2PL | Made Easy Full Syllabus Test

Question

Given problem and solution.
 

It is true that the given schedule is not serializable, what I don't understand is how it is not allowed by the 2PL protocol?

Given Schedule :

 

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

 

Under 2PL :

 

T1	T3	T3
	S(A)
	R(A)
S(A)
R(A)
		S(B)
		R(B)
		X(B)
		W(B)
		Lock Point
		U(B)
	X(A)
	W(A)
	Lock Point
	U(A)
S(A)
Lock Point
U(A)

 

Comments

Thanks for the confirmation and this question was a good one. :)

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@ankit3009 @palashbehra5

In summary can I say that,

Shared lock can be acquired even if other transaction is holding shared lock on the same variable, but it is not allowed if other transaction is holding exclusive lock on the same variable.

Exclusive lock can only be acquired iff no other transaction is holding exclusive or shared lock on the same variable.

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yes, here's the pseudocode for read-write locks. source: Navathe, Elmasri.
 

 

Answer 1

This schedule is not in 2PL since X(A) in T2 can’t be acquired before releasing all the shared lock, If you release shared lock on A in T1 (before exclusive lock of T2) then you can’t acquire in final Read Request of T1. So this is not in 2PL.

Other explanation can be:

A 2PL schedule => conflict serializable

But the above schedule is not even conflict serializable (You can check it by swapping non conflicting pair, You will run into the issue with W2(A).)

So by contraposition (Not conflict schedule => not 2PL).

Hence the above schedule is neither 2PL nor conflict Serializable.

Regarding View Serializability: T1,T3 should come before T2 (Initial Read). But according to W R sequence of T1 and T2(Final 2 operations) T2 should come before T1 leading to contradiction. Hence Not View Serializable.

 

Hence the answer is D i.e. Schedule is not Serializable.

Comments

lock up-gradation is allowed under the growing phase though right?

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When no other Transaction is holding Shared lock.

Say You are in transaction 1 and it only holds shared lock on a variable then you can upgrade this shared lock to exclusive lock.

But Given transaction 1 and transaction 2 are holding shared lock simultaneously, you can’t upgrade to exclusive lock unless the other transaction let go of the shared lock

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Just one more property just in case. Every conflict serializable schedule is view serializable.

If the schedule is not view serializable then it is not conflict serializable.

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got it, thanks.