Execution of Transactions (cont)
States as the transaction executes:
t Action v Buf(A) Buf(B) Disk(A) Disk(B)
-----------------------------------------------------
(0) BEGIN . . . 8 5
(1) READ(A,v) 8 8 . 8 5
(2) v = v*2 16 8 . 8 5
(3) WRITE(A,v) 16 16 . 8 5
(4) READ(B,v) 5 16 5 8 5
(5) v = v+1 6 16 5 8 5
(6) WRITE(B,v) 6 16 6 8 5
(7) COMMIT 6 16 6 8 5
(8) OUTPUT(A) 6 16 6 16 5
(9) OUTPUT(B) 6 16 6 16 6
(*) committed - - - 16 6
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If system crashes before (7), may need to undo disk changes.
If system crashes after (7), may need to redo disk changes.
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