Consider the following log sequence of two transactions on a bank account,…
2006
Consider the following log sequence of two transactions on a bank account, with initial balance 12000, that transfer 2000 to a mortgage payment and then apply a 5% interest.
1. T1 start
2. T1 B old=12000 new=10000
3. T1 M old=0 new=2000
4. T1 commit
5. T2 start
6. T2 B old=10000 new=10500
7. T2 commit Suppose the database system crashes just before log record 7 is written. When the system is restarted, which one statement is true of the recovery procedure?
Answer: B. We must undo log record 6 to set B to 10000 and then redo log records 2 and 3. — When the system crashes before log record 7 (T2 commit), the recovery procedure must ensure database consistency. T1 has committed (log records 1–4), so its…
- A.
We must redo log record 6 to set B to 10500
- B.
We must undo log record 6 to set B to 10000 and then redo log records 2 and 3.
- C.
We need not redo log records 2 and 3 because transaction T1 has committed.
- D.
We can apply redo and undo operations in arbitrary order because they are idempotent
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Correct answer: B
When the system crashes before log record 7 (T2 commit), the recovery procedure must ensure database consistency. T1 has committed (log records 1–4), so its changes (B from 12000 to 10000, M from 0 to 2000) are durable and do not need redo. T2 has started (record 5) and modified B to 10500 (record 6), but it did not commit. Therefore, T2's changes must be undone to restore B to 10000. The recovery process involves: 1) Undoing T2's update to B, 2) Redoing T1's changes to ensure the transfer is applied. The order is critical: undo must precede redo.