Database changes that are made via the SAP Update System and transferred to an update work process, can be carried out synchronously or asynchronously. The mode is specified in the ABAP source code of the SAP transactions and cannot be changed dynamically by the user.
The SAP transaction creates an update request with CALL FUNCTION … IN UPDATE TASK, and transfers this to an update work process. Data is then written to the update tables at the end of a database LUW.
With synchronous updates, the program that outputs the statement COMMIT WORK AND WAIT waits until the update work process outputs the status of the update. The program can then respond to errors separately.
Synchronous updates can never be processed a second time.
As the calling program waits for the result and responds to it appropriately, you can assume that the error has been intercepted. If you try to processs a synchronous update in SM13 again, you will get the message This update request cannot be carried out retrospectively (synchronous update).
Synchronous updates are indicated by the symbol in the Info column in Update Management.
Updates generated by batch input sessions are always carried out synchronously. Batch input with CALL TRANSACTION USING can be updated synchronously and asynchronously. In addition to this, every 100th update in a background job is carried out synchronously in order to avoid delays in the Update System.
With asynchronous updates, the program that outputs the statement COMMIT WORK passes the update onto the Update System and does not wait for the update process to respond.
See also: Update Techniques in the ABAP manual.