It is a physical server which is used to handle and process the user request. In SAP naming convention we define them as an Instance and it is possible to install more than one instance on a single server provided they are differentiated by the instance_number.
Instance_number is a 2 digit number that varies between [00 to 97] 98 and 99 are reserved for routing purposes.
Instances is of various types
- Database Instance.
- Central Instance.
- Dialogue Instance.
This is the Instance where database is installed.
This is the Instance where Application Server/ Tier/ Layer are installed. There will be only one instance in the entire system.
These are the instances which are used to handle the load on the central instances. We can install as many instances as possible assuming that each instance can serve up to 200 - 500 users depending upon the type of the users.