In a one-tier R/3 System configuration, all processing tasks are performed on one server, as in classic mainframe processing.
Two-tier R/3 System configurations are usually implemented using special presentation servers that are responsible solely for formatting the graphical user interface. Many R/3 System users use Windows PCs for example as presentation servers. An alternative two-tier configuration (not shown) is to install powerful desktop systems and to use these for presentation and applications also (two-tier client/server).
This type of configuration is particularly useful for processing-intensive applications (such assimulations) or for software developers, but due to the additional administration requirements is usually used for test purposes only.
In a three-tier configuration, separate servers are used for each tier. Using data from the database server,several different application servers can operate at the same time. To ensure that the load on individualservers is as even as possible and to achieve optimal performance, you can use special applicationservers for individual application areas such as distribution or financial accounting (logon and load balancing).
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 calledan instance.
A central instance is a dispatcher offering all the R/3 System processes: DVEBMGS. In the graphic,Instance C 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)
If a transaction involves the use of more than one screen, the system dialog steps shown on the preceding page are normally performed by several different dialog work processes in a dispatcher. This is known as work process multiplexing.
SAP Notes:
39412 Number of workprocesses
21960 Two instances on one computer
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