a dialog step is executed.
Dispatching and the Programming Model
The separation of application and presentation layer made it
necessary to split up application programs into dialog steps. This, and
the fact that dialog steps are dispatched to individual work processes,
has had important consequences for the programming model.
As mentioned above, a work process can only make database changes within a single database logical unit of work (LUW). A database LUW is an inseparable sequence of database operations. The contents of the database must be consistent at its beginning and end. The beginning and end of a database LUW are defined by a commit command to the database system (database commit). During a database LUW, that is, between two database commits, the database system itself ensures consistency within the database. In other words, it takes over tasks such as locking database entries while they are being edited, or restoring the old data (rollback) if a step terminates in an error.
A typical SAP application program extends over several screens and the corresponding dialog steps. The user requests database changes on the individual screens that should lead to the database being consistent once the screens have all been processed. However, the individual dialog steps run on different work processes, and a single work process can process dialog steps from other applications. It is clear that two or more independent applications whose dialog steps happen to be processed on the same work process cannot be allowed to work with the same database LUW.
Consequently, a work process must open a separate datables LUW for each dialog step. The work process sends a commit command (database commit) to the database at the end of each dialog step in which it makes database changes. These commit commands are called implicit database commits, since they are not explicitly written into the application program.
These implicit database commits mean that a database LUW can be kept open for a maximum of one dialog step. This leads to a considerable reduction in database load, serialization, and deadlocks, and enables a large number of users to use the same system.
However, the question now arises of how this method (1 dialog step = 1 database LUW) can be reconciled with the demand to make commits and rollbacks dependent on the logical flow of the application program instead of the technical distribution of dialog steps. Database update requests that depend on one another form logical units in the program that extend over more than one dialog step. The database changes associated with these logical units must be executed together and must also be able to be undone together.
The SAP programming model contains a seies of bundling techniques that allow you to group database updates together in logical units. The section of an R/3 application program that bundles a set of logically-associated database operations is called an SAP LUW. Unlike a database LUW, a SAP LUW includes all of the dialog steps in a logical unit, including the database update.
Work Processes
Work processes execute the individual dialog steps in R/3 applications. The next two sections describe firstly the structure of a work process, and secondly the different types of work process in the R/3 System. Structure of a Work Process Work processes execute the dialog steps of application programs. They are components of an application server. The following diagram shows the components of a work process:
Each work process contains two software processors and a database interface. Screen Processor In R/3 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. The screen flow logic controls a large part of the user interaction. The R/3 Basis system 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.
ABAP-Prozessor The actual processing logic of an application program is written in ABAP - SAP’s own programing 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.
Database Interface
The database interface provides the following services:
As mentioned above, a work process can only make database changes within a single database logical unit of work (LUW). A database LUW is an inseparable sequence of database operations. The contents of the database must be consistent at its beginning and end. The beginning and end of a database LUW are defined by a commit command to the database system (database commit). During a database LUW, that is, between two database commits, the database system itself ensures consistency within the database. In other words, it takes over tasks such as locking database entries while they are being edited, or restoring the old data (rollback) if a step terminates in an error.
A typical SAP application program extends over several screens and the corresponding dialog steps. The user requests database changes on the individual screens that should lead to the database being consistent once the screens have all been processed. However, the individual dialog steps run on different work processes, and a single work process can process dialog steps from other applications. It is clear that two or more independent applications whose dialog steps happen to be processed on the same work process cannot be allowed to work with the same database LUW.
Consequently, a work process must open a separate datables LUW for each dialog step. The work process sends a commit command (database commit) to the database at the end of each dialog step in which it makes database changes. These commit commands are called implicit database commits, since they are not explicitly written into the application program.
These implicit database commits mean that a database LUW can be kept open for a maximum of one dialog step. This leads to a considerable reduction in database load, serialization, and deadlocks, and enables a large number of users to use the same system.
However, the question now arises of how this method (1 dialog step = 1 database LUW) can be reconciled with the demand to make commits and rollbacks dependent on the logical flow of the application program instead of the technical distribution of dialog steps. Database update requests that depend on one another form logical units in the program that extend over more than one dialog step. The database changes associated with these logical units must be executed together and must also be able to be undone together.
The SAP programming model contains a seies of bundling techniques that allow you to group database updates together in logical units. The section of an R/3 application program that bundles a set of logically-associated database operations is called an SAP LUW. Unlike a database LUW, a SAP LUW includes all of the dialog steps in a logical unit, including the database update.
Work Processes
Work processes execute the individual dialog steps in R/3 applications. The next two sections describe firstly the structure of a work process, and secondly the different types of work process in the R/3 System. Structure of a Work Process Work processes execute the dialog steps of application programs. They are components of an application server. The following diagram shows the components of a work process:
Each work process contains two software processors and a database interface. Screen Processor In R/3 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. The screen flow logic controls a large part of the user interaction. The R/3 Basis system 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.
ABAP-Prozessor The actual processing logic of an application program is written in ABAP - SAP’s own programing 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.
Database Interface
The database interface provides the following services:- Establishing and terminating connections between the work process and the database.
- Access to database tables
- Access to R/3 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 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 the R/3 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 the R/3 System 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 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 application server. Where an R/3 System is distributed across more than one 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. Unlike Open SQL statements, Native SQL statements are not checked and converted, but instead are sent directly to the database system. Programs that use Native SQL are specific to the database system for which they were written. R/3 applications contain as little Native SQL as possible. In fact, it is only used in a few Basis 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 the Basis system. 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.
Types of Work Process Although all work processes contain the components described above, they can still be divided into different types. The type of a work process determines the kind of task for which it is responsible in the application server. It does not specify a particular set of technical attributes. The individual tasks are distributed to the work processes by the dispatcher.
Before you start your R/3 System, you determine how many work processes it will have, and what their types will be. 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 application servers.
The following diagram shows again the structure of an application server, but this time, includes the various possible work process types:
The various work processes are described briefly below. Other parts of this documentation describe the individual components of the application server and the R/3 System in more detail. Dialog Work Process Dialog work processes deal with requests from an active user to execute dialog steps. Update Work Process 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 Process Background work processes process programs that can be executed without user interaction (background jobs). Enqueue Work Process The enqueue work process administers a lock table in the shared memory area. The lock table contains the logical database locks for the R/3 System and is an important part of the SAP LUW concept. In an R/3 System, you may only have one lock table. You may therefore also only have one application server with enqueue work processes. Spool Work Process The spool work process passes sequential datasets to a printer or to optical archiving. Each application server may contain only one spool work process.
The services offered by an application server are determined by the types of its work processes. One 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 R/3 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.
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 the R/3 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 the R/3 System 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 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 application server. Where an R/3 System is distributed across more than one 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. Unlike Open SQL statements, Native SQL statements are not checked and converted, but instead are sent directly to the database system. Programs that use Native SQL are specific to the database system for which they were written. R/3 applications contain as little Native SQL as possible. In fact, it is only used in a few Basis 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 the Basis system. 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.
Types of Work Process Although all work processes contain the components described above, they can still be divided into different types. The type of a work process determines the kind of task for which it is responsible in the application server. It does not specify a particular set of technical attributes. The individual tasks are distributed to the work processes by the dispatcher.
Before you start your R/3 System, you determine how many work processes it will have, and what their types will be. 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 application servers.
The following diagram shows again the structure of an application server, but this time, includes the various possible work process types:
The various work processes are described briefly below. Other parts of this documentation describe the individual components of the application server and the R/3 System in more detail. Dialog Work Process Dialog work processes deal with requests from an active user to execute dialog steps. Update Work Process 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 Process Background work processes process programs that can be executed without user interaction (background jobs). Enqueue Work Process The enqueue work process administers a lock table in the shared memory area. The lock table contains the logical database locks for the R/3 System and is an important part of the SAP LUW concept. In an R/3 System, you may only have one lock table. You may therefore also only have one application server with enqueue work processes. Spool Work Process The spool work process passes sequential datasets to a printer or to optical archiving. Each application server may contain only one spool work process.
The services offered by an application server are determined by the types of its work processes. One 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 R/3 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.
