usually for a single application or database server within a SAP R/3 client-server environment.
There are three types of instances:-
Dialog instance
Central Instance
Database Instance
SAP System= Dialog Instance + Central Instance + 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, workprocesses also increases so that more number of users can login at a time.
Central Instance: - Central instance can also work as dialog instance. But the main thing is that it contains Enqueue and message servers. All dialog instances communicate with central instance before requesting database with 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 table is managed in central instance by enqueue service.
Database Instance: - As normal database instance accepts requests from central instance to fulfill the user's requests. As 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 C11, PRD, E56, etc.)
Logical System Names: -
When data is distributed between different systems, each system within a network has to be clearly identifiable. The "logical system" deals with this issue.
A logical system is an application system in which the applications work together on a common database. In SAP terms, the logical system is a client.
Since the logical system name is used to identify a system uniquely within the network, two systems cannot have the same name if they are connected to each other as BW systems or as source systems, or if there are plans to connect them in any way.
Example for production system logical system name might be:-
