The SAP R/3 System has a modular software architecture that follows the software-oriented client/server principle.The system distributes presentation, application logic, and data storage across hosts at different levels. This forms the basis of its scalability.
The lowest level is the database level. The data is administrated on this level using a relational database management system (RDBMS). As well as master and transaction data, it also stores programs and the metadata that describes the R/3 System and the structure of its data.
The second level is the application layer. This consists of ABAP programs. You can complement the existing SAP functions by writing your own programs using the ABAP Workbench.The third level is the presentation layer (SAPgui). It contains the user interface with which end users enter and display data. You can also use alternative front ends (AFEs) at this level.
Communication interfaces and data interfaces are different. In addition to the file I/O, the communication interfaces are the gate to external communication, while the data interfaces are used for read and write accesses to the R/3 database.
The following data interfaces are available:
Batch Input, Direct Input, CALL TRANSACTION USING, SAP-SQL (OpenSQL), Native SQL.
An instance is an administrative unit in an R/3 System. It describes which services (dialog, update, enqueue, background, spool gateway) are available . .) on the corresponding application server.
The dispatcher process on an application server coordinates tasks within an instance. It communicates with other instances in the same R/3 System via the message server, and with external systems via the gateway. As of Release 3.0, a gateway service (work process) belongs to each instance (application server). You can also install a gateway service on a non-R/3 host.
When an application server communicates (dispatcher) with external systems, it usually uses its own gateway work process. However, it may be more useful to use a gateway that runs on a dedicated host, for example, for communication with SNA, where a gateway with an add-on is required.
This is done to minimize the effort needed for configuration and maintenance (the gateway add-on is only on one host instead of each application server). You may have to use a remote gateway, for example, with remote Windows NT applications, because the remote program can only be started via this gateway.
R/3 communication always runs through the SAP gateway and is based on TCP/IP.
For R/3 communication with an IBM-R/2 System or other LU6.2 systems, where the LU6.2 transport protocol is used, you need an SAP gateway with additional SNA functions.
RFC is based on CPI-C, which means an RFC call is always transmitted in CPI-C calls.
CPI-C can be based on TCP/IP or LU6.2, depending on the platform.
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