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SAP R3 - User Request Flow - Correct me if necessary

When End user makes a request from SAP GUI i.e. SAPLOGON, if SAPLOGON is configured for group logon, Message server does the initial load balance by sending the request to only one of those AppServers in group defined for that particular user. Then Dispatcher from one of those appserver receives request and puts in queue until Work Process is free. Then allocate that request to a dialog Work process, which does the screen interpretation using DynPro, ABAP code using ABAP Processor, SQL query using Database Interface (which are part of Dialog Work Process along with Task Handler). Later all transaction steps are either written directly to SAP Database i.e. Oracle or MSSQL or mysql... as Local Update or to temp tables as Asynchronous Update and later trigger Update Work Process when available to write these from temp tables (which are vbmod, vbhdr, vberror, vbdata) into permanent tables based on Update V1 or V2 category. This is synchronous update which happens after change is committed. If the request from user is to change huge number of records and can consume lot of resources decreasing performance, such request should be dealt as Background Job. Which runs during off-peak hours (using operational mode decreasing Dia WP and increasing Btc WP). Before any update, Enque Work Process maintains a table which assigns lock on database records, these are saved in user Buffer and must be shared with other application servers via Buffer Synchronisation. So that no two AppServer modify buffered record and update database with data integrity issues.

Every AppServer must have 2 Dia WP, 2 BTC WP atleast and only one Dispatcher.

ASCS<instanceNo> or SCS<instanceNo> will have a message server and Enque server.

ASCS is ABAP and SCS is Java stack.

Central Instance is now Primary Application Server which has Database Instance.

Additional Application Server are instances which can be on same or different server to Database Instance.

Background WP are recommended to be on PAS or CI or Database Instance.

Each WP from SAP R3 AppServer has corresponding WP from Database Server to do the database query or update.

Dispatcher of one AppServers communicate with Dispatcher of another AppServer via Message Server.

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