Cumulation of Locks (SAP Library - The SAP Lock Concept (BC-CST-EQ))

Cumulation of Locks

As described under Lock Mode, there are three types of locks:

Shared (read lock): several transactions can set a read lock simultaneously (to read data that is not changed)

Exclusive (write lock): simultaneous read or write locks for this object are rejected; only the same owner (see The Owner Concept) can request the lock again. This is referred to as cumulation.

EXclusive lock that cannot be cumulated (extended write lock): this lock also can only be requested once by the same owner.

The type of lock selected is determined by the programmer of the transaction when the lock object is created. See also The SAP Lock Concept in the ABAP Documentation.

A lock can be set more than once (cumulation), if the name, argument, and lock mode of the elementary lock are identical. The cumulation counter is incremented by one with each successive cumulation and reduced by one each time a lock is released. The lock is released when the counter reaches zero.

The following graphic illustrates how locks are cumulated.

Example

This graphic is explained in the accompanying text

The lock request is accepted and written to the lock table. The cumulation counter of the dialog owner Owner_1 is 0. The counter of update owner Owner_2 is 1.

The lock request is also accepted. The cumulation counter of dialog owner Owner_1 is still 0 (because _SCOPE was set to 2); the counter of update owner Owner_2 is now 2.

This time, _SCOPE is set to 1. In other words, the cumulation counter of dialog owner Owner_1 is increased to 1 and that of update owner Owner_2 remains 2.

This time _SCOPE=3, that is, the lock has two owners. The cumulation counter of dialog owner Owner_1, therefore, is increased to 2 and that of Owner_2 is increased to 3.