CONCURRENCY CONTROL WITH LOCKING METHODS
A lock guarantees exclusive use of a data item to a current transaction. In other words, transaction T2 does not have access to a data item that is currently being used by transaction T1. A transaction acquires a lock prior to data access; the lock is released (unlocked) when the transaction is completed so that another transaction can lock the data item for its exclusive use.
Most multiuser DBMSs automatically initiate and enforce locking procedures. All lock information is managed by a lock manager.
Lock Granularity
Indicates the level of lock use. Locking can take place at the following levels: database, table, page, row or even field.
LOCK TYPES
Regardless of the level of locking, the DBMS may use different lock types:
1. Binary Locks
Have only two states: locked (1) or unlocked (0).
2. Shared/Exclusive Locks
An exclusive lock exists when access is reserved specifically for the transaction that locked the object .The exclusive lock must be used when the potential for conflict exists. A shared lock exists when concurrent transactions are granted read access on the basis of common lock. A shared lock produces no conflict as long as all the concurrent transactions are read only.
DEADLOCKS
A deadlock occurs when two transactions wait indefinitely for each other to unlock data.
The three basic techniques to control deadlocks are:
? Deadlock preventation . A transaction requesting a new lock is aborted when there is the possibility that a deadlock can occur. if the transaction is aborted , all changes made by this transaction are rolled back and all locks obtained by the transaction are released .The transaction is then rescheduled for execution.
? Deadlock detection. The DBMS periodically tests the database for deadlocks. if a deadlock is found one of the transactions is aborted (rolled back and restarted) and the other transaction are continues.
? Deadlock avoidance. The transaction must obtain all of the locks it needs before it can be executed .This technique avoids the rollback of conflicting transactions by requiring that locks be obtained in succession
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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