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Q6: Recovering from a system failure:
1- Scan the log from tail (newest) to head (oldest)
:
Create a list of committed transactions
T1,T3,T5
T4 aborted, T2 and T6 active
2- Undo updates of active and aborted transactions
(T6,A,2,3)
A = 2
(T6,X,400,0)
X = 400
(T4,A,0,1)
A = 0
(T2,Y,10,20)
Y = 10
Scan the log from head (oldest) to tail (newest):
Redo updates of committed transactions
(T1,X,0,10)
X = 10
(T3,X,10,400)
X = 400
(T3,Z,0,100)
Z = 100
(T5,A,0,2)
A = 2
Thus, T1,T3 & T5 are committed while T2,T4 & T6 are
aborted
Object value changes (last is final)
A: 2,0,2
X: 400,10,400
Y: 10
Z: 100