[DBMS] Time stamp Ordering Protocol

Question

Schedule S:- R1(A) R2(B) R3(C) R1(B) R2(C)R3(D)W1(C)W2(D)W3(E)   , following is the dependency graph

Which of the transaction is rollbacked if Time stamp order is?

a.) (T1,T2,T3)=(10,30,20)

b.)(T1,T2,T3)=(20,10,30)

Please explain approach to this?

Comments

In a) T1

b)T1 and T2 both rollback.

Is it the given answer???

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Correct.Please explain approach.

Answer 1

Firstly, the precedence graph or dependency graph arrows indicate the order in which conflict operations must be executed. Now in TO lowest TO number indicate oldest Transaction and you must be knowing that in TO protocol if latest transaction has performed a operation then older one is nit allowed it has to rollback.

Here in A) T3 has to perform the conflict operation first then the To-number of that data item will become 20 then T2 has to perform ooeration and it will make it 30. Then T1 will be performing operation on that data item but It's TO number is small than 30(TO number of that data item). Hence rollback.

Similarly it is for B).

Comments

Thanks man:). This concept was biting me for last one hour.

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I need to clear one more point.

When values are (T1,T2,T3)=(20,10,30)

Then T3 will set to 30.Now T2 will come and t2 will find some younger has done operation so it will rollback.

Now T2 will cause t1 to rollback or will T1 execute and then it check that some younger has performed write and rollback itself?I mean the reaon of T1 to rollback is because of T2 rollback or because of TO protocol ,which will cause it to rollback

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T1 rollbacks bcoz of data item had value 30, it is not bcoz of T2, If T1 had Timestamp 40 it would have gone and performed it operation and set value to 40