What is SAP Instance?

          An operational unit on a host, which constitutes of components/resources like memory, work-processes, dispatcher, gateway of a system installed.

          A 2-digit number between 00 and 99 identifies an instance.

          In this, 98 is reserved for diagnostic agent server and 97 for SAProuter

Usually instances are of three types:

• Dialog Instance: It represents the application layer (discussed on next topic). Dialog instance contains memory, work processes, dispatcher. We can install dialog instances on a new host (called Additional-application server instance) so that the newly installed dialog instance resources like memory, work processes and dispatcher will utilized. Due to this, we maintain load balance or distribution for increase of system performance.

•  Central Instance: It is a part of the system, which provides with additional services like message server and enqueue server (discussed in next topic). Actually central instance is the first started in process so that it can help with dialog instances locations and resources available. Then it will provide the information like available services to process request. Dispatcher is the medium for communication between user request and message server.

•  Database Instance: Responsible for accepting the requests from central instance to make work done.