Self framed

Question

Is the following schedule view serializable?

T1	T2	T3
R(A)
	R(A)
	W(A)
		W(A)
	W(A)

Comments

I think it is view serialisable to serial schedule T1-T3-T2

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nop, its no view serializable

R(A) of T2 -> W(A) of T3

W(A) of T3 -> W(A) of T2.

 

So,cycle.

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it is not serializable at all

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If cycle present then it is not CS .
If cycle and blind write then we have to check for view rules .
Not CS doest mean NOT VS . It can be VS .

So Cycle present is not a reason to tell the given schedule is NOT View Serialisable

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Dulqar is correct. Given schedule is view serilizable....

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it is view serialisable to serial schedule T1-T3-T2.

Only one data item A.

* T1 performs first read

* T2 perform last write.

* no producer consumer dependency.

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@Dulqar. Cycle for view srializability.

Check this

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sorry its not view serializable producer consumer conflict exsists.

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@Kaushik.P.E

What is that conflict ?

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@dulqar

T1 must start first and T2 must end first. As they perform first read and last write on A. But if that order is maintained only possible serial schedule is T1->T3->T2 (T1 and T3 contain only one operation) and the given schedule is not view equivalent to it as producer consumer order is not maintained for A.

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@Kaushik.P.E

Got it . Thank you :)

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For serial schedule T1 -> T3 -> T2 operations will be

R1(A) W3(A) R2(A) W2(A) W2(A)

reads values written by 3.

Given schedule

R1(A) R2(A) W2(A) W3(A) W2(A).

does not read value wriiten by 3.

This makes it more clear.

Answer 1

From initial reads, we can derive that T1-->T2-->T3

And from final write, we can say, T3-->T2.

No updated reads here.

There is conflict T2-->T3 and T3-->T2.

so, not view serializable.

Comments

I have said this:From initial reads, we can derive that T1-->T2-->T3.

Only from initial reads.

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why are you considering both the reads?

Only one data item A.

* T1 performs first read

* T2 perform last write.

T1 and T2 contains only one instruction. So schdule to be checked with

T1-T3-T2.

* now check for producer consumer relationship.

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I think I misunderstood your answer. I thought you are following producer consumer method(view equivalence definition). sorry..

Answer 2

The schedule is not View serializable:

First read: T1

Last write: T2

Read after write: None

Write after Write: T2->T3 and T3->T2

since there is a cycle so it is not View serializable.

Comments

WAW is not considered in view serializability except the last write dependency

i.e T3 should come before T2 for W(A)