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Fix, clarify, and simplify content type schemas (#2351)
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* Fix, clarify, and simplify content type schemas

This fixes #2349, which caught that an encoded PNG image
is encoded into a text media type.

In the process I realized some other errors, and simplified things.

* HTTP `Content-Type` is always handled by OAS
    * Media Type Object key in most cases
    * Encoding object (possibly inferred from schema) in `multipart/form-data`
* HTTP-level `Content-Encoding` is always handled by the OAS Header Object
* JSON Schema "content*" is used for embedding one media type into another
    * the encoded resource is of media type `text/plain`
    * `"contentMediaType"` is the embedded media type after decoding
    * `"contentEncoding"` is how to encode/decode binary to/from text

This removes any chance of `"contentMediaType"` conflicting with
the Media Type Object key or with `contentType` in the Encoding Object,
as they now always do different things.

Likewise, the HTTP `Content-Encoding` header (with values like
gzip, deflate, etc.) does different things than `"contentEncoding"`
(which has values like base64, base64url, quoted-printable, etc.).

The deprecated part header `Content-Transfer-Encoding` is likewise
handled in the Encoding Object, but is probably never used.

* Fix Content-Type to indicate semantics

...rather than literal content format on the wire.

* Update 3.1.0.md

Fixed a typo and changed a SHOULD to MAY.

* Update versions/3.1.0.md

* clarify default encoding content type value.

* Describe interaction between JSON Schema contentEncoding and HTTP Content-Encoding header

Co-authored-by: Mike Kistler <mkistler@us.ibm.com>

Co-authored-by: Darrel <darrmi@microsoft.com>
Co-authored-by: Mike Kistler <mkistler@us.ibm.com>
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@handrews @darrelmiller
3 people authored Feb 11, 2021
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Showing 1 changed file with 20 additions and 16 deletions.
36 changes: 20 additions & 16 deletions versions/3.1.0.md
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Original file line number Diff line number Diff line change
Expand Up @@ -1446,14 +1446,15 @@ application/json:

##### Considerations for File Uploads

In contrast with the 2.0 specification, `file` input/output content in OpenAPI is described with the same semantics as any other schema type. In contrast with the 3.0 specification, such schemas use the `contentEncoding` JSON Schema keyword rather than the `format` keyword. This keyword supports all encodings defined in [RFC4648](https://tools.ietf.org/html/rfc4648), including "base64" and "base64url", as well as "quoted-printable" from [RFC2045](https://tools.ietf.org/html/rfc2045#section-6.7).
In contrast with the 2.0 specification, `file` input/output content in OpenAPI is described with the same semantics as any other schema type.

JSON Schema also offers a `contentMediaType` keyword. However, when the media type is already specified by the
Media Type Object's key, or by the `contentType` field of an [Encoding Object](#encodingObject), the `contentMediaType` keyword SHALL be ignored if present.
In contrast with the 3.0 specification, the `format` keyword has no effect on the content-encoding of the schema. JSON Schema offers a `contentEncoding` keyword, which may be used to specify the `Content-Encoding` for the schema. The `contentEncoding` keyword supports all encodings defined in [RFC4648](https://tools.ietf.org/html/rfc4648), including "base64" and "base64url", as well as "quoted-printable" from [RFC2045](https://tools.ietf.org/html/rfc2045#section-6.7). The encoding specified by the `contentEncoding` keyword is independent of an encoding specified by the `Content-Type` header in the request or response or metadata of a multipart body -- when both are present, the encoding specified in the `contentEncoding` is applied first and then the encoding specified in the `Content-Type` header.

JSON Schema also offers a `contentMediaType` keyword. However, when the media type is already specified by the Media Type Object's key, or by the `contentType` field of an [Encoding Object](#encodingObject), the `contentMediaType` keyword SHALL be ignored if present.

Examples:

Content transferred in binary (octet-stream) SHOULD omit `schema`, as no JSON Schema type is suitable:
Content transferred in binary (octet-stream) MAY omit `schema`:

```yaml
# a PNG image as a binary file:
Expand All @@ -1474,9 +1475,12 @@ content:
image/png:
schema:
type: string
contentMediaType: image/png
contentEncoding: base64
```

Note that the `Content-Type` remains `image/png`, describing the semantics of the payload. The JSON Schema `type` and `contentEncoding` fields explain that the payload is transferred as text. The JSON Schema `contentMediaType` is technically redundant, but can be used by JSON Schema tools that may not be aware of the OpenAPI context.

These examples apply to either input payloads of file uploads or response payloads.

A `requestBody` for submitting a file in a `POST` operation may look like the following example:
Expand Down Expand Up @@ -1509,10 +1513,11 @@ requestBody:
# The property name 'file' will be used for all files.
file:
type: array
items:
contentMediaType: application/octet-stream
items: {}
```

As seen in the section on `multipart/form-data` below, the empty schema for `items` indicates a media type of `application/octet-stream`.

##### Support for x-www-form-urlencoded Request Bodies

To submit content using form url encoding via [RFC1866](https://tools.ietf.org/html/rfc1866), the following
Expand Down Expand Up @@ -1549,9 +1554,8 @@ When passing in `multipart` types, boundaries MAY be used to separate sections o
* If the property is a primitive, or an array of primitive values, the default Content-Type is `text/plain`
* If the property is complex, or an array of complex values, the default Content-Type is `application/json`
* If the property is a `type: string` with a `contentEncoding`, the default Content-Type is `application/octet-stream`
* If the JSON Schema keyword `contentMediaType` is used and no Encoding Object is present, then the Content-Type is that which is specified by `contentMediaType`, however if an Encoding Object is present, then `contentMediaType` SHALL be ignored

As with non-multipart request or response bodies, when using `contentMediaType` to specify a binary Content-Type without also using `contentEncoding`, the JSON Schema `type` keyword is omitted.
Per the JSON Schema specification, `contentMediaType` without `contentEncoding` present is treated as if `contentEncoding: identity` were present. While useful for embedding text documents such as `text/html` into JSON strings, it is not useful for a `multipart/form-data` part, as it just causes the document to be treated as `text/plain` instead of its actual media type. Use the Encoding Object without `contentMediaType` if no `contentEncoding` is required.

Examples:

Expand All @@ -1570,15 +1574,17 @@ requestBody:
type: object
properties: {}
profileImage:
# Content-Type with contentMediaType is the contentMediaType (image/png here)
# Content-Type for application-level encoded resource is `text/plain`
type: string
contentMediaType: image/png
contentEncoding: base64
children:
# default Content-Type for arrays is based on the `inner` type (text/plain here)
# default Content-Type for arrays is based on the _inner_ type (`text/plain` here)
type: array
items:
type: string
addresses:
# default Content-Type for arrays is based on the `inner` type (object shown, so `application/json` in this example)
# default Content-Type for arrays is based on the _inner_ type (object shown, so `application/json` in this example)
type: array
items:
type: object
Expand All @@ -1594,7 +1600,7 @@ A single encoding definition applied to a single schema property.
##### Fixed Fields
Field Name | Type | Description
---|:---:|---
<a name="encodingContentType"></a>contentType | `string` | The Content-Type for encoding a specific property. Default value depends on the property type: when `type` is absent and `contentMediaType` is present - the value of `contentMediaType`; when both `type` and `contentMediaType` are absent - `application/octet-stream`; for `string` with a `contentEncoding` - `application/octet-string`; for other primitive types – `text/plain`; for `object` - `application/json`; for `array` – the default is defined based on the inner type. The value can be a specific media type (e.g. `application/json`), a wildcard media type (e.g. `image/*`), or a comma-separated list of the two types.
<a name="encodingContentType"></a>contentType | `string` | The Content-Type for encoding a specific property. Default value depends on the property type: for `object` - `application/json`; for `array` – the default is defined based on the inner type; for all other cases the default is `application/octet-stream`. The value can be a specific media type (e.g. `application/json`), a wildcard media type (e.g. `image/*`), or a comma-separated list of the two types.
<a name="encodingHeaders"></a>headers | Map[`string`, [Header Object](#headerObject) \| [Reference Object](#referenceObject)] | A map allowing additional information to be provided as headers, for example `Content-Disposition`. `Content-Type` is described separately and SHALL be ignored in this section. This property SHALL be ignored if the request body media type is not a `multipart`.
<a name="encodingStyle"></a>style | `string` | Describes how a specific property value will be serialized depending on its type. See [Parameter Object](#parameterObject) for details on the [`style`](#parameterStyle) property. The behavior follows the same values as `query` parameters, including default values. This property SHALL be ignored if the request body media type is not `application/x-www-form-urlencoded` or `multipart/form-data`. If a value is explicitly defined, then the value of [`contentType`](#encodingContentType) (implicit or explicit) SHALL be ignored.
<a name="encodingExplode"></a>explode | `boolean` | When this is true, property values of type `array` or `object` generate separate parameters for each value of the array, or key-value-pair of the map. For other types of properties this property has no effect. When [`style`](#encodingStyle) is `form`, the default value is `true`. For all other styles, the default value is `false`. This property SHALL be ignored if the request body media type is not `application/x-www-form-urlencoded` or `multipart/form-data`. If a value is explicitly defined, then the value of [`contentType`](#encodingContentType) (implicit or explicit) SHALL be ignored.
Expand All @@ -1607,7 +1613,7 @@ This object MAY be extended with [Specification Extensions](#specificationExtens
```yaml
requestBody:
content:
multipart/mixed:
multipart/form-data:
schema:
type: object
properties:
Expand All @@ -1624,9 +1630,7 @@ requestBody:
description: metadata in XML format
type: object
properties: {}
profileImage:
type: string
contentMediaType: image/jpeg
profileImage: {}
encoding:
historyMetadata:
# require XML Content-Type in utf-8 encoding
Expand Down

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