What is a Function Module (FM)?
Function Modules are encapsulated ABAP procedures that are grouped in Function Groups. Modules within the same Function Group belong logically together.
They are globally available and can be called from any other ABAP program by name which must be unique.
Function Modules are created with the Function Builder (transaction SE37). The type of function module depends on the Processing Type.
Regular Function Module
A Regular Function Module is the default option. This kind of Function Module is executed immediately and synchronously on your current SAP system.
CALL FUNCTION func { parameter_list | parameter_tables }.
Remote-Enabled Function Module
Remote Function Modules can be called by other SAP and non-SAP systems utilizing the RFC protocol.
For example, you can define a Remote Function Module in an EWM system which then gets called by your ERP system to get further information about stock levels or shipment details.
CALL FUNCTION func DESTINATION dest parameter_list.
This statement calls the target system specified under dest synchronously. Available RFC destinations are configured using transaction SM59. Use the unique name of a destination for parameter dest.
The calling program continues execution once the RFC has finished. Depending on the connection this can be a performance issue.
Update Function Module
Update Function Modules are not executed immediately. They are scheduled for execution in an update work process. This update process is triggered by the statement COMMIT WORK. The statement ROLLBACK WORK deletes all update function module registrations.
CALL FUNCTION update_function IN UPDATE TASK
[EXPORTING p1 = a1 p2 = a2 ...]
[TABLES t1 = itab1 t2 = itab2 ...].
Using CALL FUNCTION .. IN UPDATE TASK is a way to bundle database changes into a single database LUW. It is also beneficial for performance since the update happens asynchronously.
What is a BAPI?
SAP BAPI stands for Business Application Programming Interface. Technically a BAPI is a Function Modules that offers a standard interface to business objects in SAP. Most of them are also RFC enabled.
Common use-cases are using a BAPI to create/read/update/delete business objects like Sales Orders, Business Partners, or Purchase Orders.
Those BAPIs behave like the SAP standard transactions for these business objects. This means that authorization checks are already in place and implemented user exits are considered.
Find further information about existing BAPIs and how to call them by running transaction BAPI.