concurrency_control - Concurrency Control Overview Three...

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Concurrency Control
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Overview b Three concurrency problems b Locking b Three concurrency problems – Revisited b Deadlock
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Concurrency b Concurrency – DBMS allow many transactions to access the same database at the same time. b A kind of control is needed to ensure that concurrent transactions do not interfere with each other.
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Three concurrency problems b The lost update problem b The uncommitted dependency problem b The inconsistent analysis problem
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The Lost Update Problem Transaction A's update is lost at time t4. Dirty write
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The Uncommitted Dependency Problem b Transaction A sees an uncommitted update at time t2. b Transaction A produces an incorrect result. Dirty Read
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The Uncommitted Dependency Problem •Transaction A become dependent on an uncommitted change at time t2. • Loses an update at time t3
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The Inconsistent Analysis Problem
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Conflict Operations b From a concurrency point of view, the primary focus on database retrievals and updates --> reads / writes b If A and B are concurrent transactions, problems occur if A and B want to read or write the same database object, say tuple t. b RR: A and B both wants to read t. Reads can not interfere with each other – No Issue.
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This note was uploaded on 04/01/2012 for the course CSE,IT 101 taught by Professor Mirunaalini during the Spring '12 term at Indian Institute of Technology, Chennai.

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concurrency_control - Concurrency Control Overview Three...

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