JSON Hypertext Application Language

4.1. Reserved Properties

5.1. href

The "href" property is REQUIRED.¶

Its value is either a URI [RFC3986] or a URI Template [RFC6570].¶

If the value is a URI Template then the Link Object SHOULD have a "templated" attribute whose value is true.¶

5.2. templated

The "templated" property is OPTIONAL.¶

Its value is boolean and SHOULD be true when the Link Object's "href" property is a URI Template.¶

Its value SHOULD be considered false if it is undefined or any other value than true.¶

5.3. type

The "type" property is OPTIONAL.¶

Its value is a string used as a hint to indicate the media type expected when dereferencing the target resource.¶

5.4. deprecation

The "deprecation" property is OPTIONAL.¶

Its presence indicates that the link is to be deprecated (i.e. removed) at a future date. Its value is a URL that SHOULD provide further information about the deprecation.¶

A client SHOULD provide some notification (for example, by logging a warning message) whenever it traverses over a link that has this property. The notification SHOULD include the deprecation property's value so that a client maintainer can easily find information about the deprecation.¶

5.5. name

The "name" property is OPTIONAL.¶

Its value MAY be used as a secondary key for selecting Link Objects which share the same relation type.¶

5.6. profile

The "profile" property is OPTIONAL.¶

Its value is a string which is a URI that hints about the profile (as defined by [RFC6906]) of the target resource.¶

5.7. title

The "title" property is OPTIONAL.¶

Its value is a string and is intended for labelling the link with a human-readable identifier (as defined by [RFC5988]).¶

5.8. hreflang

The "hreflang" property is OPTIONAL.¶

Its value is a string and is intended for indicating the language of the target resource (as defined by [RFC5988]).¶

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.¶

8.1. Self Link

Each Resource Object SHOULD contain a 'self' link that corresponds with the IANA registered 'self' relation (as defined by [RFC5988]) whose target is the resource's URI.¶

8.2. Link relations

Custom link relation types (Extension Relation Types in [RFC5988]) SHOULD be URIs (or curies) that when dereferenced in a web browser provide relevant documentation, in the form of an HTML page, about the meaning and/or behaviour of the target Resource. This will improve the discoverability of the API.¶

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"
    }
  }
}

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.¶