Assume all transactions are attempting to access the same database resource.

Transaction	Operation Requested	Lock Type Requested
T1	Read	Shared (S-lock)
T2	Read	Shared (S-lock)
T3	Write	Exclusive (X-lock)
T4	Read	Shared (S-lock)

Based on standard DBMS lock compatibility rules, which of the following statements is correct?

Correct option is A

In a DBMS locking mechanism, multiple transactions can simultaneously hold Shared Locks (S-locks) on the same resource because shared locks are compatible with each other and allow concurrent read operations.

However, an Exclusive Lock (X-lock) is incompatible with both shared and other exclusive locks because it is used for write operations and requires exclusive access to the resource.

Therefore:

• T1, T2, and T4 can all read the resource together using S-locks.
• T3 cannot obtain an X-lock while any S-lock exists on the resource.

Hence, option (a) is correct.

Important Key Points:

1. Shared Locks (S-locks) allow multiple transactions to read the same resource simultaneously.
2. Exclusive Locks (X-locks) are used for write operations.
3. An X-lock is incompatible with both S-locks and other X-locks.
4. Lock compatibility helps maintain data consistency and concurrency control in DBMS.

Knowledge Booster:

1. (b) Transaction T3 can obtain an X-lock while T1 holds an S-lock: Incorrect because an X-lock is incompatible with an existing S-lock.
2. (c) Transaction T4 can read the resource while T3 holds an X-lock: Incorrect because no shared access is allowed while an X-lock exists.
3. (d) An S-lock and an X-lock are compatible on the same resource: Incorrect because S-locks and X-locks are incompatible.
4. (e) Shared and exclusive locks are interchangeable: Incorrect because S-locks and X-locks serve different purposes and have different compatibility rules.