Thomas write rule

In computer science, in the field of Databases, the Thomas Write rule is a rule in timestamp-based concurrency control.

Given a Timestamp on a transaction T, TS(T) and Write Timestamp on an object O, WTS(O):

It states if TS(T) < WTS(O), the current write action has been made obsolete by the most recent write of O, which follows the current write according to timestamp ordering.

Given a non-conflict serializable transaction schedule:

:egin{bmatrix} T1 & T2 \ R(A) & \ &W(A) \ & Com. \ W(A) & \ Com. & end{bmatrix}

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

The Thomas Write Rule relies on the fact that T2 s write on object A is never seen by any transaction and postulates that the schedule above is equivalent to the schedule below:

:egin{bmatrix} T1 & T2 \ R(A) & \ & Com. \ W(A) & \ Com. & end{bmatrix}

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

This schedule is the same as the first and is conflict serializable.