GATE CS 2014 Set 2 — Question 39
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Databases → Transactions & Concurrency → Conflict Serializability
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Question
Consider the following schedule of transactions T1, T2, T3, T4:
Which one of the following statements is CORRECT?
| T1 | T2 | T3 | T4 |
|---|---|---|---|
| Reads(X) | |||
| Writes(X) | |||
| Commit | |||
| Writes(X) | |||
| Commit | |||
| Writes(Y) | |||
| Reads(Z) | |||
| Commit | |||
| Reads(X) | |||
| Reads(Y) | |||
| Commit |
Correct answer
(C) S is both conflict-serializable and recoverable
Solution
Conflict Serializability:
We analyze the conflicts:
For a schedule to be recoverable, if transaction reads a value written by , then must commit before commits.
We analyze the conflicts:
1. reads before writes :
2. writes before writes :
3. writes before reads :
4. writes before reads :
The precedence graph is . There are no cycles, so the schedule is conflict-serializable.Recoverability:For a schedule to be recoverable, if transaction reads a value written by , then must commit before commits.
1. reads written by . commits before reads . This is recoverable (and cascadeless).
2. reads written by . commits before reads . This is recoverable (and cascadeless).
Since all read dependencies satisfy the recoverability condition, the schedule is recoverable.Thus, is both conflict-serializable and recoverable.Continue learning with Success Tracker
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