Lets say I have service A which depends on service B. When service B is down service A returns 400 Bad Request when posting an object. Is this correct behavior? My worry is that this is misleading. There is nothing wrong with the request it just can't be processed because service B is down. Is there a better return code that would inform the user of this?
You're right, HTTP 4xx status codes are not appropriate for errors that are not caused by the client.
You should return an HTTP 5xx status code. Either "502 Bad Gateway" or "503 Service Unavailable" seems appropriate to me, depending on whether service A just acts as a proxy for service B.
If you don't want to disclose a specific reason, you can also return "500 Internal Server Error".
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