What does atomicity guarantee in the context of database transactions?

• A. Atomicity guarantees that a transaction can be partially applied and later completed.
• B. Atomicity guarantees data is never changed.
• C. Atomicity guarantees the transaction is all or nothing: either it commits completely or it aborts with no partial effects.
• D. Atomicity guarantees the database will always be consistent after a transaction.

Answer: (C)

Atomicity means a transaction is all-or-nothing: either every operation within the transaction commits and takes effect, or if any part fails, the entire transaction is aborted and all changes are rolled back. This guarantees that you won’t end up with a partial set of updates—the database state is as if the transaction never happened if it cannot complete.

Think of a money transfer that moves funds from one account to another. A successful transfer should debit one account and credit the other. With atomicity, either both updates happen and the transfer is stored, or neither update happens and the accounts remain as they were. Atomicity does not promise that data will never change, nor that the database will always be in a valid state after every transaction (that involves constraints and other ACID properties), nor does it permit partial application of a transaction. The essence is the all-or-nothing nature.