Implementing Abort(T)

• Suppose T wrote page P.

• If P was not transferred to stable storage, then deallocate its cache slot

• If it was transferred, then P’s before-image must be in stable storage (else you couldn’t undo after a system failure)

• Undo Rule - Do not flush an uncommitted update of P until P’s before-image is stable. (Ensures undo is possible.)

  • Write-Ahead Log Protocol - Do not … until P’s before-image is in the log

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