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Sunday, June 17, 2007

What is Locking ?

- When two users simultaneously attempt to access the same data record, this is
synchronised by a lock mechanism.

When dialog transactions are programmed, locks are set and released by calling certain
function modules. These function modules are generated automatically from the
definition of so-called lock objects in the ABAP/4 Dictionary.
To synchronize the access to a table by setting and removing locks, a Lock object has
to be defined in the ABAP/4 Dictionary. Activating the lock object automatically creates
function modules for setting and removing locks. These function modules must be
included when programming interactive transactions.

Lock Mechanism :

To set locks, a lock object must be defined in the ABAP/4 Dictionary. In this lock object,
those tables in which data records are to be locked by calling a lock are determined. All
tables included in a lock object must be connected to each other via foreign keys. The
key fields of the tables in a lock object form the Lock arguments for the tables. The lock
arguments are the basis for formulating the logical condition for identifying the records
to be locked.

When activating this lock object, two function modulesB with the names
ENQUEUE_ and DEQUEUE_ are generated.


Example :

Problem :

You wish to prevent a user from being able to change the name of a course or the name
of the professor with responsibility for the course at a time when another user is editing
the course description (which contains this information).


Solution :

The problem described above can be solved by defining a lock object E_UKURS. This
is done by defining primary and secondary tables in the lock object. Table UKURS is
check table of table UKRSB, so UKURS should be selected as primary table and
UKRSB as secondary table of the lock object.
The Lock argument in this case is the field combination FABNR, KRSNR, and SPRAS
(i.e Primary Key Combination).
The Lock mode Shared is to be selected here. This allows several users to access the
data simultaneously in display mode.
The lock mode in the generated function modules for setting (ENQUEUE_E_UKURS)
and releasing (DEQUEUE_E_UKURS) locks is therefore set to shared as default, but
can be overridden by calling the function modules.

If the function module ENQUEUE_E_UKURS is called with FABNR = '1' and KRSNR =
'3', the record for course 3 in faculty 1 is locked in table UKURS. Furthermore, all the
course descriptions for this course are locked in table UKRSB since field SPRAS was
not specified when the function module was called. In such cases, the lock is made
generically for a field which is not defined.
If the function module DEQUEUE_E_UKURS is now called with FABNR = '1', KRSNR =
'3' and SPRAS = 'D', the German course description is unlocked. All other course
descriptions remain locked.

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