Consider the following non serial schedule
$S : R_{1} (A), R_{3} (B), W_{1} (A), W_{2} (A), W_{2} (B), W_{3} (B)$

Which of the following protocol allows this schedule?

Thomas write rule

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Both (A) and (B)

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Neither (A) nor (B)

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Basic Timestamp protocol

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Solution

The correct option is A Thomas write rule
The order of TimeStamp is $T_{1} \to T_{3} \to T_{2}$ .

The timestamp ordering requires all the conflicts to be processed in the order of their timestamps.

$W_{2} (B) - W_{3} (B)$ is violating time stamp order and the schedule is not possible under timestamp protocol, But allowed under Thomas Write Rule which ignores $W_{3} (B)$ called Obsolete Write.

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S : W_{4} (E), R_{1} (C), R_{3} (A), R_{1} (E), W_{2} (A), R_{4} (A), W_{3} (A), R_{3} (B), W_{4} (A), W_{2} (D), W_{1} (A), R_{4} (C), W_{1} (B), R_{4} (D)

S : R_{1} (A), R_{4} (B), W_{3} (A), W_{2} (B), W_{4} (C), c o m m i t_{4}, W_{3} (C), W_{1} (A), c o m m i t 1, R_{2} (D), W_{3} (D),

R_{x} (Z) denotes Trans X Reads item

Z, W_{x} (Z) denotes Trans X writes item Z.

S_{1} : R_{1} (A), W_{1} (A), R_{2} (A), W_{2} (A), Commit T_{2}, R_{1} (B), W_{1} (B

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S_{3} : R_{2} (A), R_{1} (A), W_{1} (A), W_{2} (A), W_{2} (B) Commit T_{2}, W_{1} (B)

S : ω_{1} (x), r_{2} (x), ω_{3} (x), r_{4} (x), ω_{5} (x), r_{6} (x) ω_{7} (x), r_{9} (x), ω_{9} (x)

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