SAP R/3 - Application Server Architecture

Below picture illustrates the basic architecture of Application Server of SAP R/3.

• Depending on Organization requirements, we can maintain one Application Server or Multiple Application Servers. 
• INSTANCE: It is nothing but dispatcher and its corresponding work process.
• Application Server consists of two types of Instances.

           1. Central Instance
           2. Dialogue or Additional Instance.

• Central Instance: An instance with the Message Server is called as Central Instance.
• All instances other than Central Instance are all called as DIALOG Instances or ADDITIONAL Instances.
• Each instance is identified by 2 digit no.
• A Dialog Instance doesn't exist on its own. It is always associated with a Central Instance.
• Message Server Performs Load Balancing. It distributes the load over different available Instances.

 The internal architecture of and Instance can be seen in the below picture.

Instance has FIVE services.

• Dispatcher
• Work Process / ABAP Memory / P L Memory
• Database Connectivity
• Gateway Services
• Message Services

Dispatcher:

• It is an interface between Presentation Layer and Application Layer.
• It can understand two types of requests(Languages) :

          1) ABAP request
          2) BAPI request – Used to communicate between SAP and Non-SAP systems. (For VB-Active X and for JAVA-Java Been)


Work Process:

• It is the memory allocated in Application Server for the Presentation Layer or ABAP Memory or PL Memory.

• There are 7 types of work Processes.

           1) Dialog WP         => Screen
           2) Update WP        => DB Updates
           3) Background WP => Background Jobs
           4) Spool WP           => Printing
           5) Enqueue WP      => Lock Management
           6) Message WP      => Internal Communications
           7) Gateway WP      => External Communications
      Only the first five WP’s can be seen and configured by the Administrator.


Data Base Connectivity:    

• It is an interface between the Application Layer and DB Layer.

• This service maintains the Connection Management with Data Base.

•  It is again a collection of two sub services.

          1) Open SQL
          2) Native SQL
          Open SQL converts the ABAP request from WP into Native data base format and sends it to Native SQL where the request is processed after verification.


Gateway Services:

• It is a service in Application Server controlled by data base connectivity to redirect the transaction request into specified DB Network.