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For shorter-term needs, SQL Server achieves mutual exclusion with a spinlock. Spinlocks are used purely for mutual exclusion and never to lock user data. They are even more lightweight than latches, which are lighter than the full locks used for data and index leaf pages. The requester of a spinlock repeats its request if the lock is not immediately available. (That is, the requester spins on the lock until it is free.) Spinlocks are often used as mutexes within SQL Server for resources that are usually not busy. If a resource is busy, the duration of a spinlock is short enough that retrying is better than waiting and then being rescheduled by the operating system, which results in context switching between threads. The savings in context switches more than offsets the cost of spinning as long as you don t have to spin too long. Spinlocks are used for situations in which the wait for a resource is expected to be brief (or if no wait is expected). The sys.dm_os_tasks dynamic management view (DMV) shows a status of SPINLOOP for any task that is currently using a spinlock.
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We examine four aspects of locking user data. First we look at the mode of locking (the type of lock). I already mentioned shared, exclusive, and update locks, and I go into more detail
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about these modes as well as others. Next we look at the granularity of the lock, which speci es how much data is covered by a single lock. This can be a row, a page, an index key, a range of index keys, an extent, a partition, or an entire table. The third aspect of locking is the duration of the lock. As mentioned earlier, some locks are released as soon as the data has been accessed, and some locks are held until the transaction commits or rolls back. The fourth aspect of locking concerns the ownership of the lock (the scope of the lock). Locks can be owned by a session, a transaction, or a cursor.
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SQL Server uses several locking modes, including shared locks, exclusive locks, update locks, and intent locks, plus variations on these. It is the mode of the lock that determines whether a concurrently requested lock is compatible with locks that have already been granted. We see the lock compatibility matrix at the end of this section in Figure 10-2.
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Shared locks are acquired automatically by SQL Server when data is read. Shared locks can be held on a table, a page, an index key, or an individual row. Many processes can hold shared locks on the same data, but no process can acquire an exclusive lock on data that has a shared lock on it (unless the process requesting the exclusive lock is the same process as the one holding the shared lock). Normally, shared locks are released as soon as the data has been read, but you can change this by using query hints or a different transaction isolation level.
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Exclusive Locks
SQL Server automatically acquires exclusive locks on data when the data is modi ed by an INSERT, UPDATE, or DELETE operation. Only one process at a time can hold an exclusive lock on a particular data resource; in fact, as you see when we discuss lock compatibility later in this chapter, no locks of any kind can be acquired by a process if another process has the requested data resource exclusively locked. Exclusive locks are held until the end of the transaction. This means the changed data is normally not available to any other process until the current transaction commits or rolls back. Other processes can decide to read exclusively locked data by using query hints.
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