intent__locking - Concurrency (Intent Locking)...

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Concurrency (Intent Locking) P.Mirunalini, A.P,CSE SSNCE
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Isolation Levels b Serializability guarantees isolation for ACID properties. b Protocols used to guarantee serializability reduce the degree of concurrency or overall system throughput. b System supports variety of Isolation levels. b Isolation level to a given transaction is defined as the degree of interference the transaction in question is prepared to tolerate on the part of concurrent transactions.
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Isolation Levels b If serializability to be guaranteed --> amount of interfernce that can be tolerated is none --> Isolation level should be maximum b Higher the isolation level, less the interference --> lower concurrency b Lower the isolation level, more the interference --> higher concurrency
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Lower isolation level- Violations b If a transaction executes at a lower isolation level following three violations may occur. s Dirty Read s NonRepeatable read s Phantoms
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Dirty Read b A transaction T1 may read the update of a transaction T2, which has not commited yet. b If T2 fails and is aborted ,then T1 would have read a value that does not exist and is incorrect.
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Dirty Read-Example
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NonRepeatable Read b A Nonrepeatable Read occurs if transaction T1 retrieves a different result from the each read s Transaction T1 reads an item s Transaction T2 reads and updates the same item s Transaction T1 reads the same item again, but now it has a new, modified value s Expected (good) behavior if the transactions were serialized:
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NonRepeatable Read
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Phantom Read b A Phantom Read occurs if transaction T1 obtains a different result from each Select for the same criteria s Transaction T1 executes search on certain criteria and retrieve m items from a table s
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intent__locking - Concurrency (Intent Locking)...

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