Microsoft® SQL Server™ 2005 Administrator's Companion

Transaction Locking

SQL Server uses an object called a lock to allow synchronized access by multiple users that attempt to access the same piece of data at the same time. Locking helps to ensure logical integrity of transactions and data. Locks are managed internally by SQL Server lock manager and are acquired on a per-user-connection basis. When a user connection acquires (or owns) a lock on a resource, the lock indicates that the user has the right to use that resource. Resources that can be locked by a user include a row of data, a page of data, an extent (eight pages), a table, a file, or an entire database. For example, assuming the default isolation level of read committed is used, if the user holds a lock on a data page, another user cannot ...