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What are Lock Objects, and their Differences
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Answer Posted / uma mb
Lock objects (their developer given names should begin with
EZ or EY) are resident in the ABAP dictionary once they are
created and activated by the ABAP developer. Each lock
object, on activation, creates two function modules –
ENQUEUE_EZ<given name> and DEQUEUE_EZ<given name>.
In ABAP dictionary, we can create a lock object by stating
the table name (primary table) we want to lock, its lock
mode (there are three types of lock mode E, S and X
described at the end of this answer) and lock parameters
giving the primary key fields if only a single table is
involved or foreign key field(s) to join this table with
any other required (secondary) table(s) as the process
might demand.
To understand why lock objects are required / needed is
best comprehended by understanding the LOCK (locking of
data about to be changed by business transaction) concept
and lock management first.
Locking of data (in tables or resources) prior to making a
change to the data becomes very necessary to maintain
consistency of data when several people are working on the
same data (table / application) adding, changing and
deleting (records or table entries) records to it. A very
good example of this kind of high end activity on a table
data is seen in a multi-user environment like airline
booking system. A person queries the database for currently
available seats on a flight. If seats are available indeed,
then a booking has to be done. Between the time of query of
seats availability and time of actually booking the
required seats, many seconds/minutes elapse and in the
meantime another person might have already booked thus
bring the balance of seats down and rendering earlier read
balance incorrect. Therefore, when the query is fired, the
user locks the table entry (or entries as the case might
be). After this locking, nobody else gets access to the
same entry until the entire booking is done and balance
seats are decreased and lock released finally.
It is imperative to realize that locks once set must get
released as soon as the need for them is over so that
resources (tables) are released for others to use. There is
a default internal locking mechanism (database lock
administration) of ABAP but this is not sufficient (the
reason is these locks are implicitly released whenever a
screen changes and this would harm the integrity of a
business SAP transaction under process/progression which
could run through a series of dialog steps/screens).
Before firing a SELECT query to enquire seat availability,
developer must (a) issue a call to function module
ENQUEUE_EZ<given name> to logically lock the table
(s)/resource(s). (b) On successful locking, book the
tickets, reduce balance of seats available and (c) then
issue a call to function module DEQUEUE_EZ<given name> to
release the logical lock placed on the global lock table
(the enqueue work process coordinates this activity with
the global lock table). If locking is not successful, the
transaction has to be exited.
Lock Modes: S for Shared Read Lock.
X for Extended Write (Non-cumulative) lock
E for Exclusive Write lock (Cumulate)
E – When this lock is set, then the user (aka owner of
lock) who sets it can only have exclusive access and edit
records locked. A request for any other write lock or
read / shared lock is rejected. Only the lock owner can set
the lock again (cumulate).
S – Shared read lock. Several users can read the locked
record(s). A write lock request is rejected (to
prevent ‘dirty read’ situation). However, additional read
share locks from other users are accepted by the system.
X - This type of write lock can be requested only once
even by the same SAP transaction/lock owner. All other
subsequent lock requests of any kind, even by the lock
owner fails.
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