Introduction: The Hard Parts

It is minute 30 of the interview.

The resources are named. The operation list is on the board. One create operation has a request, response, and ordinary validation errors.

Then the interviewer changes the question:

"What happens if the caller sends the payment twice?"

The API has not gained a new feature. The existing promise is being tested under uncertainty.

That is the subject of this chapter. Chapters 2 and 3 established what callers can request and how results can be delivered