In concurrency control, what is the primary difference between a "shared lock" and an "exclusive lock"?

Question

BlackTom AI · Accepted Answer

A shared lock permits multiple transactions to read the same data item concurrently, while an exclusive lock grants a single transaction the right to both read and modify the data and prevents other transactions from accessing it.

1) Relevant knowledge points and concepts
- Concurrency control: ensures correctness when multiple transactions run concurrently.
- Locks: mechanisms to control access to data items; common types are shared (read) and exclusive (write).
- Lock compatibility: shared locks are compatible with other shared locks but not with exclusive locks; exclusive locks are incompatible with both shared and exclusive locks held by others.
- Two-phase locking (2PL): protocols use shared and exclusive locks to guarantee serializability.

2) Step-by-step reasoning
- If Transaction T1 wants to read data D, it can acquire a shared lock on D. If Transaction T2 also wants to read D, it can also obtain a shared lock because reads do not conflict.
- If Transaction T3 wants to write D, it must acquire an exclusive lock. An exclusive lock cannot be granted while any shared or exclusive locks by others exist, so T3 must wait until T1 and T2 release their shared locks.
- Once T3 holds an exclusive lock, no other transaction can read or write D until T3 releases the lock.

3) Calculations / intermediate steps
- No numerical calculations are required; the logic is based on compatibility: Shared vs Shared = allowed; Shared vs Exclusive = conflict; Exclusive vs Exclusive = conflict.

4) Why the correct answer is correct
- The core distinction is read-sharing: shared locks allow concurrent read access, exclusive locks prevent any concurrent access so the lock holder can safely perform updates.

5) Why the other options are incorrect
- Fat vs thin clients: unrelated to lock semantics.
- Vertical vs horizontal partitioning: refers to data modeling, not lock types.
- Permanent vs expiring after 60 seconds: locks are not defined by a fixed expiration in general concurrency theory.

6) Conclusion
This confirms the correct answer: "A shared lock allows others to read data, while an exclusive lock prevents all access."

In concurrency control, what is the primary difference between a "shared lock" and an "exclusive lock"?单项选择题

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