I have a REST API to manage a booking system I'm searching how to manage this situation : A customer can book a time slot : A TimeSlot resource is created and linked to a Person resource. In order to create the link between a time lot and a person, the REST client send a POST request on the TimeSlot resource But if too many people booked the same slot (let's say the limit is 5 links), it must be impossible to create more associations. How can I handle this business restriction ? Can I return a 404 status code with a JSON response detailing the error with a status code ? Is it a RESTFul approach ? EDIT : Like suggested below I used status 409 Conflict in addition to a JSON response detailing the error