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:
- <math>\begin{bmatrix}
T1 & T2 \\ R(A) & \\
&W(A) \\
& Com. \\ W(A) & \\ Com. & \end{bmatrix}<math>
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:
- <math>\begin{bmatrix}
T1 & T2 \\ R(A) & \\ & Com. \\ W(A) & \\ Com. & \end{bmatrix}<math>
Text: T1:R(A), T2 Commit, T1: W(A), T1 Commit.
This schedule is the same as the first and is conflict serializable.