Atomicity

• A transaction is all or nothing — if any step fails, the entire transaction is rolled back and the database is left unchanged
• Implemented via undo logs (also called rollback logs): the database records the inverse of each operation so it can reverse partial changes on failure

BEGIN;
  UPDATE accounts SET balance = balance - 20 WHERE id = 1;
  UPDATE accounts SET balance = balance + 20 WHERE id = 2;
COMMIT; -- both succeed, or neither does