In a local update, the update program is run by the same work process that processed the request. The dialog user has to wait for the update to finish before entering further data. This kind of update is useful when you want to reduce the amount of access to the database. The disadvantage of local updates is their parallel nature. The updates can be processed by many different work processes, unlike asynchronous or synchronous update, where the update is serialized due to the fact that there are fewer update work processes (and maybe only one).
You switch to local update using the ABAP statement SET UPDATE TASK LOCAL. This statement sets a "local update switch". When it is set, the system interprets CALL FUNCTION IN UPDATE TASK as a request for local update. The update is processed in the same work process as the dialog step containing the COMMIT WORK. The transaction waits for the update to finish before continuing.
As an example, suppose you have a program that uses asynchronous update that you normally run in dialog mode. However, this time you want to run it in the background. Since the system response time is irrelevant when you are running the program in the background, and you only want the program to continue processing when the update has actually finished, you can set the SET UPDATE TASK LOCAL switch in the program. You can then use a system variable to check at runtime whether the program is currently running in the background.
By default, the local update switch is not set, and it is reset after each COMMIT WORK or ROLLBACK WORK. You therefore need to include a SET UPDATE TASK LOCAL statement at the beginning of each SAP LUW.
If you reset data within the local update, the ROLLBACK WORK statement applies to both the dialog and the update part of the transaction, since no new SAP LUW is started for the update.