GATE CS 2021 Set 1 — Question 23

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MSQ+1 / -0MediumLog-Based RecoveryTransactions & ConcurrencyDatabases

Databases → Transactions & Concurrency → Log-Based Recovery

Last updated 5 September 2026

Question

Suppose a database system crashes again while recovering from a previous crash. Assume checkpointing is not done by the database either during the transactions or during recovery. Which of the following statements is/are correct?

Correct answer (A) The same undo and redo list will be used while recovering again. Solution The correct answer is (A) . Reasoning: 1. Idempotence of Recovery: Database recovery processes (such as ARIES) are designed to be idempotent. This means that if the system crashes during the recovery phase, the recovery process can simply be restarted from the beginning (or the last valid checkpoint) without causing inconsistency. The end result will be the same as if the recovery had succeeded on the first attempt. 2. Log Invariance: The transaction log is stored on stable storage. When the system crashes during recovery, the log records from the original transactions remain unchanged. Since no new user transactions are processed during recovery, the history of operations to be analyzed remains the same. 3. Absence of Checkpoints: The question states that no checkpointing is done. This forces the recovery process to scan the log from the beginning (or the start of the earliest uncommitted transaction) every time it restarts. 4. Same Lists: Because the log content is identical and the starting point is the same, the Analysis Phase of the recovery algorithm will identify the exact same set of "winner" (to be redone) and "loser" (to be undone) transactions. Consequently, the undo and redo lists constructed will be identical to those from the previous (failed) recovery attempt. Thus, the system can recover successfully, operations will be re-applied/undone as necessary (idempotently), and consistency will be restored. Turn this into a strength. Explore AI-powered practice and doubt support with Success Tracker.

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Correct answer (A) The same undo and redo list will be used while recovering again. Solution The correct answer is (A) . Reasoning: 1. Idempotence of Recovery: Database recovery processes (such as ARIES) are designed to be idempotent. This means that if the system crashes during the recovery phase, the recovery process can simply be restarted from the beginning (or the last valid checkpoint) without causing inconsistency. The end result will be the same as if the recovery had succeeded on the first attempt. 2. Log Invariance: The transaction log is stored on stable storage. When the system crashes during recovery, the log records from the original transactions remain unchanged. Since no new user transactions are processed during recovery, the history of operations to be analyzed remains the same. 3. Absence of Checkpoints: The question states that no checkpointing is done. This forces the recovery process to scan the log from the beginning (or the start of the earliest uncommitted transaction) every time it restarts. 4. Same Lists: Because the log content is identical and the starting point is the same, the Analysis Phase of the recovery algorithm will identify the exact same set of "winner" (to be redone) and "loser" (to be undone) transactions. Consequently, the undo and redo lists constructed will be identical to those from the previous (failed) recovery attempt. Thus, the system can recover successfully, operations will be re-applied/undone as necessary (idempotently), and consistency will be restored.

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GATE CS 2021 Set 1 — Question 23

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