Consider the following transactions T1, T2, and T3, with operations Read(X) and Write(X), where X is a data item:T1: Read(X), Write(X)T2: Write(X)T3: Read(X), Write(X)In the given scenario, which schedule is an example of an irrecoverable schedule?T1, T2, T3T1, T3, T2T2, T1, T3T3, T1, T2
Question
Consider the following transactions T1, T2, and T3, with operations Read(X) and Write(X), where X is a data item:T1: Read(X), Write(X)T2: Write(X)T3: Read(X), Write(X)In the given scenario, which schedule is an example of an irrecoverable schedule?T1, T2, T3T1, T3, T2T2, T1, T3T3, T1, T2
Solution
An irrecoverable schedule is one where a transaction Tj reads a data item X that is written by a transaction Ti before Ti commits, and Tj commits before Ti. This is dangerous because if Ti fails and needs to be rolled back after Tj has committed, it would be impossible to undo the effects of Tj, leading to an inconsistent database state.
Let's analyze the given schedules:
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T1, T2, T3: T1 reads and writes X, then T2 writes X, and finally T3 reads and writes X. This is not an irrecoverable schedule because there is no instance where a transaction reads a data item written by another transaction before it commits.
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T1, T3, T2: T1 reads and writes X, then T3 reads and writes X, and finally T2 writes X. This is not an irrecoverable schedule because there is no instance where a transaction reads a data item written by another transaction before it commits.
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T2, T1, T3: T2 writes X, then T1 reads and writes X, and finally T3 reads and writes X. This is an irrecoverable schedule because T1 reads X that is written by T2 before T2 commits.
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T3, T1, T2: T3 reads and writes X, then T1 reads and writes X, and finally T2 writes X. This is not an irrecoverable schedule because there is no instance where a transaction reads a data item written by another transaction before it commits.
So, the schedule T2, T1, T3 is an example of an irrecoverable schedule.
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