APPLICATION SERVER ARCHITECTURE

                  An application server is a set of executables that collectively interpret the ABAP/4 programs and manage the input and output for them.

                  If your ABAP/4 program requests information from the database, the application server will format the request and send it to the database server.

                  All requests that come in from presentation servers are directed first to the dispatcher.

Dispatcher:

• The dispatcher is the link between the work processes and the users logged onto the application server.


• The dispatcher writes the task first to the dispatcher queue.


• The dispatcher pulls the requests from the queue on a first-in, first-out basis.


• Each request is then allocated to the first available work process.

Work process:

• An application server contains work processes, which are components that can run an application

• It handles one request at a time.

• To perform any processing for a user’s request, a work process needs to address two special memory areas:

                     1.The user context -It is a memory area that contains information about the user. 

                     2. The program roll area- It is a memory area that contains information about the        programs execution.

Understanding a User Context:

•  A user context is memory that is allocated to contain the characteristics of a user that is logged on the R/3 system.

•  It holds information needed by R/3 about the user, such as:

1.  The user’s current settings
2.  The user’s authorizations
3.  The names of the programs the user is currently running

• When a user logs on, a it is allocated for that logon. When they log off, it is freed.

Understanding a Roll Area:

• A roll area is memory that is allocated by a work process for an instance of a program.

•  It holds information needed by R/3 about the program’s execution, such as:

1. The values of the variables
2. The dynamic memory allocations
3. The current program pointer

•  Each time a user starts a program, a roll area is created for that instance of the program.

•  If two users run the same program at the same time, two roll areas will exist-one for each user. The roll area is freed when the program ends.

Types of Work Processes:

• There are seven types of work processes.

•  Each handles a specific type of request.

• The type of work processes and the types of requests that they handle are shown in Table.

WP Type	Request Type
D (Dialog)	·         Dialog requests ·         Deals with requests from an active user to execute dialog steps. ·         Responsible for user interaction in the SAP R/3 interface. ·         Sends user req to db and displays output of req to the user
V (Update)	·         Requests to update data in the database. ·         Responsible for consistency in asynchronous data changes.
B (Background)	·         Background jobs ·         It process programs that can be executed without user interaction ie in background at a specific time.
S (Spool)	·         Print spool requests for printing job. ·         It contains information about printer and the printing format.
E (Enqueue)	·         Logical lock requests. ·         It administers a lock table in the shared memory area. ·         It manages simultaneous database access my multiple application servers.
M (Message)	·         Routes messages between application servers within an R/3 system. ·         Used for Logon purpose and load balancing.
G (Gateway)	·         Funnels messages into and out of the R/3 system. ·         Used for transport of bigger amount of data between application servers as well as external (non SAP) systems that communicates with SAP.

Components of a Work Process:

•  Each work process is composed of the following:

1.  A task handler - All requests pass through the task handler, which then funnels the request to the appropriate part of the work process
2. An ABAP/4 Processor - interprets and executes the ABAP/4 Programs.
3. A screen interpreter - controls a large part of the user interaction. The R/3 Basis system contains a special language for programming screen flow logic. The screen processor executes the screen flow logic.
4. A database interface - handles the job of communicating with the database.