What is an Instance?

Sap An instance is a group of resources such as

  • Memory
  • Work Processes
  • Dispatcher
  • Gateway

Usually for a single application or database server within an SAP R/3 client-server environment.

The instance number is a 2 digit number that varies between [00 to 97] 98 and 99 are reserved 

for routing purposes.


There are three types of instances:-

  1. Dialog instance
  2. Central Instance
  3. Database Instance


For one SAP system, all three instances share the same directory.

  • Dialog Instance: - Dialog instance exists in the application layer. Its purpose is to maintain the load on the server. Dialog instance exists on the different host. If a number of dialog instance increases hardware resources, dispatcher, work processes also increases so that more number of users can login at a time.
  • Central Instance: - Central instance can also work as a dialog instance. But the main thing is that it contains Enqueue and message servers. All dialog instances communicate with the central instance before requesting the database with the message server. When an instance is started, the dispatcher process attempts to establish a connection to the message server so that it can announce the services it provides (DIA, BTC, SPO, UPD, etc.).Lock the table is managed in a central instance by enqueue service.

  • Database Instance: - A normal database instance accepts requests from the central instance to fulfill the user's requests. As a lock management system provided by enqueue server, it will provide service to users.

What is SID?

SID is a unique identification code for every R/3 installation (SAP system) consisting of a 

database server & several application servers. SID stands for SAP System Identification. 

SAPSID — a three-character code such as PRD, DEV, QAS, etc.)