[ShivaniX]

What are presentation, application and database servers in SAP R/3?

[srinivas_naik]

SAP R/3 architecture is based on a three-tier client/server model:
·Presentation Server
·Application Server
·Database Server
Though it is a three-tier architecture model, it is not restricted only in three-tier; it can go up to multi-tier client-server system. In SAP the software components are arranged in tiers and function depending on their position. SAP R/3 must have at least one presentation server, one application server and exactly one database server.
Presentation Server

Out of the three-tie, presentation server is the one which runs on user workstation. The SAP graphical user interface (SAP GUI) is run on this layer. No application logic is processed in this layer. SAP GUI does not adhere to the style guidelines of its host system. The layer contains the software component that makes up the SAP GUI (graphical user interface). This layer is the interface between the R/3 system and its users. The SAP R/3 uses the SAP GUI to provide 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 SAP GUI component is running, it remains linked to user’s terminal session in the R/3 system.
Application Server
Another tier in the SAP R/3 system is the application server where the actual business logic is being executed. It sends the data to be presented to the user, to the Presentation layer. It comprises the business administration "know-how" of the system and processes pre–defined and user–defined application programs, such as OLTP and the implementation of decision support queries. Application servers are usually connected via a local area network with the database server.
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. Not practical, you only need one application server to run an R/3 system. But 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.
Database Server
The next tier is the database server where the actual RDBMS lies. This layer holds the system – wide database and the central booking process of the SAP R/3 architecture. The Database layer is comprised of a central database system, which contains all the data in the R/3 system. The database system has two components – the database management system (DBMS) and the database itself. The R/3 support the database system from the suppliers, such as ADABAS D, DB2/400 (on AS/400), DB2 Common Server, DB2/MVS, INFORMIX, Microsoft SQL Server, ORACLE, and ORACLE Parallel Server; SAP does not manufacture its own database.
The database not only contains the master data and transaction data from your business applications, but also the data for the entire R/3 system is stored here.