(Quick Reference) # 10.12 Hypermedia as the Engine of Application State Version: 6.0.0 ## Table of Contents ## 10.12 Hypermedia as the Engine of Application State HATEOAS, an abbreviation for Hypermedia as the Engine of Application State, is a common pattern applied to REST architectures that uses hypermedia and linking to define the REST API. Hypermedia (also called Mime or Media Types) are used to describe the state of a REST resource, and links tell clients how to transition to the next state. The format of the response is typically JSON or XML, although standard formats such as Atom and/or HAL are frequently used. ## 10.12.1 HAL Support HAL is a standard exchange format commonly used when developing REST APIs that follow HATEOAS principals. An example HAL document representing a list of orders can be seen below:
    {
        "_links": {
            "self": { "href": "/orders" },
            "next": { "href": "/orders?page=2" },
            "find": {
                "href": "/orders{?id}",
                "templated": true
            },
            "admin": [{
                "href": "/admins/2",
                "title": "Fred"
            }, {
                "href": "/admins/5",
                "title": "Kate"
            }]
        },
        "currentlyProcessing": 14,
        "shippedToday": 20,
        "_embedded": {
            "order": [{
                "_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"
            }]
        }
    }
#### Exposing Resources Using HAL To return HAL instead of regular JSON for a resource you can simply override the renderer in `grails-app/conf/spring/resources.groovy` with an instance of `grails.rest.render.hal.HalJsonRenderer` (or `HalXmlRenderer` for the XML variation):
    import grails.rest.render.hal.*
    beans = {
        halBookRenderer(HalJsonRenderer, rest.test.Book)
    }
You will also need to update the acceptable response formats for the resource so that the HAL format is included. Not doing so will result in a 406 - Not Acceptable response being returned from the server. This can be done by setting the `formats` attribute of the `Resource` transformation:
    import grails.rest.*

    @Resource(uri='/books', formats=['json', 'xml', 'hal'])
    class Book {
        ...
    }
Or by updating the `responseFormats` in the controller:
    class BookController extends RestfulController {
        static responseFormats = ['json', 'xml', 'hal']

        // ...
    }
With the bean in place requesting the HAL content type will return HAL:
    $ curl -i -H "Accept: application/hal+json" http://localhost:8080/books/1

    HTTP/1.1 200 OK
    Server: Apache-Coyote/1.1
    Content-Type: application/hal+json;charset=ISO-8859-1

    {
      "_links": {
        "self": {
          "href": "http://localhost:8080/books/1",
          "hreflang": "en",
          "type": "application/hal+json"
        }
      },
      "title": "\"The Stand\""
    }
To use HAL XML format simply change the renderer:
    import grails.rest.render.hal.*
    beans = {
        halBookRenderer(HalXmlRenderer, rest.test.Book)
    }
#### Rendering Collections Using HAL To return HAL instead of regular JSON for a list of resources you can simply override the renderer in `grails-app/conf/spring/resources.groovy` with an instance of `grails.rest.render.hal.HalJsonCollectionRenderer`:
    import grails.rest.render.hal.*
    beans = {
        halBookCollectionRenderer(HalJsonCollectionRenderer, rest.test.Book)
    }
With the bean in place requesting the HAL content type will return HAL:
    $ curl -i -H "Accept: application/hal+json" http://localhost:8080/books
    HTTP/1.1 200 OK
    Server: Apache-Coyote/1.1
    Content-Type: application/hal+json;charset=UTF-8
    Transfer-Encoding: chunked
    Date: Thu, 17 Oct 2013 02:34:14 GMT

    {
      "_links": {
        "self": {
          "href": "http://localhost:8080/books",
          "hreflang": "en",
          "type": "application/hal+json"
        }
      },
      "_embedded": {
        "book": [
          {
            "_links": {
              "self": {
                "href": "http://localhost:8080/books/1",
                "hreflang": "en",
                "type": "application/hal+json"
              }
            },
            "title": "The Stand"
          },
          {
            "_links": {
              "self": {
                "href": "http://localhost:8080/books/2",
                "hreflang": "en",
                "type": "application/hal+json"
              }
            },
            "title": "Infinite Jest"
          },
          {
            "_links": {
              "self": {
                "href": "http://localhost:8080/books/3",
                "hreflang": "en",
                "type": "application/hal+json"
              }
            },
            "title": "Walden"
          }
        ]
      }
    }
Notice that the key associated with the list of `Book` objects in the rendered JSON is `book` which is derived from the type of objects in the collection, namely `Book`. In order to customize the value of this key assign a value to the `collectionName` property on the `HalJsonCollectionRenderer` bean as shown below:
    import grails.rest.render.hal.*
    beans = {
        halBookCollectionRenderer(HalCollectionJsonRenderer, rest.test.Book) {
            collectionName = 'publications'
        }
    }
With that in place the rendered HAL will look like the following:
    $ curl -i -H "Accept: application/hal+json" http://localhost:8080/books
    HTTP/1.1 200 OK
    Server: Apache-Coyote/1.1
    Content-Type: application/hal+json;charset=UTF-8
    Transfer-Encoding: chunked
    Date: Thu, 17 Oct 2013 02:34:14 GMT

    {
      "_links": {
        "self": {
          "href": "http://localhost:8080/books",
          "hreflang": "en",
          "type": "application/hal+json"
        }
      },
      "_embedded": {
        "publications": [
          {
            "_links": {
              "self": {
                "href": "http://localhost:8080/books/1",
                "hreflang": "en",
                "type": "application/hal+json"
              }
            },
            "title": "The Stand"
          },
          {
            "_links": {
              "self": {
                "href": "http://localhost:8080/books/2",
                "hreflang": "en",
                "type": "application/hal+json"
              }
            },
            "title": "Infinite Jest"
          },
          {
            "_links": {
              "self": {
                "href": "http://localhost:8080/books/3",
                "hreflang": "en",
                "type": "application/hal+json"
              }
            },
            "title": "Walden"
          }
        ]
      }
    }
#### Using Custom Media / Mime Types If you wish to use a custom Mime Type then you first need to declare the Mime Types in `grails-app/conf/application.groovy`:
    grails.mime.types = [
        all:      "*/*",
        book:     "application/vnd.books.org.book+json",
        bookList: "application/vnd.books.org.booklist+json",
        ...
    ]
| It is critical that place your new mime types after the 'all' Mime Type because if the Content Type of the request cannot be established then the first entry in the map is used for the response. If you have your new Mime Type at the top then Grails will always try and send back your new Mime Type if the requested Mime Type cannot be established.
Then override the renderer to return HAL using the custom Mime Types:
    import grails.rest.render.hal.*
    import grails.web.mime.*

    beans = {
        halBookRenderer(HalJsonRenderer, rest.test.Book, new MimeType("application/vnd.books.org.book+json", [v:"1.0"]))
        halBookListRenderer(HalJsonCollectionRenderer, rest.test.Book, new MimeType("application/vnd.books.org.booklist+json", [v:"1.0"]))
    }
In the above example the first bean defines a HAL renderer for a single book instance that returns a Mime Type of `application/vnd.books.org.book+json`. The second bean defines the Mime Type used to render a collection of books (in this case `application/vnd.books.org.booklist+json`).
| `application/vnd.books.org.booklist+json` is an example of a media-range (http://www.w3.org/Protocols/rfc2616/rfc2616.html - Header Field Definitions). This example uses entity (book) and operation (list) to form the media-range values but in reality, it may not be necessary to create a separate Mime type for each operation. Further, it may not be necessary to create Mime types at the entity level. See the section on "Versioning REST resources" for further information about how to define your own Mime types.
With this in place issuing a request for the new Mime Type returns the necessary HAL:
    $ curl -i -H "Accept: application/vnd.books.org.book+json" http://localhost:8080/books/1

    HTTP/1.1 200 OK
    Server: Apache-Coyote/1.1
    Content-Type: application/vnd.books.org.book+json;charset=ISO-8859-1

    {
      "_links": {
        "self": {
          "href": "http://localhost:8080/books/1",
          "hreflang": "en",
          "type": "application/vnd.books.org.book+json"
        }
      },
      "title": "\"The Stand\""
    }
#### Customizing Link Rendering An important aspect of HATEOAS is the usage of links that describe the transitions the client can use to interact with the REST API. By default the `HalJsonRenderer` will automatically create links for you for associations and to the resource itself (using the "self" relationship). However you can customize link rendering using the `link` method that is added to all domain classes annotated with `grails.rest.Resource` or any class annotated with `grails.rest.Linkable`. For example, the `show` action can be modified as follows to provide a new link in the resulting output:
    def show(Book book) {
        book.link rel:'publisher', href: g.createLink(absolute: true, resource:"publisher", params:[bookId: book.id])
        respond book
    }
Which will result in output such as:
    {
      "_links": {
        "self": {
          "href": "http://localhost:8080/books/1",
          "hreflang": "en",
          "type": "application/vnd.books.org.book+json"
        }
        "publisher": {
            "href": "http://localhost:8080/books/1/publisher",
            "hreflang": "en"
        }
      },
      "title": "\"The Stand\""
    }
The `link` method can be passed named arguments that match the properties of the `grails.rest.Link` class. ## 10.12.2 Atom Support Atom is another standard interchange format used to implement REST APIs. An example of Atom output can be seen below:
    <?xml version="1.0" encoding="utf-8"?>
    <feed xmlns="http://www.w3.org/2005/Atom">

     <title>Example Feed</title>
     <link href="http://example.org/"/>
     <updated>2003-12-13T18:30:02Z</updated>
     <author>
       <name>John Doe</name>
     </author>
     <id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</id>

     <entry>
       <title>Atom-Powered Robots Run Amok</title>
       <link href="http://example.org/2003/12/13/atom03"/>
       <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
       <updated>2003-12-13T18:30:02Z</updated>
       <summary>Some text.</summary>
     </entry>

    </feed>
To use Atom rendering again simply define a custom renderer:
    import grails.rest.render.atom.*
    beans = {
        halBookRenderer(AtomRenderer, rest.test.Book)
        halBookListRenderer(AtomCollectionRenderer, rest.test.Book)
    }
## 10.12.3 Vnd.Error Support Vnd.Error is a standardised way of expressing an error response. By default when a validation error occurs when attempting to POST new resources then the errors object will be sent back allow with a 422 respond code:
    $ curl -i -H "Accept: application/json"  -H "Content-Type: application/json" -X POST -d "" http://localhost:8080/books

    HTTP/1.1 422 Unprocessable Entity
    Server: Apache-Coyote/1.1
    Content-Type: application/json;charset=ISO-8859-1

    {
      "errors": [
        {
          "object": "rest.test.Book",
          "field": "title",
          "rejected-value": null,
          "message": "Property [title] of class [class rest.test.Book] cannot be null"
        }
      ]
    }
If you wish to change the format to Vnd.Error then simply register `grails.rest.render.errors.VndErrorJsonRenderer` bean in `grails-app/conf/spring/resources.groovy`:
    beans = {
        vndJsonErrorRenderer(grails.rest.render.errors.VndErrorJsonRenderer)
        // for Vnd.Error XML format
        vndXmlErrorRenderer(grails.rest.render.errors.VndErrorXmlRenderer)
    }
Then if you alter the client request to accept Vnd.Error you get an appropriate response:
    $ curl -i -H "Accept: application/vnd.error+json,application/json" -H "Content-Type: application/json" -X POST -d "" http://localhost:8080/books
    HTTP/1.1 200 OK
    Server: Apache-Coyote/1.1
    Content-Type: application/vnd.error+json;charset=ISO-8859-1

    [
        {
            "logref": "book.nullable,
            "message": "Property [title] of class [class rest.test.Book] cannot be null",
            "_links": {
                "resource": {
                    "href": "http://localhost:8080/rest-test/books"
                }
            }
        }
    ]
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