Which principle states that completed transactions should remain stable/permanent even during system failure?

• A. Durability
• B. Availability
• C. Consistency
• D. Isolation

Answer: (A)

Durability is the principle that ensures once a transaction is committed, its effects are saved to non-volatile storage and will survive crashes or power failures. This means the changes are not lost even if the system goes down right after the commit, because they’ve been persisted, often via write-ahead logs and durable storage across replicas. For example, a completed money transfer will still be reflected in the database after a restart.

The other properties relate to different guarantees: availability is about the system being accessible, consistency ensures all nodes see the same data after operations, and isolation prevents concurrent transactions from interfering with each other. None of them alone guarantees that a committed transaction remains permanent after a failure.