If there's a chance that the links will exist in the future, both 400 Bad Request and 403 Forbidden would be incorrect: their relevant sections in RFC 2616 both have instructions that the client SHOULD NOT repeat the request. The difference between the two is that, in the case of 400 Bad Request, the server did not understand what the client was trying to do, whereas with 403 Forbidden the server did understand the request, but is refusing to fulfill it (ever).
If you want to indicate to the client that the request was understood, but you just won't handle it at the time of the request (but make no claim about handling it in the future), 404 Not Found would be the most appropriate response to send.
If you want to indicate to the client that request was understood, but the request does not follow predetermined rules or guidelines (like your example of a client not providing two numbers within 20% of each other), you want to use 409 Conflict, which is intended to indicate to the client that the request needs to be fixed in order to be handled properly.
You should separate good faith non-conformity from bad-faith non-conformity, though. If you're trying to protect your application from requests by a bad actor, they're almost certainly not going to care what response code you send: they'll just keep constructing URLs until they hit something they like.
When in doubt, consult the documentation. Reviewing the W3C definitions for HTTP Status codes, gives us this:
200 OK - The request has succeeded. The information returned with the response is dependent on the method used in the request.
404 Not Found - The server has not found anything matching the Request-URI.
In the context of your API, it very much depends on how queries are created and how objects are retrieved. But, my interpretation has always been that:
- If I ask for a particular object, and it exists return
200 code, if it doesn't exist return the correct 404 code.
- But, if I ask for a set of objects that match a query, a null set is a valid response and I want that returned with a
200 code. The rationale for this is that the query was valid, it succeeded and the query returned nothing.
So in this case you are correct, the service isn't searching for "a specific thing" it is requesting a particular thing, if that thing isn't found say that clearly.
I think Wikipedia puts it best:
200 OK - ... The actual response will depend on the request method used. In a GET request, the response will contain an entity corresponding to the requested resource.
404 Not Found - The requested resource could not be found but may be available again in the future. Subsequent requests by the client are permissible.
Seems pretty clear to me.
Regarding the example requests
/GoalTree/GetByDate?versionDate=...
/GoalTree/GetById?versionId=...
For the format, you said, you always return the nearest revision to that date. It will never not return an object, so it should always be returning 200 OK. Even if this were able to take a date range, and the logic were to return all objects within that timeframe returning 200 OK - 0 Results is ok, as that is what the request was for - the set of things that met that criteria.
However, the latter is different as you are asking for a specific object, presumably unique, with that identity. Returning 200 OK in this case is wrong as the requested resource doesn't exist and is not found.
Regarding choosing status codes
- 2xx codes Tell a User Agent (UA) that it did the right thing, the request worked. It can keep doing this in the future.
- 3xx codes Tell a UA what you asked probably used to work, but that thing is now elsewhere. In future the UA might consider just going to the redirect.
- 4xx codes Tell a UA it did something wrong, the request it constructed isn't proper and shouldn't try it again, without at least some modification.
- 5xx codes Tell a UA the server is broken somehow. But hey that query could work in the future, so there is no reason not to try it again. (except for 501, which is more of a 400 issue).
You mentioned in a comment using a 5xx code, but your system is working. It was asked a query that doesn't work and needs to communicate that to the UA. No matter how you slice it, this is 4xx territory.
Consider an alien querying our solar system
Alien: Computer, please tell me all planets that humans inhabit.
Computer: 1 result found. Earth
Alien: Computer, please tell me about Earth.
Computer: Earth - Mostly Harmless.
Alien: Computer, please tell me about all planets humans inhabit, outside the asteroid belt.
Computer: 0 results found.
Alien: Computer, please destroy Earth.
Computer: 200 OK.
Alien: Computer, please tell me about Earth.
Computer: 404 - Not Found
Alien: Computer, please tell me all planets that humans inhabit.
Computer: 0 results found.
Alien: Victory for the mighty Irken Empire!
Best Answer
Return
200 OKand (optionally) include some status information in your JSON indicating that some of the entities no longer exist.If you return no entities, you force your user to create a new request that excludes the missing entities. Just note in your documentation that your endpoint will not return entities that no longer exist, which is the most intuitive behavior anyway.