GATE CS 2015 Set 2 — Question 39

MCQ+2 / -0.67MediumLog-Based RecoveryTransactions & ConcurrencyDatabases

Databases → Transactions & Concurrency → Log-Based Recovery

Last updated 5 September 2026

Question

Consider a simple checkpointing protocol and the following set of operations in the log. (start, T4); (write, T4, y, 2, 3); (start, T1); (commit, T4); (write, T1, z, 5, 7); (checkpoint); (start, T2); (write, T2, x, 1, 9); (commit, T2); (start, T3), (write, T3, z, 7, 2); If a crash happens now and the system tries to recover using both undo and redo operations, what are the contents of the undo list and the redo list?

Correct answer (A) Undo: T3, T1; Redo: T2 Solution At the time of the crash: Undo List : Transactions that were active (started but not committed) at the time of the crash. T1 started before checkpoint and has not committed. T3 started after checkpoint and has not committed. So, Undo List = {T1, T3}. Redo List : Transactions that committed after the last checkpoint (or were active at checkpoint and committed later). T4 committed before the checkpoint, so its changes are already saved to the database. It does not need to be redone. T2 started after the checkpoint and committed before the crash. It needs to be redone. So, Redo List = {T2}. Turn this into a strength. Explore AI-powered practice and doubt support with Success Tracker.

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Correct answer (A) Undo: T3, T1; Redo: T2 Solution At the time of the crash: Undo List : Transactions that were active (started but not committed) at the time of the crash. T1 started before checkpoint and has not committed. T3 started after checkpoint and has not committed. So, Undo List = {T1, T3}. Redo List : Transactions that committed after the last checkpoint (or were active at checkpoint and committed later). T4 committed before the checkpoint, so its changes are already saved to the database. It does not need to be redone. T2 started after the checkpoint and committed before the crash. It needs to be redone. So, Redo List = {T2}.

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