Instance Server versus Application Server

An instance is a group of R/3 services that are started and stopped together. Usually, an instance is one dispatcher with its work processes, although other standalone services such as a gateway can be called an instance.

A Central instance is a dispatcher offering all the R/3 System processes: DVEBMGS.

Instance shows all the processes except the gateway (G).

An R/3 application server is a computer where one or more R/3 instances are running.

An R/3 System consists of one or more R/3 instances. The instances can run on one or more computers. Each instance belongs to exactly one R/3 System.

From the hardware point of view, however, an application server can be defined as a computer on which at least one dispatcher, also called a dialog instance, is running.

The following restrictions apply to the number of each type of work process:

·         Dialog (D): each dispatcher needs at least 2 dialog work processes.

·         Spool (S): at least 1 per R/3 System (more than 1 per dispatcher allowed)

·         Update (V): at least 1 per R/3 System (more than 1 per dispatcher allowed)

·         Background (B): at least 2 per R/3 System (more than 1 per dispatcher allowed)

·         Enqueue (E): exactly 1 per R/3 System (only 1 E work process is required and allowed)