The difference between the logical view and a hardware- or software-oriented view of NetWeaver AS ABAP is that not all of the components shown can be assigned to a particular hardware or software unit. The following illustration represents this logical view in form of a block diagram.
The following is a list of the tasks of the three logical components in NetWeaver AS ABAP.
The kernel and administration services component is a runtime environment for all ABAP applications that is hardware-, operating system- and database-independent. The ABAP runtime environment is written principally in C and C++. However, some low-level parts are also written in ABAP itself. The tasks of the kernel and administration services component are as follows:
All ABAP applications run on software processors (virtual machines) within this component.
User and process administration
NetWeaver AS ABAP is a multi-user environment, and each user can run several independent ABAP applications. In short, this component is responsible for the tasks that usually belong to an operating system. Users log onto the NW AS and run ABAP applications within it. In this way, they do not come into contact with the actual operating system of the host. NetWeaver AS ABAP is the only user of the host operating system.
Each NetWeaver AS ABAP is linked to a database system, consisting of a database management system (DBMS) and the database itself. The ABAP applications do not communicate directly with the database. Instead, they use administration services.
ABAP applications can communicate both with other SAP Systems and with external systems. It is also possible to access ABAP applications from external systems using a BAPI interface. The services required for communication are all part of the kernel and administration services component.
administration of NetWeaver AS ABAP
The component contains programs that allow you to monitor and control the NW AS while it is running, and to change its runtime parameters.
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 these application programs. It is fully integrated in NetWeaver AS ABAP and, like other ABAP applications, is itself written in ABAP.
The presentation components are responsible for the interaction between the ABAP applications and the user (input and output) and for desktop component integration (such as word processing and spreadsheets) into the NW AS.
The following illustration represents the software-oriented view of an SAP System. In an ABAP-based SAP System, NetWeaver AS ABAP comprises all SAP GUI components and the ABAP application servers.
An SAP 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 SAP System contains the following software layers:
The database layer, which is accessed by NetWeaver AS, consists of a central database system which in turn is made up of the database management system (DBMS) and the database itself.
The database does not only contain the master data and transaction data from your ABAP application programs; all data for NetWeaver AS ABAP is stored there. For example, the database contains the control and Customizing data that determines how NetWeaver AS runs and the ABAP application programs themselves. The components (program texts, screen definitions, menus, function modules, and so on) are stored in a special part of the database known as the Repository, and are therefore also referred to as Repository objects. You work with them in the ABAP Workbench.
The software components of the application layer of NetWeaver AS ABAP consist of one or more ABAP application servers and a message server. Each application server contains a set of services used to run the NW AS. Theoretically, you only need one application server to run a NW AS. 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 a NW AS. 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.
This layer is the interface between the SAP System and its users. Using its software components referred to as SAP GUI (Graphical User Interface) this layer provides an intuitive graphical 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 SAP GUI component is running, it remains linked to a user’s terminal session in NetWeaver AS ABAP.
This software-oriented view can be expanded to include further layers, such as an Internet Transaction Server (ITS).
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 system 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 ABAP application server on a single host. A common configuration is to run the database system and a single ABAP 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.
The distribution of the SAP System over three layers means that the system load is also distributed. This leads to better system performance. Another benefit is the high scalability achieved due to the fact that you can distribute the software components of an SAP System among different hardware units virtually without any restrictions. This is particularly valuable in the application layer, where you can easily adapt NetWeaver AS ABAP to meet increasing demand by installing further ABAP application servers.
The architecture of the SAP System allows you to install the application layer and the database layer on separate hosts and let them communicate using the network. This considerably reduces the load placed on the database system which contains the central data storage for NetWeaver AS ABAP and therefore needs to fulfill high demands while the system is running.
Likewise, the separation of the presentation and the application layer reduces the load placed on the ABAP application servers by isolating user entry processing and data output formatting from actual program execution. It is important to note that the SAP GUI and the ABAP application server are set up in a way that reduces the movement of data between the two layers to the smallest possible degree. As a result, the components of the presentation layer can even be used on hosts that have slow connections to application servers a long way away.
The following illustration represents a SAP GUI user view of the SAP System:
For the user, the visible components of the SAP System are those that appear as a window on the screen. These windows are created by the presentation layer of NetWeaver AS ABAP.
Before the user logs onto the SAP System, he or she must start a SAP GUI utility called SAP Logon, which is installed at the front end. In SAP Logon, the user chooses one of the available SAP Systems. The program then connects to the message server of NetWeaver AS ABAP in the SAP System selected and obtains the address of a suitable (most lightly-used) ABAP application server. It then starts a SAP GUI, connected to that application server. The SAP Logon program is then no longer required for this connection.
SAP GUI starts the logon screen. Once the user has successfully logged on, it displays the initial screen of the SAP System in a window on the screen. Within SAP GUI, each window is represented as a session. After logging on, the user can open up to five further sessions within the single SAP GUI. These behave almost like independent SAP applications. 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 and interact with each other).
The user can open other SAP GUIs, using SAP Logon, to log onto the same system or another SAP System. The individual SAP GUIs and corresponding terminal sessions of a user are totally independent. This means that you can have SAP GUIs representing the presentation layers of several SAP Systems open on your desktop computer.