HTTP Response Codes

HTTP Response Codes

HTTP defines a set of standard response codes on requests, they are largely grouped as follows:

  • 1xx: compatibility with older HTTP, typically of no concern

  • 2xx: success

  • 3xx: redirection (the resource is at another location, or is unchanged since last requested)

  • 4xx: client errors (the client did something wrong)

  • 5xx: server errors (the server failed in a way that was unexpected)

2xx Success Codes

  • Synchronous resource creation

  • Response status code must be 201 Created

  • Must return a Location header with the URI of the created resource

  • Should return a representation of the resource in the body

  • Asynchronous resource creation

    • Response status code must be 202 Accepted

    • Must return a Location header set to one of the following:
      • the URI of the resource to be created, if known.

      • the URI of a status resource that the client can use to query the progress of the asynchronous operation.

  • Synchronous resource deletion

  • Response status code must be 204 No Content

  • For all other successful requests, the return code should be 200 OK.

  • If a request attempts to put a resource into a state which it is already in (for example, locking an instance which is already locked), the return code should be in the 2xx Successful range (usually matching the return code which would be given if the state had changed). It is not appropriate to use 409 Conflict when the resulting state of the resource is as the user requested.

5xx Server Error Codes

These codes represent that the server, or gateway, has encountered an error or is incapable of performing the requested method. They indicate to a client that the request has resulted in an error that exists on the server side and not with the client.

They should be used to indicate that errors have occurred during the request process which cannot be resolved by the client alone. The nature of each code in the 5xx series carries a specific meaning and they should be fully researched before deploying.

The server must not return server-side stacktraces/traceback output to the end user. Tracebacks and stacktraces belong in server-side logs, not returned via the HTTP API to an end user.

Failure Code Clarifications

  • If the request results in the OpenStack user exceeding his or her quota, the return code should be 403 Forbidden. Do not use 413 Request Entity Too Large.

  • For badly formatted requests, the return code should be 400 Bad Request. Do not use 422 Unprocessable Entity.

    • If the API limits the length of a property that is a collection, the return code should be 400 Bad Request when the request exceeds the length limit. The client should adjust requests to achieve success, and shouldn’t expect to repeat the request and have it work. Do not use 403 Forbidden for this case, because this is different than exceeding quota – for a subsequent request to succeed when quotas are exceeded the server environment must change.

  • If a request contains a reference to a nonexistent resource in the body (not URI), the code should be 400 Bad Request. Do not use 404 NotFound because RFC 7231#section-6.5.4 (section 6.5.4) mentions the origin server did not find a current representation for the target resource for 404 and representation for the target resource means a URI. A good example of this case would be when requesting to resize a server to a non-existent flavor. The server is the resource in the URI, and as long as it exists, 404 would never be the proper response. 422 Unprocessable Entity is also an option for this situation but do not use 422 because the code is not defined in RFC 7231 and not standard. Since the 400 response code can mean a wide range of things, it is extremely important that the error message returned clearly indicates that the resource referenced in the body does not exist, so that the consumer has a clear understanding of what they need to do to correct the problem.

  • If a request contains an unexpected attribute in the body, the server should return a 400 Bad Request response. Do not handle the request as normal by ignoring the bad attribute. Returning an error allows the client side to know which attribute is wrong and have the potential to fix a bad request or bad code. (For example, additionalProperties should be false on JSON-Schema definition)

  • Similarly, if the API supports query parameters and a request contains an unknown or unsupported parameter, the server should return a 400 Bad Request response. Invalid values in the request URL should never be silently ignored, as the response may not match the client’s expectation. For example, consider the case where an API allows filtering on name by specifying ‘?name=foo’ in the query string, and in one such request there is a typo, such as ‘?nmae=foo’. If this error were silently ignored, the user would get back all resources instead of just the ones named ‘foo’, which would not be correct. The error message that is returned should clearly indicate the problem so that the user could correct it and re-submit.

  • If a request is made to a known resource URI, but the HTTP method used for the request is not supported for that resource, the return code should be 405 Method Not Allowed. The response should include the Allow header with the list of accepted request methods for the resource.

  • If a request is made which attempts to perform an action on a resource which is already performing that action and therefore the request cannot be fulfilled (for example, snapshotting an instance which is already in the process of snapshotting), the return code should be 409 Conflict.

  • A 500 Internal Server Error should not be returned to the user for failures due to user error that can be fixed by changing the request on the client side. 500 failures should be returned for any error state that cannot be fixed by a client, and requires the operator of the service to perform some action to fix. It is also possible that this error can be raised deliberately in case of some detected but unrecoverable error such as a MessageQueueTimeout from a failure to communicate with another service component, an IOError caused by a full disk, or similar error.


If an error response body is returned, it must conform to the Errors guideline.

Common Mistakes

There are many common mistakes that have been made in the implementations of RESTful APIs in OpenStack. This section attempts to enumerate them with reasons why they were wrong, and propose future alternatives.

Use of 501 - Not Implemented

Some time in the Folsom era projects started using 501 for “Feature Not Implemented” - Discussion on openstack-dev

This is a completely incorrect reading of HTTP. “Method” means something very specific in HTTP, it means an HTTP Method. One of GET / HEAD / POST / PUT / PATCH / OPTIONS / TRACE.

The purpose of the 501 error was to indicate to the client that POST is not now, and never will be an appropriate method to call on any resource on the server. An appropriate client action is to blacklist POST and ensure no code attempts to use this. This comes from the early days of HTTP where there were hundreds of commercial HTTP server implementations, and the assumption that all HTTP methods would be handled by a server was not something the vendors could agree on. This usage was clarified in RFC RFC 7231#section-6.6.2 (section 6.6.2).

If we assume the following rfc statement to be true: “This is the appropriate response when the server does not recognize the request method and is not capable of supporting it for any resource.” that is irreconcilable with a narrower reading, because we’ve said all clients are correct in implementing “never send another POST again to any resource”. It’s as if saying the “closed” sign on a business means both, closed for today, as well as closed permanently and ok for the city to demolish the building tomorrow. Stating that either is a valid reading so both should be allowed only causes tears and confusion.

We live in a very different world today, dominated by Apache and Nginx. As such 501 is something you’d be unlikely to see in the wild. However that doesn’t mean we can replace it’s definition with our own.

Going forward projects should use a 400 ‘BadRequest’ response for this condition, plus a more specific error message back to the user that the feature was not implemented in that cloud. 404 ‘NotFound’ may also be appropriate in some situations when the URI will never exist. However one of the most common places where we would return “Feature Not Implemented” is when we POST an operation to a URI of the form /resource/{id}/action. Clearly that URI is found, however some operations on it were not supported. Returning a 404 (which is by default cachable) would make the client believe /resource/{id}/action did not exist at all on the server.

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