400 BAD request HTTP error code meaning?

 

I have a JSON request which I'm posting to a HTTP URL.

Should this be treated as 400 where requestedResource field exists but "Roman" is an invalid value for this field? 

[{requestedResource:"Roman"}]

Should this be treated as 400 where "blah" field doesn't exist at all? 

[{blah:"Roman"}]

A 400 means that the request was malformed. In other words, the data stream sent by the client to the server didn't follow the rules.

In the case of a REST API with a JSON payload, 400's are typically, and correctly I would say, used to indicate that the JSON is invalid in some way according to the API specification for the service.

By that logic, both the scenarios you provided should be 400's.

Imagine instead this were XML rather than JSON. In both cases, the XML would never pass schema validation--either because of an undefined element or an improper element value. That would be a bad request. Same deal here.

From w3.org

10.4.1 400 Bad Request

The request could not be understood by the server due to malformed syntax. The client SHOULD NOT repeat the request without modifications.

Selecting a HTTP response code is quite an easy task and can be described by simple rules. The only tricky part which is often forgotten is paragraph 6.5 from RFC 7231:

Except when responding to a HEAD request, the server SHOULD send a representation containing an explanation of the error situation, and whether it is a temporary or permanent condition.

Rules are as following:

  • If request was successful, then return 2xx code (3xx for redirect). If there was an internal logic error on a server, then return 5xx. If there is anything wrong in client request, then return 4xx code.
  • Look through available response code from selected category. If one of them has a name which matches well to your situation, you can use it. Otherwise just fallback to x00 code (200, 400, 500). If you doubt, fallback to x00 code.
  • Return error description in response body. For 4xx codes it must contain enough information for client developer to understand the reason and fix the client. For 5xx because of security reasons no details must be revealed.
  • If client needs to distinguish different errors and have different reaction depending on it, define a machine readable and extendible error format and use it everywhere in your API. It is good practice to make that from very beginning.
  • Keep in mind that client developer may do strange things and try to parse strings which you return as human readable description. And by changing the strings you will break such badly written clients. So always provide machine readable description and try to avoid reporting additional information in text.

    • So in your case I'd returned 400 error and something like this if "Roman" is obtained from user input and client must have specific reaction: 

    {
    "error_type" : "unsupported_resource",
    "error_description" : ""Roman" is not supported"
}

    or a more generic error, if such situation is a bad logic error in a client and is not expected, unless developer made something wrong: 

    {
    "error_type" : "malformed_json",
    "error_description" : ""Roman" is not supported for "requestedResource" field"
}
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