ITPEC FE Morning October 2019 Question 31

Database Lock Compatibility — shared locks allow concurrent reads; exclusive locks block everything.

Lock compatibility matrix for concurrency control:

• •Shared + Shared → permitted (multiple readers OK)
• •Shared + Exclusive → denied
• •Exclusive + Shared → denied
• •Exclusive + Exclusive → denied

Why (a) is correct:
- A shared lock (S-lock) is for reading — multiple transactions can hold shared locks on the same resource simultaneously

Why not others:
- (b) Shared + Exclusive is denied — a writer would conflict with existing readers
- (c) Exclusive + Shared is denied — a reader would see uncommitted (dirty) data
- (d) Exclusive + Exclusive is denied — two concurrent writes cause data corruption

Key rule: Only shared locks are compatible with each other. Any involvement of an exclusive lock blocks all other lock requests.

AI-generated — may contain errors