Database-Management-System
March 26, 2024Question 5086 – Communication
March 26, 2024Database-Management-System
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Question 257
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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?
(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?
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Undo : T3, T1; Redo T2
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Undo : T3, T1; Redo T2, T4
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Undo : none; Redo T2, T4, T3 T1
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Undo : T3, T1, T4; Redo T2
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Question 257 Explanation:
As T1 & T3 are not yet committed they must be undone. The transactions which are after the latest checkpoint must be redone. So T2 must be redone. No need to redo the records which are before the last checkpoint, so T4 need not be redone.
Correct Answer: A
Question 257 Explanation:
As T1 & T3 are not yet committed they must be undone. The transactions which are after the latest checkpoint must be redone. So T2 must be redone. No need to redo the records which are before the last checkpoint, so T4 need not be redone.