Strict 2PL

Question

Which of the following schedules are allowed by Strict 2PL protocol?

1.   r₁(A); r₂(B); r₃(C); w₁(B); C₁; w₂(C); C₂; w₃(D); C₃
2.   r₁(A); r₂(B); r₃(C); w₁(B); w₂(C); w₃(A); C₁; C₂; C₃
3.   r₁(A); r₂(B); r₃(C); r₁(B); r₂(C); r₃(D); C₃; w₁(C); C₁; w₂(D); C₂
4.   r₁(A); r₂(B); r₃(C); w₁(B); C₁; w₂(C); C₂; w₃(D); C₃

Comments

Yup exactly I was going To post my doubt for option1 and 4 is to why they are not strict 2PL

They are 2 strict PL  right???

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For Option 1 and option 4 This image is right???

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@bad_engineer

Could you answer my query after Lokesh's answer?

Answer 1

here, S(*) = shared lock and X(*) = Exclusive lock

For 1 and 4 (both are same)

T1	T2	T3
S(A) R(A)
	S(B) R(B)
		S(C) R(C) X(D) U(C)
	X(C) U(B)
X(B) W(B) commit U(B) U(A)
	W(C) commit U(C)
		W(D) commit U(D)

For 3rd schedule

T1	T2	T3
S(A) R(A)
	S(B) R(B)
		S(C) R(C)
S(B) R(B)
	S(C) R(C)
		S(D) R(D) commit U(C) U(D)
	X(D) U(C)
X(C) W(C) commit U(C) U(B) U(A)
	W(D) commit U(D) U(B)

Comments

@Rahul. Why are you keeping T2 and T1 waiting ? If you keep them waiting, the output may not be same as original schedule as per your method.

Consider this schedule:

               R(B)

R(B)

W(B)

              W(B)

By ur method, serializability order is T2-> T1.

But, this schedule isnt conflict serializzable.

Also, conflict serilazable schedules not possible under 2PL:

http://infolab.stanford.edu/~nsample/cs245/handouts/hw6sol/sol6.html

Also, we are working on a static schedule( i.e. we know the entire transaction). What about real time trasactions? Not possible

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Okay then as per you reference this schedule must not be allowed. I am now confused myself.

@anirban post the answer to make it clear.

I think sushant you are right. But still doubtful

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If schedule under 2PL then it is CS , right ?