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# Thomas write rule

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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:

$\begin{bmatrix} T1 & T2 \\ R(A) & \\  &W(A) \\  & Com. \\ W(A) & \\ Com. & \end{bmatrix}$

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:

$\begin{bmatrix} T1 & T2 \\ R(A) & \\ & Com. \\ W(A) & \\ Com. & \end{bmatrix}$

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

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

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