Explain what ACID means and what each property actually guarantees.

ACID describes the guarantees a transaction gives you. Atomicity means all of the writes land or none do. Consistency means the transaction moves the database from one valid state to another, respecting its constraints. Isolation means concurrent transactions do not see each other's partial work, to the degree the isolation level promises. Durability means that once a commit returns, the data survives a crash.

Atomicity is all or nothing: if I debit one account and credit another, either both writes commit or neither does. Consistency means the transaction leaves the database satisfying its constraints, so foreign keys and check constraints hold on either side of it. Isolation is the interesting one, because it is a dial rather than a promise. Most databases default to read committed, which still allows non repeatable reads and phantom rows, so if you assume serializable behavior you will eventually ship a race. Durability means that once the commit returns, the write survives a power cut, usually because it hit the write ahead log and was flushed. The place this bit me was a coupon redemption endpoint. We checked the remaining uses and then decremented in two separate statements under read committed, and two concurrent requests both saw one use left. The fix was a single conditional update with a where clause on the remaining count, which made the check and the write atomic.