@apiture/openapi-down-convert

OpenAPI Down Convert

openapi-down-convert is a tool to down-convert an API definition document from OpenAPI 3.1 to OpenAPI 3.0.

• OpenAPI Specification (OAS) 3.1.0 Release describes the changes from OAS 3.0 to OAS 3.1.
• See Migrating from OpenAPI 3.0 to 3.1.0 for going in the other direction. This tool helps "undo" some of those transformations.

Warning: This is not a fully robust tool. It does the minimal work necessary for OAS 3.1 API documents in order to support tools such as openapi-generator which do not support OAS 3.1. It only supports the OAS 3.1 features that the Apiture APIs use.

Warning: Down converting yields a loss in fidelity. Some API information is lost.

Installation and Synopsis

The tool is implemented with Node.js.

⤓ NPM Installation

npm i @apiture/openapi-down-convert

import { Converter, ConverterOptions } from './converter';
const options : ConverterOptions = { verbose: false,
                                     deleteExampleWithId: true,
                                     allOfTransform: false };
  const converter = new Converter(oas31Document, options);
  try {
    const oas30Document = converter.convert();
    ...
    }
  } catch (ex) {
    // handle the exception
  }

Since the use case for the converter is in build pipelines and CLI use, this operation is synchronous and does not use async/await/Promises.

⤓ Command Line

Install:

npm i -g @apiture/openapi-down-convert

then to use:

openapi-down-convert --input openapi-3.1.yaml --output openapi-3.0.yaml

Command line options:

Usage: openapi-down-convert [options]

Options:
  -i, --input <input-file>               A OpenAPI 3.1 file name. Defaults to "openapi.yaml"
  -o, --output <output-file>             The output file, defaults to stdout if omitted
  -a, --allOf                            If set, convert complex $ref in JSON schemas to allOf
  --authorizationUrl <authorizationUrl>  The authorizationUrl for openIdConnect -> oauth2 transformation
  --tokenUrl <tokenUrl>                  The tokenUrl for openIdConnect -> oauth2 transformation
  -d, --delete-examples-with-id          If set, delete any JSON Schema examples that have an `id` property
  -s, --scopes <scopes>                  If set, this JSON/YAML file describes the OpenID scopes
  -v, --verbose                          Verbose output
  -V, --version                          output the version number
  -h, --help                             display help for command

The verbose mode logs the changes to standard error output stream.

The tool only supports local file-based documents, not URLs. Download such files to convert:

openapi-down-convert --input <(curl -s https://my.host/path/openapi-3.1.yaml) \
                     --output openapi-3.0.yaml

OpenAPI Specifications Transformations

Here is a list of the transformations the tool performs:

⤓ openapi declaration

Change openapi: 3.1.x to openapi: 3.0.3

⤓ openIdConnect security definitions

Replace openIdConnect security definition with an oauth2 security requirement They are close enough, as far as code generation (such as openapi-generator) is concerned - it just means an Authorization: header must have a valid token. Use the options to specify the authorizationUrl and tokenUrl for the oauth2 security definition.

TODO: Fetch the openIdConnect connection info and extract the authorization and token URLs from it.

    accessToken:
      type: openIdConnect
      description: ...
      openIdConnectUrl: 'https://auth.apiture.com/openidConnectDiscovery'

becomes something like:

accessToken:
  type: oauth2
  description: "OpenIDConnect authorization code flow via https://auth.apiture.com/openidConnectDiscovery"
  flows:
    authorizationCode:
      authorizationUrl: <authorizationUrl option>
      tokenUrl: <tokenUrl option>
      scopes:
        scope1: Allow the application to access your personal profile data.
        scope2: Allow the application to send email on your behalf.
        scope3: >-
          TODO: describe the 'scope3' scope.
        scope4: >-
          TODO: describe the 'scope4' scope.

The tool scans all the security objects in all the operations to build a list of the used scopes. The descriptions default to TODO: describe the 'x' scope unless the --scopes scopes.yaml option is used; that file can list the descriptions for the scopes as simple scopeName: scope description pairs:

scope1: Allow the application to access your personal profile data.
scope2: Allow the application to send email on your behalf.

⤓ $ref object rewrites

For all schemas which contain a $ref object with siblings (description, other schema elements), replace the $ref: uri with allOf . For example:

myProperty:
  description: Blah blah
  $ref: '#/components/schemas/MyArray'

becomes:

mySchema:
  description: Blah blah
  allOf:
    - $ref: '#/components/schemas/MyArray'

This also applies to the schema used in parameters or in requestBody objects and in responses.

Note This transformation is disabled by default because it breaks openapi-generator 5.4 in cases where the referenced schema is an array. It generates Typescript types for such cases as

  myProperty: Array | null;
model/Model.ts:26:23 - error TS2314: Generic type 'Array<T>' requires 1 type argument(s).

which should be

  incompleteAccounts: Array<string> | null;

To enable, use the allOfTransform: true option in the Converter constructor or the --allOf command line argument. When disabled, the $ref is instead simplified to a JSON reference.

Other (non-JSON Schema) OAS 3.1 $ref objects can have description and summary. $ref for non-schema objects in OAS 3.0 cannot have description and summary. openapi-down-convert simply removes description and summary to yield a valid JSON reference as required by OAS 3.0. (The resulting OpenAPI will use the description in the $ref target.)

⤓ info.license.identifier

Remove info.license.identifier.

⤓ JSON Schema related changes

OAS 3.0 uses an earlier JSON Schema version (Draft 4). The tool convert examples in schemas to a single example.

As a special case, if the resulting example includes an id, it is deleted if the --delete-examples-with-id CLI option is set. This addresses Spectral issue 2081.

⤓ Convert const to enum

Convert JSON Schema that uses const to an enum with one value. For example

components:
  schema:
     version:
        description: The API version.
        type: string
        const: '1.0.0'

becomes

components:
  schema:
     version:
        description: The API version.
        type: string
        enum:
          - '1.0.0'

Convert type arrays to nullable

If a schema has a type array of exactly two values, and one of them is the string 'null', the type is converted to the non-null string item, and nullable: true added to the schema.

For example:

    myResponse:
      title: My Response
      description: Response from an API operation
      type: [ object, 'null' ]
      allOf:
        ...

becomes

    myResponse:
      title: My Response
      description: Response from an API operation
      type: object
      nullable: true
      allOf:
        ...

and

    myResponse:
      title: My Response
      description: Response from an API operation
      type: arrray
      items:
        type: [ 'string', 'null' ]
        ...

becomes

    myResponse:
      title: My Response
      description: Response from an API operation
      type: arrray
      items:
        type: string
        nullable: true
        ...

This transformation does not handle more complex type array scenarios such as

   type: [ number, string, boolean, 'null']

To support that, the schema would need to be recast using oneOf, but this is not trivial due to other schema attributes that may be possible (properties, allOf etc.)

(Contributions welcome.)

Remove unevaluatedProperties

The tool removes the unevaluatedProperties value, introduced in later versions of JSON Schema, as this is not supported in OAS 3.0 JSON Schema Draft 4 used in OAS 3.0.

    myResponse:
      title: My Response
      description: Response from an API operation
      type: object
      unevaluatedProperties: false
      allOf:
        ...

becomes

    myResponse:
      title: My Response
      description: Response from an API operation
      type: object
      unevaluatedProperties: false
      allOf:
        ...

Remove schema $id and $schema

The tool removes any $id or $schema keywords that may appear inside schema objects.

Unsupported down conversions

• openapi-down-convert does not convert exclusiveMinimum and exclusiveMaximum as defined in JSON Schema 2012-12; these handled differently in JSON Schema Draft 4 used in OAS 3.0. Contributions welcome!
• Webhooks are not addressed. Contributions welcome!
• The tool only supports self-contained documents. It does not follow or resolve external $ref documents embedded in the source document.