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Recovery using an undo log

Example

step action log record 1 begin(T1) 2 read(A, v) 3 v = v * 2 4 write(A, v) update(T1, A, 20) 5 read(B, v) 6 v = v + 1 7 write(B, v) update(T1, B, 51) 8 flush() 9 output(A) CRASH

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