Thomas Write Rule

Question

Consider the following schedule:

The possible values of P & Q for which the above schedule is allowed under Thomas write rule but not under basic timestamp ordering protocol if timestamp(T₂ )>timestamp(T₁ )

1.   W(B),W(C)
2.   W(A),W(A)
3.   Both (a)&(b)
4.   None of these

Comments

option 2 ?

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Option 1 is valid in both

Option 2 is invalid in Basic TimeStamp but valid in TWR

So Anwer should be 2.

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Can u pls explain why it is so . Im not able to understand it :(

Answer 1

Answer : 4 (None)

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Thomas’ write rule :

Suppose that transaction Ti issues write(Q).

1. If TS(Ti) < R-timestamp(Q), then the value of Q that Ti is producing was previously needed, and it had been assumed that the value would never be produced. Hence, the system rejects the write operation and rolls Ti back.

2. If TS(Ti) < W-timestamp(Q), then Ti is attempting to write an obsolete value of Q. Hence, this write operation can be ignored.

3. Otherwise, the system executes the write operation and sets W-timestamp(Q) to TS(Ti).

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Thus, None of the options A and B is correct. Because A makes the schedule legal under both Basic TSO protocol and Thomas write TSO protocol.

And B makes the schedule illegal under both Basic TSO protocol and Thomas write TSO protocol because when T1 issues W(A) in Q statement, then according to Condition_1 of Thomas Rule TS(T1) < (Read_TS(A) = TS(T2))  thus , the system rejects the write operation and rolls T1 back.

So, The correct answer is None.

Comments

So answer be A option bcz it is legal.

Answer 2

Option 1 : We haven't any W->W or W->R or R->W dependency from T2 to T1, So Option 1 is valid for Simple TimeStamp. and so, valid for TWR (bcz in TWR we can even ignore W->W dependency).

Option 2 : There will be W(A)->W(A) dependency from T2 to T1, which voilate Simple TimeStamp Rule but valid for TWR (bcz TWR will be voilate only if W->R or R->W dependency is here from T2 to T1)

So Option 2

Note: It should be given that TimeStamp of T2 is greater than that of T1.

Comments

if you say the option b is valid then what about R2(A) ->W1(A)? (bold and underlined below)

T1                          T2

R1(A)
R1(B)
R1(C)
	R2(A)
	R2(B)
	W2(A)
W1(A)
	R(C)

@Arjun Sir, Please help. 

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I think correct ans is NOA,

because TWR only save us from WW conflict, not even in RW or WR