If Commit(Tk) < Start(Tk),Tkcompletes its execution beforeTistarted, the serializability is maintained.•If Start(Ti) < Commit(Tk) < Commit(Ti) and updateset(Ti) andupdateset(Tk) do not intersectb.Validation test for first-committer-wins scheme with W-timestampsfor data items: If a transactionTiwrites a data itemQ, then theW-timestamp(Q) is set to Commit(Ti). For the validation test of atransactionTito pass, following condition must hold:•For each data itemQwritten byTi, W-timestamp(Q) < Start(Ti)c.First-updater-wins scheme:i.For a data itemQwritten byTi, the W-timestamp is assigned thetimestamp when the write occurred inTiii.Since the validation is done after acquiring the exclusive locksand the exclusive locks are held till the end of the transaction,the data item cannot be modified inbetween the lock acquisitionand commit time. So, the result of validation test for a transactionwould be the same at the commit time as that at the update time.iii.Because of the exclusive locking, at the most one transaction canacquire the lock on a data item at a time and do the validationtesting. Thus, two or more transactions can not do validationtesting for the same data item simultaneously.
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