How does the transactional outbox pattern guarantee that a business change and its event are never inconsistent?

Instead of trying to make a database save and a message publish atomic with each other directly, which is hard because they're different systems, the pattern writes the event into a plain database table, the outbox, in the exact same local database transaction as the business change itself. Local database transactions are already atomic through ordinary database guarantees, so the business row and the outbox row either both commit or neither does — there's no window where one succeeds without the other. A separate relay process then reads unpublished outbox rows afterward and delivers them to a message broker, retrying as needed. Because the durable record of the event survived the one transaction that mattered, a crash in the relay or the broker only delays delivery; it never loses the event.