What is an instance? How many types of instances are there? Explain them.
An instance is an administrative unit that combines SAP system components proving one or more services. The services provided by an instance are started or stopped together. You use a common instance profile to set parameters of all the components of an instance. Each instance has its own buffer area. An instance runs on one physical computer, but there can be multiple instances on one computer. An instance is identified by the system ID(SID) and the instance number. There are two types of instances :
The central instance of the SAP system is distinguish by the fact that it offers services that no other instance of the system offers. For the AS ABAP, these are the message server and the enqueue work process. For the AS Java you can recognize the central instance by the software deployment manager (SDM).
All other instances of the system are typically called dialog instances.