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If lock requests for the same entity are queued, then once a shared lock is granted, any queued shared locks may also be granted. If an exclusive lock is found next on the queue, it must wait until all shared locks have been released. As with exclusive locks, these shared locks should be held for the
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queue. This would ensure that any possible waiter would get equal chance to obtain the lock and not be locked out. To further speed up the process, if an entity has gone to sleep waiting for a lock, performance is improved if the entity is notified of the grant, instead of discovering it on some sort
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when retrieved for editing or updating. Anyone attempting to retrieve the same record for editing is denied write access because of the lock (although, depending on the implementation, they may be able to view the record without editing it). Once the record is saved or edits are canceled, the lock
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can contain multiple entries. Shared locks allow all holders to read the contents of the record knowing that the record cannot be changed until after the lock has been released by all holders. Exclusive locks cannot be obtained when a record is already locked (exclusively or shared) by another
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whenever a record is being modified by any user, so that no other user can save data. This prevents records from being overwritten incorrectly, but allows only one record to be processed at a time, locking out other users who need to edit records at the same time.
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The application or system should be designed such that any lock is held for the shortest time possible. Data reading, without editing facilities, does not require a lock, and reading locked records is usually permissible.
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is achieved if each individual account may be taken by a clerk. This would allow any customer to be serviced without waiting for another customer who is accessing a different account. This is analogous to a
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If the clerks can remove one page from the ledger, containing the account of the current customer (plus several other accounts), then multiple customers can be serviced
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If the bank clerks (to follow the illustration above) are serving two customers, but their accounts are contained in one ledger, then the entire ledger, or one or more
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Exclusive locks are exclusively held by a single entity, usually for the purpose of writing to the record. If the locking schema was represented by a list, the
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To allow several users to edit a database table at the same time and also prevent inconsistencies created by unrestricted access, a single record can be
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would contain only one entry. Since this type of lock effectively blocks any other entity that requires the lock from processing, care must be used to:
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Record locks need to be managed between the entities requesting the records such that no entity is given too much service via successive
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not hold the lock across system or function calls where the entity is no longer running on the processor – this can lead to deadlock;
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The introduction of granular (subset) locks creates the possibility for a situation called
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for two different transactions. Clerks 1 and 2 both retrieve (i.e., copy) the account's
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is released. Records can never be saved so as to overwrite other changes, preserving
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ensure that if the entity is unexpectedly exited for any reason, the lock is freed.
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A thorough and authoritative description of locking was written by
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is the technique of preventing simultaneous access to data in a
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among multiple database users. This is the "I" in the acronym
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In database management theory, locking is used to implement
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Distributed
Transaction Processing: Concepts and Techniques
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ensure the lock is held for the shortest time possible;
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Shared locks differ from exclusive locks in that the
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227:database, a record is typically called a "row".
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131:The classic example is demonstrated by two
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109:Learn how and when to remove this message
338:Gray, Jim & Reuter, Andreas (1993),
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16:Solution for concurrent database access
135:clerks attempting to update the same
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47:adding citations to reliable sources
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283:Non-holders of the lock (a.k.a.
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99:December 2009
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41:Please help
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302:holder list
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213:granularity
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313:See also
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179:Jim Gray
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346:375–437
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83:scholar
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248:grants
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223:In a
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172:ACID
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