Explain Thomas' Write rule.  Show how it is different from timestamp ordering protol

Ans: In timestamp-based concurrency control the thomas Write rule is a rule. Given a Timestamp on a transaction T, TS (T) and Write Timestamp on an object O, WTS (O):

 It says if TS(T) < WTS(O), the current write action has been made obsolete through the most  recent write of O, that follows the current write as per to timestamp ordering.

 Given a non-conflict serializable transaction schedule:

 Text: T1:R(A), T2:W(A), T2 Commit, T1: W(A), T1 Commit.

 The Thomas Write Rule based on the fact that T1's write on object A is never seen through any transaction and postulates which is schedule above  is equivalent to the schedule below where T2 takes place strictly after T1, and that therefore the write of T1 can be ignored:

 Text: T1: R (A), T1: W (A), T1 Commit, T2: W (A), T2 Commit.

 This schedule has similar effect as the first and is conflict serializable