SAP based the architecture of R/3 on a three-tier client/server model.
- Presentation Server
- Application Server
- Database Server
- Presentation Server
Application Server
An application server is a set of executables that collectively interpret the ABAP/4 programs and manage the input and output for them. When an application server is started, these executables all start at the same time. When an application server is stopped, they all shut down together. The number of processes that start up when you bring up the application server is defined in a single configuration file called the application server profile.
Each application server has a profile that specifies its characteristics when it starts up and while it is running. For example, an application sever profile specifies:
- Number of processes and their types
- Amount of memory each process may use
- Length of time a user is inactive before being automatically logged off
If your ABAP/4 program requests information from the database, the application server will format the request and send it to the database server.
Discovering the Database Server
The database server is a set of executables that accept database requests from the application server. These requests are passed on to the RDBMS (Relation Database Management System). The RDBMS sends the data back to the database server, which then passes the information back to the application server. The application server in turn passes that information to your ABAP/4 program.
There is usually a separate computer dedicated to house the database server, and the RDBMS may run on that computer also, or may be installed on its own computer.
Configuring the Servers
In a three-tier client/server configuration, the presentation servers, applications servers, and database server all run on separate machines. This is the most common configuration for large systems, and is common in production.
In the distribution presentation configuration, the application and database servers are combined on one computer and the presentation servers run separately. This is used for smaller systems, and is often seen on a development system.
In the two-tier client/server configuration, the presentation and application servers are combined and the database server is separate. This configuration is used in conjunction with other application servers. It is used for a batch server when the batch is segregated from the online servers. A SAPGUI is installed on it to provide local control.
When all servers are combined onto a single machine, you have a central configuration. This is rarely seen because it describes a standalone R/3 system with only a single user.
Defining an R/3 System
The simplest definition of an R/3 system is “one database.” In one R/3 system, there is only one database. To expand the definition, R/3 is considered to be all of the components attached to that one database. One R/3 system is composed of one database server accessing a single database, one or more application servers, and one or more presentation servers. By definition, it is all of the components attached to one database. If you have one database, you have one system. If you have one system, you have one database. During an implementation, there is usually one system (or one database) assigned to development, one or more systems designated for testing, and one assigned to production.
The term R/3 system landscape denotes a description of the number of systems within an SAP installation and how they are designated, such as development, test, or production.
Defining an R/3 Instance
When you hear someone say the word instance, most of the time, that person will be referring to an application server. The term instance is synonymous with application server.
The term central instance refers to the database server. If an application server and database server both reside on the same machine, the term central instance refers to the computer on which both reside.
In the most general terms, an instance is a server. It is a set of R/3 processes providing services to the R/3 system.
