Exclusive lock: The locked data can only be displayed or edited by a single user. A request for another exclusive lock or for a shared lock is rejected.
Shared lock: More than one user can access the locked data at the same time in display mode. A request for another shared lock is accepted, even if it comes from another user. An exclusive lock is rejected.
Exclusive but not cumulative: Exclusive locks can be requested several times from the same transaction and are processed successively. In contrast, exclusive but not cumulative locks can be called only once from the same transaction. All other lock requests are rejected.
Different types of lock modes...
Several users (transactions) can access locked data at the same time in display mode. Requests from further shared locks are accepted, even if they are from different users. An exclusive lock set on an object that already has a shared lock will be rejected.
An exclusive lock protects the locked object against all types of locks from other transactions. Only the same lock owner can reset the lock (accumulate).
Exclusive but not cumulative lock
X (eXclusive non-cumulative)
Whereas exclusive locks can be requested several times by the same transaction and released one by one, an exclusive, non-cumulative lock can only be requested once by the same transaction. Each further lock request will be rejected.
Optimistic locks initially behave like shared locks and can be converted into exclusive locks. See Optimistic Locks.
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SM12:Tcode for lock management
Managing lock entries lets you monitor your system with regards to lock logic. You can determine which locks are being used currently. Locks that have had the backup flag set are highlighted in color
You can detect and correct problems by deleting locks that you no longer want. You can also later analyze the cause of lock table overflows using the History
Mode: you can see which type the lock object is ( S: shared lock, E: exclusive lock, X: exclusive and not cumulative, O: optimistic lock). The various lock modes are described in section Functions of the SAP Lock Concept.
Table: The Table column contains the tables in which rows are locked.
Lock argument:the Lock argument column, you can see the argument (key field) of the lock entry.
This corresponds to the entries in the lock table.
The lock entries are shown in different colors:
Blue means that the locks have already been transferred to the update task (see also The Owner Concept), with the result that the backup flag is set. These locks are also rewritten to the lock table when the enqueue server is restarted.
Black means that the lock (still) belongs to the dialog owner. The backup flag is not set.
By choosing Edit ® Sort by, you can display the locks according to user, time, table, or host system (host).
By double-clicking a lock entry, you can display detailed information, including the host name and number of the SAP System in which the lock was generated.
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