Consider the following statements: Statement I: Conservative 2PL is a…
2022
Consider the following statements:
Statement I: Conservative 2PL is a deadlock-free protocol.
Statement II: Thomas's write rule enforces conflict serializability.
Statement III: Timestamp ordering protocol ensures serializability based on the order of transaction timestamps.
Which of the following is correct ?
- A.
Statement I, Statement II true and Statement III false
- B.
Statement I, Statement III true and Statement II false
- C.
Statement I, Statement II false and Statement III true
- D.
Statement I, Statement II and Statement III true
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Correct answer: B
Answer: Statement I is true; Statement II is false; Statement III is true.
Statement I — Conservative two-phase locking is deadlock-free.
Why: In conservative (static) 2PL a transaction obtains all required locks before it starts executing. Because no transaction holds a lock while waiting for another lock, circular wait cannot occur, so deadlocks are prevented.
Statement II — Thomas's write rule does not enforce conflict serializability (so the statement is false).
Why: Thomas's write rule is a relaxation of basic timestamp ordering that allows obsolete writes (writes with timestamps older than the most recent read or write) to be ignored instead of causing aborts. This improves concurrency but can permit schedules that are not conflict-serializable; at best it preserves a weaker form of serializability (e.g., view serializability in cases).
Statement III — Timestamp ordering protocol ensures serializability based on transaction timestamps.
Why: The basic timestamp ordering protocol assigns each transaction a timestamp and orders conflicting operations according to these timestamps; this enforces a serial schedule equivalent to timestamp order, guaranteeing serializability.
Conclusion: Statement I and Statement III are true; Statement II is false.