ISRO2018-55

Question

Let us assume that transaction $T1$ has arrived before transaction $T2$. Consider the schedule $s=r1(A);r2(B): w2(A);  w1(B)$

Which of the following is true?

1. Allowed under basic timestamp protocol.
2. Not allowed under basic timestamp protocols because $T1$ is rolled back.
3. Not allowed under basic timestamp protocols because $T2$ is rolled back.
4. None of these

Comments

here the order is T1 followed by T2, this must be maintained but again there is a conflict operation r2(B) -> w1(B), which is showing T2 is followed by T1 which is incorrect hence T1 must be rolled back.

OPTION(B) is correct

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option B is correct.

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given schedule is allowed under Thomas write rule?

Answer 1

• if a schedule is conflict serializable schedule then it is allowed in basic timestamp protocol but the given schedule is not conflict serializable so it is not allowed in basic timestamp.
• in basic timestamp ordering protocol, the conflicting operation should be executed in the timestamp order but in given schedule $\mathbf{T_2}$ is violating this. So we need to rollback.
• 

Comments

T2 is violating because T1 has already read A and T2 now cannot do any operation on A because T1 has older timestamp than T2 and T1 has accessed it first?

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Better explanation.

Answer 2

According to basic timestamp ordering protocol conflicting operation should be executed in the timestamp order.

Here T2 performing read on B, than its timestamp will be newer and when older transcation perform Write on B i.e T1 than it voilated our condition. so

option B is right ans,

Comments

Yes B is definitely right :)

Answer 3

r1(A)-->w2(A) conflict serializable

T2 is rolled back

Answer is (C)

Comments

Yes m also thinking same