I need to set particular resource accessible via REST Api to one of defined states - idle, running or stopped. How to inform a REST API client that record is already in requested state? E.g. client wants to start the record and that record is already running? He needs to know that record is running and operation is not processable.
It's definitively client error, so status code from 4xx family should be the answer. I have an idea to use 400, 409 or 412 but not sure.
How to inform a REST API client that record is already in requested state
Closest I could find is
409 Conflict
The request could not be completed due to a conflict with the current state of the resource. This code is only allowed in situations where it is expected that the user might be able to resolve the conflict and resubmit the request. The response body SHOULD include enough information for the user to recognize the source of the conflict. Ideally, the response entity would include enough information for the user or user agent to fix the problem; however, that might not be possible and is not required.
Conflicts are most likely to occur in response to a PUT request. For example, if versioning were being used and the entity being PUT included changes to a resource which conflict with those made by an earlier (third-party) request, the server might use the 409 response to indicate that it can't complete the request. In this case, the response entity would likely contain a list of the differences between the two versions in a format defined by the response Content-Type.
emphasis mine