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The length of time that a lock is held depends primarily on the mode of the lock and the transaction isolation level in effect. The default isolation level for SQL Server is Read Committed. At this level, shared locks are released as soon as SQL Server has read and processed the locked data. In Snapshot isolation, the behavior is the same shared locks are released as soon as SQL Server has read the data. If your transaction isolation level is Repeatable Read or Serializable, shared locks have the same duration as exclusive locks; that is, they are not released until the transaction is over. In any isolation level, an exclusive lock is held until the end of the transaction, whether the transaction is committed or rolled back. An update lock is also held until the end of the transaction unless it has been promoted to an exclusive lock, in which case the exclusive lock, as is always the case with exclusive locks, remains for the duration of the transaction. In addition to changing your transaction isolation level, you can control the lock duration by using query hints. I discuss query hints for locking, brie y, later in this chapter.
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Lock duration is also directly affected by the lock ownership. Lock ownership has nothing to do with the process that requested the lock, but you can think of it as the scope of the lock. There are four types of lock owners, or lock scopes: transactions, cursors, transaction_ workspaces, and sessions. The lock owner can be viewed through the request_owner_type column in the sys.dm_tran_locks view. Most of our locking discussion deals with locks with a lock owner of TRANSACTION. As we ve seen, these locks can have two different durations depending on the isolation level and lock mode. The duration of shared locks in Read Committed isolation is only as long as the locked data is being read. The duration of all other locks owned by a transaction is until the end of the transaction. A lock with a request_ownertype value of CURSOR must be requested explicitly when the cursor is declared. If a cursor is opened using a locking mode of SCROLL_LOCKS, a cursor lock is held on every row fetched until the next row is fetched or the cursor is closed. Even if the transaction commits before the next fetch, the cursor lock is not released. In SQL Server 2008, locks owned by a session must also be requested explicitly and apply only to APPLICATION locks. A session lock is requested using the sp_getapplock procedure. Its duration is until the session disconnects or the lock is released explicitly. Transaction_workspace locks are acquired every time a database is accessed, and the resource associated with these locks is always a database. A workspace holds database locks for sessions that are enlisted into a common environment. Usually, there is one workspace per session, so all DATABASE locks acquired in the session are kept in the same workspace object. In the case of distributed transactions, multiple sessions are enlisted into the same workspace, so they share the database locks. Every process acquires a DATABASE lock with an owner of SHARED_TRANSACTION_ WORKSPACE on any database when the process issues the USE command. The exception is any processes that use master or tempdb, in which case no DATABASE lock is taken. That lock isn t released until another USE command is issued or until the process is disconnected. If a process attempts to ALTER, RESTORE, or DROP the database, the DATABASE lock acquired has an owner of EXCLUSIVE_TRANSACTION_WORKSPACE. SHARED_TRANSACTION_ WORKSPACE and EXCLUSIVE_TRANSACTION_WORKSPACE locks are maintained by the same workspace and are just two different lists in one workspace. The use of two different owner names is misleading in this case.
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