A partial schedule is: Time T1 T2 --- --- --- 1 read(A) 2 write(A) 3 read(C) 4 write(C) 5 read(B) 6 write(B) 7 read(A) 8 commit 9 read(B) T1 fails immediately after time 9. Which is correct?

Abort T2 then restart T1 and T2 to ensure atomicity

The schedule is recoverable and nothing else is needed

GATE 2015 CS (CS3) – Question 51

Databases · Transactions and Concurrency Control · 2 marks · Multiple choice

A partial schedule is:

Time	T1	T2
1	read(A)
2	write(A)
3		read(C)
4		write(C)
5		read(B)
6		write(B)
7		read(A)
8		commit
9	read(B)

T1 fails immediately after time 9. Which is correct?

Abort T2 then restart T1 and T2 to ensure atomicity
The schedule is nonrecoverable and cannot ensure transaction atomicity
Only abort and restart T2
The schedule is recoverable and nothing else is needed

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Correct answer: (B) The schedule is nonrecoverable and cannot ensure transaction atomicity

Explanation

T2 reads A written by uncommitted T1 and then commits at time eight. T1 subsequently fails, invalidating a value already used by committed T2. Recoverability requires a reader to commit only after the writer it depends on commits; that condition is violated. Therefore the schedule is nonrecoverable, B.

GATE 2015 CS (CS3) – Question 51

Explanation

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