An instance is a unit that can (and has to) be started and stopped as a whole. It is made up of several OS Processes, memory areas used by individual process and shared by all the processes, and files.



Primary and Additional Application Servers The business logic is executed in an SAP instance. It can be an ABAP instance (logic is executed by work processes) or Java instance (logic is executed by application threads in a server node)



An SAP system can be made up of several such instances. The first instance is called Primary Application Server (it used to be called Central Instance). Instances after PAS, that can execute business logic, are called Additional Application Servers (it used to be called as Dialog Instances)



Central Services To keep track of which instance is up and is least loaded, SAP uses a Message Server. So if a logon is made via Message Server, it checks which instance is least used and logs the user on to that server. You can group the type of users, provide them a logon group, and assign a set of instances to a logon group for a higher degree of control.



When a business user is executing a transaction, it would be making changes to several tables. To ensure that the records that are being changed by one transaction is not changed by any other transaction, a lock is placed on the record. The locks on tables, or even other objects are handled by an Enqueue Service.



Message Service and the Enqueue Service are both part of SAP Central Services. ABAP systems have their own central services instance - ASCS and for Java systems have central services instances as SCS.



ASCS and SCS also control SAP licensing. An SAP system can have only one central services instance. They are the central node of a star network topology.



Database Instance All the change made by PAS and AAS are persisted to database. The DB has a set of services clubbed together to form a Database Instance.