The Transactional Outbox

1. The Dual Write

A service handles a request, changes its own state, and tells the rest of the world. Written directly, that is two operations against two systems.

order = db.insert(order)          # system one
broker.publish(OrderCreated(...)) # system two

There is no transaction spanning both, so there are four outcomes and two of them are wrong.

• Both succeed. Correct.
• Both fail. Correct, nothing happened.
• The database commits and the publish fails. The order exists and nobody downstream knows. No warehouse pick, no confirmation email, no analytics row.