Layers of SAP and what are Instances in SAP

SAP i.e. System Applications and Products in Data Processing is the largest ERP software available in market these days. Through this article I would like to cover some the basics of SAP systems.
For operating business application software’s we have three layers:
  • Presentation Layer:  This is used by end users for viewing the front end on the screen.
  • Application Layer: For executing application level programs.
  • Database Layer: For storage of data at database level this layer is used.

There are 3 types of architecture in SAP
  • 1 tier configuration: All the three layers are present on one laptop/desktop. So frontend can run the application and execute the application and also use it for data storage.
  • 2 tier configuration: In this the front end i.e. presentation layer is at different server, application and database works from same server.
  • 3 tier configuration: All the three layers operate from different servers. In this end user has front end software installed on their personal system and it operates SAP application and database which are installed on different servers.

SAP Business Suite also uses even more than 3 tier architecture where 4th layer can be web services.

What is an Instance?

An instance is an administrative unit that combine SAP components to provide different services. Start and stop of these services occur together only. Any system in SAP is defined by SID i.e. system ID and different instances have their respective instance number. We can set the different parameters for different instances in instance specific profile. Instance is also known as application server.
In three tier architecture while installation of application only we have the option of giving different server/host details for database. Usually database has same ID (DB ID) i.e. as system ID.

Two types of instances: Central Instance: Central Instance has some of the functionalities which other instances doesn’t have, it comprises of Message Server and Enqueue work processes in case of ABAP application server and SDM (Software Deployment Manager) in case of JAVA application server. In newer versions of SAP, Central Services Instance is there which has services of AS JAVA, Message server and Enqueue work process. AS ABAP central instance does not exist since the central services can be moved to AS ABAP in case of high availability.

All other instances are Dialog instances. Dialog instances consists of all the work processes except message server and enqueue server.

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