Editing SAP R/3 (section)

===Application server===
This server contains the SAP applications. In systems with two layers, this server forms part of the database server. Application server can be set up for online users, for background processing, or for both.

An application server is a collection of executables that collectively interpret the ABAP/4 (Advanced Business Application Programming / 4th Generation) 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 server 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.

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, only one application server is needed 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.

The application server exists to interpret ABAP/4 programs, and they only run there. If an ABAP/4 program requests information from the database, the application server will send the request to the database server.