recoverable doubt closed

Question

In the following T1 and T2 are transaction and A is an object ,which of the following  has the potential of becoming of irrecoverable schedules?

A)  T2 writes A after T1 wrote A; T1 is uncommitted

B) T2 reads A after T1 wrote A;  T1 is uncommitted

C )  T2 writes A after T1 wrote A; T1 is committed

D)   T2 reads A after T1 read A; T1 is uncommitted

Comments

mam in option A there is nowhere Read after Read

I think it's option D

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i think even option B is also not the answer,

i know the condition for recoverability,

r(x) in T2 is reading data from w(A), and there is no Commit operation, TRUE

but w(A) in T1 is blind write. even if the transaction fails after w(A) in T1, r(A) in T2 will be reading same value as it read before failing. so this transaction is recoverable.

if initially r(A) would have been there in T1, then it may have been irrecoverable.

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@Shaik Masthan, please see the option b, is it recoverable ?

Answer 1

B must be the answer.

Irrecoverability is when a committed transaction has to be rolled back .

A cannot be the answer , since T2 is writing A irrespective of the changes made by T1(This is called Blind Write).

B has to be the answer , since T2 is reading what T1 wrote and T1 is not committed till now . So there's a potential chance that T2 would be needed to rollback if T1 failed , to maintain consistency.

C cannot be answer since transaction is committed and D cannot be answer since Read Read are not conflicting opertations