ACID Transactions

A transaction groups several operations so they succeed or fail as one unit — ACID is the set of guarantees that makes that safe.

A transaction bundles multiple reads and writes into a single unit — either the whole thing takes effect, or none of it does. ACID is the classic acronym for the guarantees a transactional database gives you.

Fig. 6 — The four ACID guarantees

Click a letter to see what it actually guarantees.

BEGIN;
UPDATE accounts SET balance = balance - 100 WHERE id = 'a';
UPDATE accounts SET balance = balance + 100 WHERE id = 'b';
COMMIT;

If the process crashes between the two UPDATEs, atomicity guarantees the first one gets rolled back too — you never end up with money debited from one account and not credited to the other.