The Dual-Write Problem and the Outbox

1. The Bug Hiding in the Obvious Code

Checkout finishes an order. The code does the two things every event-driven design needs:

db.transaction {
  insert order          // 1. write to my database
}
broker.publish(orderPlacedEvent)   // 2. tell the world

This code is wrong. The bug has a name interviewers expect you to know: the dual-write problem.

Steps 1 and 2 write to two different systems. One is our database. The other is a broker, the piece of infrastructure that carries events to other services. No single transaction covers both.