Which of the following concurrency anomalies is also known as a read-write…
2018
Which of the following concurrency anomalies is also known as a read-write conflict?
Answer: C. Unrepeatable read — Concept: Concurrency-control theory names a conflict between two transactions on the same data item by the chronological order of their two operations: a read…
- A.
Dirty read
- B.
Lost update problem
- C.
Unrepeatable read
- D.
Uncommitted dependency
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Correct answer: C
Concept: Concurrency-control theory names a conflict between two transactions on the same data item by the chronological order of their two operations: a read followed later by another transaction’s write is a read-write (R-W) conflict; a write followed later by another transaction’s read is a write-read (W-R) conflict; and a write followed later by another transaction’s write is a write-write (W-W) conflict.
Application: Trace the operations for an unrepeatable read on an item X:
Transaction T1 reads item X.
Transaction T2 writes/updates item X and commits.
T1 reads item X again, later in the same execution, and gets a different value than its first read.
The two operations on X therefore occur in the order read (T1) → write (T2), which is exactly the read-then-write ordering that defines a read-write conflict — so “read-write conflict” is the alternate name for unrepeatable read.
Cross-check against the other anomalies:
Anomaly | Operation order on the item | Conflict type |
|---|---|---|
Dirty read / uncommitted dependency (same anomaly) | write (uncommitted) → read | write-read (W-R) |
Lost update problem | write → write | write-write (W-W) |
Unrepeatable read | read → write | read-write (R-W) |
Result: Only the unrepeatable read matches the read-then-write ordering that defines a read-write conflict, so the correct answer is unrepeatable read.