Internet-Draft JSON Hypertext Application Language September 2023
Kelly Expires 30 March 2024 [Page]
Workgroup:
Network Working Group
Internet-Draft:
draft-kelly-json-hal-09
Published:
Intended Status:
Informational
Expires:
Author:
M. Kelly
Stateless

JSON Hypertext Application Language

Abstract

This document proposes a media type for representing resources and their relations with hyperlinks.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on 30 March 2024.

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Table of Contents

1. Introduction

There is an emergence of non-HTML HTTP applications ("Web APIs") which use hyperlinks to direct clients around their resources.

The JSON Hypertext Application Language (HAL) is a standard which establishes conventions for expressing hypermedia controls, such as links, with JSON [RFC4627].

HAL is a generic media type with which Web APIs can be developed and exposed as series of links. Clients of these APIs can select links by their link relation type and traverse them in order to progress through the application.

HAL's conventions result in a uniform interface for serving and consuming hypermedia, enabling the creation of general-purpose libraries that can be re-used on any API utilising HAL.

The primary design goals of HAL are generality and simplicity. HAL can be applied to many different domains, and imposes the minimal amount of structure necessary to cover the key requirements of a hypermedia Web API.

2. Requirements

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 [RFC2119].

3. HAL Documents

A HAL Document uses the format described in [RFC4627] and has the media type "application/hal+json".

Its root object MUST be a Resource Object.

For example:

GET /orders/523 HTTP/1.1
Host: example.org
Accept: application/hal+json

HTTP/1.1 200 OK
Content-Type: application/hal+json

{
  "_links": {
    "self": { "href": "/orders/523" },
    "warehouse": { "href": "/warehouse/56" },
    "invoice": { "href": "/invoices/873" }
  },
  "currency": "USD",
  "status": "shipped",
  "total": 10.20
}

Here, we have a HAL document representing an order resource with the URI "/orders/523". It has "warehouse" and "invoice" links, and its own state in the form of "currency", "status", and "total" properties.

4. Resource Objects

A Resource Object represents a resource.

It has two reserved properties:

(1)
"_links": contains links to other resources.
(2)
"_embedded": contains embedded resources.

All other properties MUST be valid JSON, and represent the current state of the resource.

4.1. Reserved Properties

4.1.2. _embedded

The reserved "_embedded" property is OPTIONAL

It is an object whose property names are link relation types (as defined by [RFC5988]) and values are either a Resource Object or an array of Resource Objects.

Embedded Resources MAY be a full, partial, or inconsistent version of the representation served from the target URI.

6. Example Document

The following is an example document representing a list of orders

GET /orders HTTP/1.1
Host: example.org
Accept: application/hal+json

HTTP/1.1 200 OK
Content-Type: application/hal+json

{
  "_links": {
    "self": { "href": "/orders" },
    "next": { "href": "/orders?page=2" },
    "find": { "href": "/orders{?id}", "templated": true }
  },
  "_embedded": {
    "orders": [{
        "_links": {
          "self": { "href": "/orders/123" },
          "basket": { "href": "/baskets/98712" },
          "customer": { "href": "/customers/7809" }
        },
        "total": 30.00,
        "currency": "USD",
        "status": "shipped",
      },{
        "_links": {
          "self": { "href": "/orders/124" },
          "basket": { "href": "/baskets/97213" },
          "customer": { "href": "/customers/12369" }
        },
        "total": 20.00,
        "currency": "USD",
        "status": "processing"
    }]
  },
  "currentlyProcessing": 14,
  "shippedToday": 20
}

Here, the order list document provides a "next" link directing to the next page, and a "find" link containing a URI Template which can be expanded with an 'id' variable to go directly to a specific order.

It also has two embedded resources, "orders". Each of these has its own links to the associated "basket" and "customer" resources, and properties showing their "total", "currency" and "status".

Additionally, the order list resource has its own properties "currentlyProcessing" and "shippedToday".

7. Media Type Parameters

7.1. profile

The media type identifier application/hal+json MAY also include an additional "profile" parameter (as defined by [RFC6906])

HAL documents that are served with the "profile" parameter still SHOULD include a "profile" link belonging to the root resource.

8. Recommendations

8.3. HAL curies

HAL etablishes a mechanism called "curies" which allows for link relation types that are compact and more human readable (eg. "acme:widgets"), whilst still offering a way that they MAY be expanded into a dereferencable URI providing documentation (eg. "https://docs.acme.com/relations/widgets")

To this end, HAL documents have a reserved link relation type called "curies".

HAL curies are established for a given Resource Object via an array of Link Objects with the "curies" reserved link relation type. These links contain a URI Template with the token 'rel', and are named via the "name" property.

The following demonstrates the relation "https://docs.acme.com/relations/widgets" being abbreviated to "acme:widgets" using curies:

{
  "_links": {
    "self": { "href": "/orders" },
    "curies": [{
      "name": "acme",
      "href": "https://docs.acme.com/relations/{rel}",
      "templated": true
    }],
    "acme:widgets": { "href": "/widgets" }
  }
}

HAL curies can be used to create versioned link relation types like so:

{
  "_links": {
    "self": { "href": "/" },
    "curies": [{
      "name": "v1",
      "href": "https://docs.example.com/relations/v1/{rel}",
      "templated": true
    },{
      "name": "v2",
      "href": "https://docs.example.com/relations/v2/{rel}",
      "templated": true
    }],
    "v1:orders": {
      "href": "https://api.example.com/orders",
      "deprecation": "https://dev.example.com/deprecations/v1-orders"
    },
    "v2:orders": { "href": "https://api.example.com/order-list" }
  }
}

In cases where an embedded Resource defines its own curies which conflict with those of its parent then, for links within this resource, these are overwritten and SHOULD take precedence over the curies of the parent.

8.4. Hypertext Cache Pattern

The "hypertext cache pattern" allows servers to use embedded resources to dynamically reduce the number of requests a client makes, improving the efficiency and performance of the application.

Clients MAY be automated for this purpose so that, for any given link relation, they will read from an embedded resource (if present) in preference to traversing a link.

To activate this client behaviour for a given link, servers SHOULD add an embedded resource into the representation with the same relation.

Servers SHOULD NOT entirely "swap out" a link for an embedded resource (or vice versa) because client support for this technique is OPTIONAL.

The following examples shows the hypertext cache pattern applied to an "author" link:

Before:

{
  "_links": {
    "self": { "href": "/books/the-way-of-zen" },
    "author": { "href": "/people/alan-watts" }
  }
}

After:

{
  "_links": {
    "self": { "href": "/blog-post" },
    "author": { "href": "/people/alan-watts" }
  },
  "_embedded": {
    "author": {
      "_links": { "self": { "href": "/people/alan-watts" } },
      "name": "Alan Watts",
      "born": "January 6, 1915",
      "died": "November 16, 1973"
    }
  }
}

9. Security Considerations

TBD

10. IANA Considerations

TBD

11. Normative References

[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[RFC3986]
Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, , <https://www.rfc-editor.org/info/rfc3986>.
[RFC4627]
Crockford, D., "The application/json Media Type for JavaScript Object Notation (JSON)", RFC 4627, DOI 10.17487/RFC4627, , <https://www.rfc-editor.org/info/rfc4627>.
[RFC5988]
Nottingham, M., "Web Linking", RFC 5988, DOI 10.17487/RFC5988, , <https://www.rfc-editor.org/info/rfc5988>.
[RFC6570]
Gregorio, J., Fielding, R., Hadley, M., Nottingham, M., and D. Orchard, "URI Template", RFC 6570, DOI 10.17487/RFC6570, , <https://www.rfc-editor.org/info/rfc6570>.
[RFC6906]
Wilde, E., "The 'profile' Link Relation Type", RFC 6906, DOI 10.17487/RFC6906, , <https://www.rfc-editor.org/info/rfc6906>.

Appendix A. Acknowledgements

Thanks to Darrel Miller, Mike Amundsen, and everyone in hal-discuss for their suggestions and feedback.

The author takes all responsibility for errors and omissions.

Appendix B. Frequently Asked Questions

B.1. How should a client know the meaning/structure/semantics/type of a resource?

There are two main approaches to solving this problem. Both involve exposing additional documentation describing the resource which may be human and/or machine readable (i.e. an HTML page and/or a JSON Schema document). The difference between the two approaches is in where that URI is shared with the client, which is either:

(1)
The URI that was the preceding link relation type.
(2)
A 'profile' link from the resource itself.

B.2. Where can I find libraries for working with HAL?

A list of libraries is maintained here: https://github.com/mikekelly/hal_specification/wiki/Libraries

B.3. Why are the reserved properties prefixed with an underscore?

We elected for a prefix character to minimise risk of collisions with properties that represent the resource's state, and underscore was the character picked.

Another reason for prefixing the reserved properties is to make it visually apparent that the reserved properties are distinct from standard properties belonging to the resource.

B.4. Are all underscore-prefixed properties reserved?

No, HAL only reserves the names detailed in this specification.

B.5. Why does HAL have no forms?

Omitting forms from HAL was an intentional design decision that was made to keep it focused on linking for APIs. HAL is therefore a good candidate for use as a base media type on which to build more complex capabilities. An additional media type is planned for the future which will add form-like controls on top of HAL.

Author's Address

Mike Kelly
Stateless
URI: https://stateless.group/