Friday, March 4, 2011

SQL Server: Difference Between Locking, Blocking and Dead Locking


Like ever, today’s article of Pinal Dave was interesting and informative. After, our mutual discussion between our DBAs and Developers on Pinal Dave topic of Concurrency and Isolation. I felt that most of us are intermingling three equally sounding words. Those are LOCKING, BLOCKING and DEAD LOCKING.
Lets try to revisit these concepts with some simple analogies.
LOCKING occurs when connection needs access to a piece of data in database and it’s necessary for SQL Server when managing multiple connections. Just assume an example of your garage, when you park your car in garage, basically you are locking the place of garage.

BLOCKING occurs when two connections need access to same piece of data concurrently and one connection is blocked because at a particular time, only one connection can have access. Just like, you stop (block) your car on a traffic signal because some other car or cars are using the crossing area.
DEAD LOCK occurs when one connection is blocked and waiting for a second to complete his work, but on other side, second connection is also waiting for first connection to release the lock. Just like, you need to cross the signal area but same time someone else from opposite side also want to cross the signal. Now, you need a way which other is holding and other need way where your car is.
That is why, one should be clear that locking is integral part of SQL Server to handle concurrency, blocking is bad when one connection/transaction is waiting unnecessary for a long time, and deadlocking is a phenomenon which should never occur.

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