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Distributed_One_Copy_Serializability_with_Snap

Distributed_One_Copy_Serializability_with_Snap -...

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Unformatted text preview: Distributed One¡Copy Serializability with Snapshot Isolation Aspen Olmsted¢ CSCE £¤¥ Fall ¤¦§§ Background Isolation Isolation is a property that defines how¡when the changes made by one operation become visible to other concurrent operations Concurrency phenomena There are three different read phenomena when Transaction ¡ reads data that Transaction ¢ might have changed and one update phenomena when two transactions update the same date item£ ● Lost Updates ● Dirty Reads ● Non¤repeatable reads ● Phantom reads Concurrency Control Ensure that correct results for concurrent operations are generated ● Pessimistic concurrency control ● Optimistic concurrency control Pessimistic Concurrency Control Block an operation of a transaction¡ if it may cause violation of the rules¡ until the possibility of violation disappears¢ Optimistic Concurrency Control Assumes that multiple transactions can complete without affecting each other¡ and that therefore transactions can proceed without locking the data resources that they affect¢ Before committing¡ each transaction verifies that no other transaction has modified its data¢ If the check reveals conflicting modifications¡ the committing transaction rolls back¢ Lost Update ¡Write¢Write Conflict£ A lost update occurs when a transaction is allowed to overwrite data from a row that has been modified by another running transaction¤ Lost Update Example Transaction ¡ ● Update Course Set students = ¡¢ Where id = £CSCE¤¥¦£ ● Update Semester Set Start = £¥¢¡¥§¡=¢¡§¢¨£ where id = £SP¥¢¡¥£ ● Commit Transaction ¥ ● Update Course Set students = ¡¥ Where id = £CSCE¤¥¦£ ● Commit Dirty Reads A dirty read occurs when a transaction is allowed to read data from a row that has been modified by another running transaction and not yet committed¡ Dirty Read Example Transaction ¡ ● Select students from Course where id = ¢CSCE£¤¥¢ Transaction ¤ ● Update Course Set students = ¡¤ Where id = ¢CSCE£¤¥¢ ● Rollback Non¡repeatable reads A non¡repeatable read occurs¢ when during the course of a transaction¢ a row is retrieved twice and the values within the row differ between reads£ Non¡repeatable Read Example Transaction ¢ ● Select students from Course where id = £CSCE¤¥¦£ ● Select students from Course where id = £CSCE¤¥¦£ Transaction ¥ ● Update Course Set students = ¢¥ Where id = £CSCE¤¥¦£ ● Commit Phantom reads...
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Distributed_One_Copy_Serializability_with_Snap -...

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