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Undo Logging

Rules

1. Write update(T, x, v) to the log before the new value of x is written to the database.

2. Write commit(T) to the log (soon) after updated pages have been written to the database.

For any changed element

1. Write the undo record to the log.

2. Write the new value of the element to the database.

3. (Eventually) write the commit record to the log.

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