August 21st, 2009
Advance Topics, Concurrency Control
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T. Advance Topics
Concurrency is when two users try to update the same data at nearly the same time.
RDBMS systems use locking to prevent users from reading data that is being changed by other users. This is why a SELECT statement may need the option FOR UPDATE. The locking ensures transaction integrity and database consistency. Concurrency transparency means that data locking makes each transaction appear as though it is the only transaction. Sometimes each user is trying to access data locked by the other user causing a deadlock. Most RDBMS systems unlock one user, process that user, and then unlock the other user.
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