Transactions & Isolation

Atomicity — all of a transaction's changes apply, or none do. A crash mid-transfer cannot leave money debited from one account and not credited to the other.

Consistency — the database moves from one valid state to another, enforcing constraints. Weaker than it sounds: it means your declared constraints hold, nothing more.

Durability — once committed under the configured guarantee, the change survives the failures that guarantee covers. With synchronous local durability, commits wait for persistent storage; asynchronous settings deliberately trade some of that guarantee for latency.

Isolation is the interesting one. It governs what concurrent transactions can see of each other, and unlike the others it is not a single guarantee — it is a dial, with real anomalies at every setting below the top.