Errors

Description

Errors are a crucial part of the developer experience when using an API. As developers learn the API they inevitably run into errors. The quality and consistency of the error messages returned to them will play a large part in how quickly they can learn the API, how they can be more effective with the API, and how much they enjoy using the API.

This document describes an emerging standard within OpenStack for providing structured error responses that can be consistently processed and include coding that will eventually allow errors to be searched for on the web and in documentation using the value of the code field. Such codes help to distinguish the causes of different errors in response to the same HTTP method and URI, even when the same HTTP status is returned.

Note

Services choosing to add these structured error responses are advised that doing so is not considered a backwards incompatible change and are encouraged to add them without needing to version the service. However, when a code is added to a specific response, subsequent change to that code, on that response, is an incompatbile change.

Errors JSON Schema

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in RFC 2119.

{
  "$schema": "http://json-schema.org/draft-04/schema#",
  "id": "http://specs.openstack.org/openstack/api-wg/errors-schema.json#",
  "type": "object",
  "properties": {
    "errors": {
      "type": "array",
      "description": "An ordered list of errors with the most recent error first.",
      "minItems": 1,
      "items": {
        "type": "object",
        "description": "Additional information about problems encountered while performing an operation.",
        "properties": {
          "request_id": {
            "type": "string",
            "description": "A unique identifier for this particular occurrence of the problem. If this property is present, it MUST match the X-Openstack-Request-Id header of the response in which it occurred."
          },
          "code": {
            "type": "string",
            "description": "A service-specific error code. The general form of the code is service-type.error-code. service-type MUST be the service type as defined by the service types authority at http://specs.openstack.org/openstack/service-types-authority. error-code is defined by service project team and MUST only consist of lowercase alpha, numeric, '.', and '-' characters."
          },
          "status": {
            "type": "integer",
            "description": "The HTTP status code applicable to this problem. It MUST match the status code of the response in which it occurred."
          },
          "title": {
            "type": "string",
            "description": "A short, human-readable summary of the problem. It SHOULD NOT change from occurrence to occurrence of the problem, except for purposes of localization."
          },
          "detail": {
            "type": "string",
            "description": "A human-readable explanation specific to this occurrence of the problem."
          },
          "links": {
            "type": "array",
            "description": "An array that MUST contain at least one Link Description Object with a 'rel': 'help' and an 'href' that links to a resource that can help the user as defined by http://specs.openstack.org/openstack/api-wg/guidelines/errors.html#errors-documentation",
            "minItems": 1,
            "items": { "$ref": "http://json-schema.org/draft-04/links" }
          }
        },
        "required": [
          "code",
          "status",
          "title",
          "detail",
          "links"
        ]
      }
    }
  },
  "required": [
    "errors"
  ]
}

Errors JSON Example

{
  "errors": [
    {
      "request_id": "1dc92f06-8ede-4fb4-8921-b507601fb59d",
      "code": "orchestration.create-failed",
      "status": 418,
      "title": "The Stack could not be created",
      "detail": "The Stack could not be created because of error(s) in other parts of the system.",
      "links": [
        {
          "rel": "help",
          "href": "TODO(sdague): example href"
        }
      ]
    },
    {
      "request_id": "d413ea12-dfcd-4009-8fad-229b475709f2",
      "code": "compute.scheduler.no-valid-host-found",
      "status": 403,
      "title": "No valid host found",
      "detail": "No valid host found for flavor m1.xlarge.",
      "links": [
        {
          "rel": "help",
          "href": "TODO(sdague): example href"
        }
      ]
    }
  ]
}

Note

This example is completely contrived. This is not how Orchestration or Compute responds with errors. It merely illustrates how a service might chain together errors.

Errors Documentation

The intention of the code is twofold:

  • To provide a convenient and memorable phrase that a human can use when communicating with other humans about an error they’ve experienced, or use when searching documentation or their favorite search engine for references to the error.
  • To act as flow control in client code where the same HTTP status code may be used to indicate multiple conditions. A common case is when a 409 Conflict may indicate several different ways in which the desired state cannot be accommodated and the handling of the conflict should differ.

To satisfy both of these requirements the strings used for the error codes need to be self-describing and human-readable while also distinct from one another. Avoiding abbreviation is recommended.

As an example, consider a service that provides a URI, /servers/{uuid}. There are at least two different ways that a 404 Not Found response may happen when making a GET request against that URI. One is that no server having {uuid} currently exists. The other is that the URI has been entered incorrectly (e.g., /server/{uuid}). These conditions should result in different codes. Two possible codes for these cases are compute.server.not_found and compute.uri.not_found, respectively.

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