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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.
- T1 start
- T1 B old=12000 new=10000
- T1 M old=0 new=2000
- T1 commit
- T2 start
- T2 B old=10000 new=10500
- 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?
- 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
- c)3.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
Correct answer is option 'B'. Can you explain this answer?
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Most Upvoted Answer
Consider the following log sequence of two transactions on a bank acco...
Understanding the Recovery Procedure
When a database system crashes during transactions, the recovery procedure ensures data consistency. In this scenario, we have two transactions, T1 and T2, manipulating a bank account balance.
Log Sequence Overview
- Initial Balance: 12000
- Transaction T1:
- Step 1: Transfer 2000 to a mortgage payment (B old=12000, new=10000)
- Step 2: Commit T1
- Transaction T2:
- Step 3: Apply 5% interest (B old=10000, new=10500)
- Step 4: Commit T2
Crash Occurrence
The crash happens just before log record 7 is written, meaning T2 has not fully completed its operation.
Analyzing the Options
- Option A: Redo log record 6 (B set to 10500) is incorrect as T2 is not fully committed.
- Option B: Undo log record 6 (reset B to 10000) and then redo records 2 and 3. This is the correct action because T2's changes aren’t valid without T2’s commitment.
- Option C: This is incorrect because while T1 committed, T2 did not, thus T1’s effects must be preserved through redoing its operations.
- Option D: This is misleading; although redo and undo can often be applied in various orders, correct sequencing is crucial in this case to maintain data integrity.
Conclusion
Hence, the correct answer is option B. The recovery procedure must first undo T2’s uncommitted changes to ensure the account balance reflects only committed transactions. This maintains the integrity of the database state post-crash.
When a database system crashes during transactions, the recovery procedure ensures data consistency. In this scenario, we have two transactions, T1 and T2, manipulating a bank account balance.
Log Sequence Overview
- Initial Balance: 12000
- Transaction T1:
- Step 1: Transfer 2000 to a mortgage payment (B old=12000, new=10000)
- Step 2: Commit T1
- Transaction T2:
- Step 3: Apply 5% interest (B old=10000, new=10500)
- Step 4: Commit T2
Crash Occurrence
The crash happens just before log record 7 is written, meaning T2 has not fully completed its operation.
Analyzing the Options
- Option A: Redo log record 6 (B set to 10500) is incorrect as T2 is not fully committed.
- Option B: Undo log record 6 (reset B to 10000) and then redo records 2 and 3. This is the correct action because T2's changes aren’t valid without T2’s commitment.
- Option C: This is incorrect because while T1 committed, T2 did not, thus T1’s effects must be preserved through redoing its operations.
- Option D: This is misleading; although redo and undo can often be applied in various orders, correct sequencing is crucial in this case to maintain data integrity.
Conclusion
Hence, the correct answer is option B. The recovery procedure must first undo T2’s uncommitted changes to ensure the account balance reflects only committed transactions. This maintains the integrity of the database state post-crash.
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Community Answer
Consider the following log sequence of two transactions on a bank acco...
We must undo log record 6 to set B to 10000 and then redo log records 2 and 3 bcoz system fail before commit operation. So we need to undone active transactions(T2) and redo committed transactions (T1) Note: Here we are not using checkpoints. Checkpoint : Checkpoint is a mechanism where all the previous logs are removed from the system and stored permanently in a storage disk. Checkpoint declares a point before which the DBMS was in consistent state, and all the transactions were committed. Recovery: When a system with concurrent transactions crashes and recovers, it behaves in the following manner − =>The recovery system reads the logs backwards from the end to the last checkpoint. =>It maintains two lists, an undo-list and a redo-list. =>If the recovery system sees a log with and or just , it puts the transaction in the redo-list. =>If the recovery system sees a log with but no commit or abort log found, it puts the transaction in undo-list. All the transactions in the undo-list are then undone and their logs are removed. All the transactions in the redo-list and their previous logs are removed and then redone before saving their logs. So Answer is B redo log records 2 and 3 and undo log record 6
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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. T1 start T1 B old=12000 new=10000 T1 M old=0 new=2000 T1 commit T2 start T2 B old=10000 new=10500 T2 commitSuppose 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?a)We must redo log record 6 to set B to 10500b)We must undo log record 6 to set B to 10000 and then redo log records 2 andc)3.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 idempotentCorrect answer is option 'B'. Can you explain this answer?
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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. T1 start T1 B old=12000 new=10000 T1 M old=0 new=2000 T1 commit T2 start T2 B old=10000 new=10500 T2 commitSuppose 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?a)We must redo log record 6 to set B to 10500b)We must undo log record 6 to set B to 10000 and then redo log records 2 andc)3.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 idempotentCorrect answer is option 'B'. Can you explain this answer? for Computer Science Engineering (CSE) 2024 is part of Computer Science Engineering (CSE) preparation. The Question and answers have been prepared according to the Computer Science Engineering (CSE) exam syllabus. Information about 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. T1 start T1 B old=12000 new=10000 T1 M old=0 new=2000 T1 commit T2 start T2 B old=10000 new=10500 T2 commitSuppose 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?a)We must redo log record 6 to set B to 10500b)We must undo log record 6 to set B to 10000 and then redo log records 2 andc)3.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 idempotentCorrect answer is option 'B'. Can you explain this answer? covers all topics & solutions for Computer Science Engineering (CSE) 2024 Exam. Find important definitions, questions, meanings, examples, exercises and tests below for 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. T1 start T1 B old=12000 new=10000 T1 M old=0 new=2000 T1 commit T2 start T2 B old=10000 new=10500 T2 commitSuppose 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?a)We must redo log record 6 to set B to 10500b)We must undo log record 6 to set B to 10000 and then redo log records 2 andc)3.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 idempotentCorrect answer is option 'B'. Can you explain this answer?.
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. T1 start T1 B old=12000 new=10000 T1 M old=0 new=2000 T1 commit T2 start T2 B old=10000 new=10500 T2 commitSuppose 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?a)We must redo log record 6 to set B to 10500b)We must undo log record 6 to set B to 10000 and then redo log records 2 andc)3.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 idempotentCorrect answer is option 'B'. Can you explain this answer? for Computer Science Engineering (CSE) 2024 is part of Computer Science Engineering (CSE) preparation. The Question and answers have been prepared according to the Computer Science Engineering (CSE) exam syllabus. Information about 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. T1 start T1 B old=12000 new=10000 T1 M old=0 new=2000 T1 commit T2 start T2 B old=10000 new=10500 T2 commitSuppose 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?a)We must redo log record 6 to set B to 10500b)We must undo log record 6 to set B to 10000 and then redo log records 2 andc)3.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 idempotentCorrect answer is option 'B'. Can you explain this answer? covers all topics & solutions for Computer Science Engineering (CSE) 2024 Exam. Find important definitions, questions, meanings, examples, exercises and tests below for 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. T1 start T1 B old=12000 new=10000 T1 M old=0 new=2000 T1 commit T2 start T2 B old=10000 new=10500 T2 commitSuppose 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?a)We must redo log record 6 to set B to 10500b)We must undo log record 6 to set B to 10000 and then redo log records 2 andc)3.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 idempotentCorrect answer is option 'B'. Can you explain this answer?.
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ample number of questions to practice 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. T1 start T1 B old=12000 new=10000 T1 M old=0 new=2000 T1 commit T2 start T2 B old=10000 new=10500 T2 commitSuppose 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?a)We must redo log record 6 to set B to 10500b)We must undo log record 6 to set B to 10000 and then redo log records 2 andc)3.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 idempotentCorrect answer is option 'B'. Can you explain this answer? tests, examples and also practice Computer Science Engineering (CSE) tests.
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