2 phase locking

Question

how does a 2 phase locking protocol cause deadlock ?

and why is it conflict serializable

Comments

condition for deadlock on 2PL locking

X_L(A)

               X_L(B)

X_L(B)

               X_L(A)

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@ashwin

but in this case its not conflict serializable too right ?

Answer 1

Conflict  serializable it possible in Basic 2PL because it has following feature:

1> All locks should be acquired in growing phase only.

2->locks will be release in shrinking phase.

because a lock has got acquired in growing phase it can not be acquired again. so you will not get something like.

T1	T2
x(A)
WRITE A
RELEASE A
	X(A)
	WRITE A
	RELEASE A
GET A READ LOCK OR WRITE LOCK(NOT POSSIBLE)

because you can get locks only in growing phase once you release the lock you cant acquire locks after, this will ensure conflict serializablty.

Comments

@red devil

got that point...well said :)

then how can 2PL  cause a deadlock ?

and shouldnt conflict serializability and deadlock be connected?

if a schedule is conflict serializable then obviously it doesnt form a loop( which happens due to mutual waiting) and hence shouldnt have deadlocks, but they do  have............how ?

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This is leading to deadlock but is following 2PL 

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@red devil

so only if it releases its locks...it cannot acquire again right ?

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yes that is why there are growing and shrinking phase.