If commit t k start t k t k completes its execution

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If Commit( T k ) < Start( T k ), T k completes its execution before T i started, the serializability is maintained. If Start( T i ) < Commit( T k ) < Commit( T i ) and update set( T i ) and update set( T k ) do not intersect b. Validation test for first-committer-wins scheme with W-timestamps for data items: If a transaction T i writes a data item Q , then the W-timestamp( Q ) is set to Commit( T i ). For the validation test of a transaction T i to pass, following condition must hold: For each data item Q written by T i , W-timestamp( Q ) < Start( T i ) c. First-updater-wins scheme: i. For a data item Q written by T i , the W-timestamp is assigned the timestamp when the write occurred in T i ii. Since the validation is done after acquiring the exclusive locks and the exclusive locks are held till the end of the transaction, the data item cannot be modified inbetween the lock acquisition and commit time. So, the result of validation test for a transaction would be the same at the commit time as that at the update time. iii. Because of the exclusive locking, at the most one transaction can acquire the lock on a data item at a time and do the validation testing. Thus, two or more transactions can not do validation testing for the same data item simultaneously.
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  • Spring '13
  • Dr.Khansari
  • TI, Two-phase locking

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