Lock Mechanism 

You can synchronize access by several programs to the same data with a logical lock mechanism. This lock mechanism fulfills two main functions:

·        A program can tell other programs which data records it is just reading or changing.

·        A program can prevent itself from reading data that is just being changed by another program.

The data records of a table to be locked are defined by a logical condition. When a lock is set, this logical condition is entered in a lock table. This entry is retained until it is removed by the program or the program comes to an end. All the locks set by a program are thus removed at the end of the program.

When accessing data records, the records just being edited by other programs can be identified by the entry in the lock table. Such an entry for the lock must define a number of fully specified key fields, that is either a value is passed for the key field or this field is locked generically.

To set locks, a lock object must be defined in the ABAP Dictionary. When this lock object is activated, two function modules (see Function Modules for Lock Requests) are generated with the names ENQUEUE_<lockobjectname> and DEQUEUE_<lockobjectname>.

If data records are to be locked, you must call function module ENQUEUE_<lockobjectname>. The values of the key fields that specify the records to be locked are passed for all the tables contained in the lock object when the function module is called. There is a generic lock if a value is not passed for all the key fields. The function module writes the appropriate lock entry (see Example for Lock Objects). If another program also requests a lock, it will be accepted or rejected depending on the lock mode (see Lock Collisions). The program can then react to this situation.

Locked data records can be unlocked by calling function module DEQUEUE_<lockobjectname>. The key values and the lock mode used to set the lock must be passed to the function module.

This lock procedure requires that all programs used cooperate. Inconsistencies can occur if a program reads or changes data without having previously locked it. When a lock is set, the data records are only protected against changes by another program if this program also requests a lock before accessing the data.

Instead of writing lock requests or lock releases directly in the lock table, it is also possible to collect them first in a local lock container. The collected locks can be sent at a later time as a group. A parameter of the relevant function module controls whether a lock request or lock release is sent directly.

You can find further information about the lock concept and how lock management works in the documentation The SAP Lock Concept.