Locking and Blocking in SQL Server

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Tuning SQL Server (eBook)

Tuning SQL Server (Second Edition)

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The majority of modern relational database management systems (RDBMSs) make use of lock-based concurrency. Lock-based concurrency is the approach based on which the Database Engine of a RDBMS ensures that no actions of committed transactions are lost. This is actually what Locking is.

SQL Server locks resources using different lock modes that determine how the resources can be accessed by concurrent transactions. SQL Server uses the following lock modes:


In RDBMSs with lock-based concurrency, such as SQL Server, there are many cases where blocking can occur. Blocking takes place when one server process id (SPID) holds a lock on one resource and a second SPID tries to place a lock of a conflicting type on the same resource. Most of the times, each blocking incident does not take much time and it is a natural behavior of RDBMs that use lock-based concurrency in order to help ensure data integrity.

There are sometimes however undesirable "blocking" incidents. Such incidents are:


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The SQL Server and .NET Hub

Reference: The SQL Server and .NET Hub (http://www.sqlnethub.com)

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