Sql server update-lock
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Update locks in SQL Server are used to prevent deadlocks that could occur when multiple transactions try to concurrently update the same resource. Without update locks, transactions would first acquire shared locks before updating data. However, this could lead to deadlocks if two transactions simultaneously tried to convert their shared locks to exclusive locks in order to perform an update. To avoid this, SQL Server uses update locks, which are incompatible with other locks and ensure that only one transaction can update a resource at a time by first acquiring an update lock before converting it to an exclusive lock.
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- 2. BACKGROUND • Locks are used by SQL Server DB engine to protect a resource from modification by concurrent transactions simultaneously. • Among many locks used by SQL Server Update lock is one of the least understood locks. • This lock is used by SQL Server Database Engine to prevent a kind of deadlock which would have been possible if update lock was not there. 2
- 3. WITH NO UPDATE LOCKS • If Update lock was not there then a transaction would acquire a shared lock before updating a resource. • As shared locks are compatible, another concurrent transaction would also place a shared lock on the same resource. • Now to update the resource, first transaction will try to convert the shared lock to an exclusive lock. 3
- 4. WITH NO UPDATE LOCKS CONTINUED…… • But the first transaction will fail to acquire an exclusive lock because of shared lock placed by second transaction. • Similarly if the second transaction tries to acquire an exclusive lock on the same row , it will also fail because of shared lock placed by the first transaction resulting in deadlock. • This scenario is possible in Repeatable Read and Serialization Isolation. 4
- 5. WITH UPDATE LOCKS • To overcome this kind of deadlock, SQL Server introduced update lock which is not compatible with other locks. • Only one transaction can acquire an update lock at a time on a resource. • Now just before updating the resource , the update lock is converted to an exclusive lock. 5
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