Joshua T Kalis he/him/his

How to: Hypermedia


Why Even Hypermedia?

Hypermedia is not a "silver bullet" and will incur some up-front cost in development time. Some reasons to invest in a hypermedia approach are:

  1. Externalize Knowledge - loosen the coupling between provider and consumer.
  2. Change Resilience - alleviate the need to version APIs for many reasons.
  3. Centralize business logic into the API rather than split between clients and provider.

REST Level-set

  1. Everything is a Resource - Every URL (endpoint) within an API - up to (excluding) the queryString - is a distinct Resource within the API.
  2. The PirateNinja Pattern - Entities and Entity Collections - The Entities are resources supporting - at least - Read, and optionally Create, Update, or Delete. They are the "things" - the nouns - that are generally interesting and revolve around business concepts for which the API exists. - The Collections are meta-resources that enable organization and management of groups Entities.
  3. Avoid Nesting - Nesting resources will often expose data relationships that might change over time and will then present the need for breaking changes to the API. - Deeply nested - beyond a single layer - will introduce uncertainty about the relationship between parent and child.

Siren

The examples in this workshop will use the Siren hypermedia type: https://github.com/kevinswiber/siren.


Hypermedia Affordances

  1. Resource Properties (Siren section: properties)
  2. Links not Identifiers (Siren section: links)
  3. Extended Properties (Siren section: entities)
  4. Capability Descriptions / Transformations (Siren section: actions)

Requirements

Let's design a RESTful API for cataloging Books solving for:

1 Specifically, when adding Books, the Author(s) and Genre(s) must already be added to their lists so a relationship can be established.


A Book


Book - Resource Representation application/json

A basic, non-hypermedia, representation might look like this.

{
  "title": "Microservice Architecture",
  "authors": [
    "Irakli Nadareishvili",
    "Ronnie Mitra",
    "Matt McLarty",
    "Michael Amundsen"
  ],
  "genre": [
    "Computer Science"
  ],
  "IBSN": "9781491956250"
}

The Bare Essentials

Representations like this should be viewed as the least amount of data necessary; but much more information can be provided.


Additional Context

Some additional information that is very helpful are:

  1. Links - What are some related, or supporting, Resources? - Is there additional information about a Resource?
  2. Control - Do I have access to add an Entity to the Collection? - How would changes to an Entity be submitted?
  3. Meta - How many Entities are in a Collection? - Is this Resource composed of others?

Book - Siren Entity Representation - properties

These are the only properties of the resource Book that aren't themselves other resources that have a relationship with with this one. Some additional properties that might be appropriate to a Book might be: Pages, Publish Date, Summary, etc.

{
  "properties": {
    "title": "Microservice Architecture",
    "IBSN": "9781491956250"
  }
}

The link to the Book isn't a property of the book at all; it is a feature of the API. There are some additional links that can/should be included for available navigation of API clients.

{
  "links": [
    {
      "class": ["book"],
      "href": "/books/9781491956250",
      "rel": ["item", "self"]
    },
    {
      "href": "/books",
      "rel": ["collection", "parent"]
    },
    {
      "href": "/",
      "rel": ["index"]
    }
  ]
}

Book - Siren Entity Representation - entities

Related resources - Authors and Genres - are included to provide the relevant context for the Book. Additionally they provide a way to find more information about the resources that they are through their href.

{
  "entities": [
    {
      "class": ["genre"],
      "href": "/genres/1234",
      "rel": ["item", "related"],
      "properties": {
        "name": "Computer Science"
      }
    },
    {
      "class": ["author"],
      "href": "/authors/1234",
      "rel": ["item", "related"],
      "properties": {
        "name": "Irakli Nadareishvili"
      }
    },
    {
      "class": ["author"],
      "href": "/authors/2345",
      "rel": ["item", "related"],
      "properties": {
        "name": "Ronnie Mitra"
      }
    },
    {
      "class": ["author"],
      "href": "/authors/3456",
      "rel": ["item", "related"],
      "properties": {
        "name": "Matt McLarty"
      }
    },
    {
      "class": ["author"],
      "href": "/authors/4567",
      "rel": ["item", "related"],
      "properties": {
        "name": "Michael Amundsen"
      }
    }
  ]
}

Book - Siren Entity Representation - actions

Resource actions will be associated with the client requesting the resource; meaning, only actions that the client is allowed to perform will be returned. Including the actions in the resource representations like this makes them explicit and generally accessible to any client without needing to communicating requirements from person to person.

{
  "actions": [
    {
      "class": ["delete"],
      "name": "delete-book",
      "title": "Delete Book",
      "method": "DELETE",
      "href": "/books/9781491956250"
    },
    {
      "class": ["update"],
      "name": "update-book",
      "title": "Edit Book",
      "method": "PUT",
      "href": "/books/9781491956250",
      "type": "application/x-www-form-urlencoded",
    },
    {
      "class": ["view"],
      "name": "view-book",
      "title": "Microservice Architecture",
      "href": "/books/9781491956250",
      "fields": [
        {
          "name": "title",
          "title": "Title",
          "type": "text",
          "value": "Microservice Architecture"
        },
        {
          "name": "ISBN",
          "title": "ISBN",
          "type": "text",
          "value": "9781491956250"
        },
        {
          "name": "genres",
          "title": "Genre(s)",
          "type": "list",
          "value": [
            "/genres/1234"
          ]
        },
        {
          "name": "authors",
          "title": "Author(s)",
          "type": "list",
          "value": [
            "/authors/1234",
            "/authors/2345",
            "/authors/3456",
            "/authors/4567"
          ]
        }
      ]
    }
  ]
}

Book - Siren Entity Representation - full

This representation is considerably more verbose than the simple, non-hypermedia, version but it also carries with it far more useful information than the other, enabling far more capability with less coupling between provider and client.

{
  "actions": [
    {
      "class": ["delete"],
      "name": "delete-book",
      "title": "Delete Book",
      "method": "DELETE",
      "href": "/books/9781491956250"
    },
    {
      "class": ["update"],
      "name": "update-book",
      "title": "Edit Book",
      "method": "PUT",
      "href": "/books/9781491956250",
      "type": "application/x-www-form-urlencoded",
    },
    {
      "class": ["view"],
      "name": "view-book",
      "title": "Microservice Architecture",
      "href": "/books/9781491956250",
      "fields": [
        {
          "name": "title",
          "title": "Title",
          "type": "text",
          "value": "Microservice Architecture"
        },
        {
          "name": "ISBN",
          "title": "ISBN",
          "type": "text",
          "value": "9781491956250"
        },
        {
          "name": "genres",
          "title": "Genre(s)",
          "type": "list",
          "value": [
            "/genres/1234"
          ]
        },
        {
          "name": "authors",
          "title": "Author(s)",
          "type": "list",
          "value": [
            "/authors/1234",
            "/authors/2345",
            "/authors/3456",
            "/authors/4567"
          ]
        }
      ]
    }
  ],
  "entities": [
    {
      "class": ["genre"],
      "href": "/genres/1234",
      "rel": ["item", "related"],
      "properties": {
        "name": "Computer Science"
      }
    },
    {
      "class": ["author"],
      "href": "/authors/1234",
      "rel": ["item", "related"],
      "properties": {
        "name": "Irakli Nadareishvili"
      }
    },
    {
      "class": ["author"],
      "href": "/authors/2345",
      "rel": ["item", "related"],
      "properties": {
        "name": "Ronnie Mitra"
      }
    },
    {
      "class": ["author"],
      "href": "/authors/3456",
      "rel": ["item", "related"],
      "properties": {
        "name": "Matt McLarty"
      }
    },
    {
      "class": ["author"],
      "href": "/authors/4567",
      "rel": ["item", "related"],
      "properties": {
        "name": "Michael Amundsen"
      }
    }
  ],
  "links": [
    {
      "class": ["book"],
      "href": "/books/9781491956250",
      "rel": ["item", "self"]
    },
    {
      "href": "/books",
      "rel": ["collection", "parent"]
    },
    {
      "href": "/",
      "rel": ["root"]
    }
  ],
  "properties": {
    "title": "Microservice Architecture",
    "IBSN": "9781491956250"
  }
}

The Collection of Books


Books - Siren Collection Representation - properties

The properties of a resource reflect the unique attributes of the Collection and nothing outside of itself; the Entities are resources unto themselves and therefor are not part of the properties. Providing data such as: pageCount, pageSize, EntityCount, etc. helps clients gain an appropriate level of understanding of the collection.

{
  "properties": {
    "EntityCount": 1,
    "title": "All Books",
    "page": 1,
    "pageCount": 1,
    "pageSize": 100,
    "startOfCollection": true,
    "endOfCollection": true
  }
}

The links for a collection will often include navigational links that can be followed for traversing long lists of resources. Again, links are provided with URLs so the client doesn't need to know how to construct them; this makes client logic simpler and less coupled to the API.

{
  "links": [
    {
      "href": "/books",
      "rel": ["collection", "self"]
    },
    {
      "href": "/books?page=1",
      "rel": ["chapter", "collection", "first"]
    },
    {
      "href": "/books?page=1",
      "rel": ["chapter", "collection", "last"]
    },
    {
      "href": "/",
      "rel": ["index", "parent"]
    }
  ]
}

Books - Siren Collection Representation - entities

Entity representations, when included within the Collection, are necessarily different than when on their own because at the level of the collection less information is necessary because if more information is needed about a resource the link should be followed for that resource. The collection is only for listing all comprised Entities.

{
  "entities": [
    {
      "class": ["book"],
      "href": "/books/9781491956250",
      "title": "Microservice Architecture"
    }
  ]
}

Books - Siren Collection Representation - actions

The availability of an action in a resource representation is indication that the client has access to execute the action; there is no need to provide a definition for an action to a client that can not perform the action.

{
  "actions": [
    {
      "class": ["add"],
      "name": "add-book",
      "title": "Add Book",
      "method": "POST",
      "href": "/books",
      "type": "application/x-www-form-urlencoded",
      "fields": [
        {
          "name": "title",
          "title": "Title",
          "type": "text",
          "value": ""
        },
        {
          "name": "ISBN",
          "title": "ISBN",
          "type": "text",
          "value": ""
        },
        {
          "name": "genres",
          "title": "Genre(s)",
          "type": "list",
          "value": []
        },
        {
          "name": "authors",
          "title": "Author(s)",
          "type": "list",
          "value": []
        }
      ]
    }
  ]
}

Books - Siren Collection Representation - full

{
  "actions": [
    {
      "class": ["add"],
      "name": "add-book",
      "title": "Add Book",
      "method": "POST",
      "href": "/books",
      "type": "application/x-www-form-urlencoded",
      "fields": [
        {
          "name": "title",
          "title": "Title",
          "type": "text",
          "value": ""
        },
        {
          "name": "ISBN",
          "title": "ISBN",
          "type": "text",
          "value": ""
        },
        {
          "name": "genres",
          "title": "Genre(s)",
          "type": "list",
          "value": []
        },
        {
          "name": "authors",
          "title": "Author(s)",
          "type": "list",
          "value": []
        }
      ]
    }
  ],
  "entities": [
    {
      "class": ["book"],
      "href": "/books/9781491956250",
      "title": "Microservice Architecture"
    }
  ],
  "links": [
    {
      "href": "/books",
      "rel": ["collection", "self"]
    },
    {
      "href": "/books?page=1",
      "rel": ["chapter", "collection", "first"]
    },
    {
      "href": "/books?page=1",
      "rel": ["chapter", "collection", "last"]
    },
    {
      "href": "/",
      "rel": ["root", "parent"]
    }
  ],
  "properties": {
    "EntityCount": 1,
    "title": "All Books",
    "page": 1,
    "pageCount": 1,
    "pageSize": 100,
    "startOfCollection": true,
    "endOfCollection": true
  }
}

/


The Root Resource

Many APIs ignore the root resource but for hypermedia APIs it is possibly the most important resource because it is the one URL that will never change. The root resource will return the list of all top-level and most used resources offered.

{
  "class": ["root"],
  "actions": [
    {
      "class": ["search", "authors"],
      "name": "search-authors",
      "title": "Search Authors",
      "method": "GET",
      "href": "/authors",
      "fields": [
        {
          "name": "name",
          "title": "Author Name",
          "type": "text"
        }
      ]
    },
    {
      "class": ["search", "books"],
      "name": "search-books",
      "title": "Search Books",
      "method": "GET",
      "href": "/books",
      "fields": [
        {
          "name": "title",
          "title": "Book Title",
          "type": "text"
        }
      ]
    },
    {
      "class": ["search", "genres"],
      "name": "search-genres",
      "title": "Search Genres",
      "method": "GET",
      "href": "/genres",
      "fields": [
        {
          "name": "genre",
          "title": "Genre",
          "type": "text"
        }
      ]
    }
  ],
  "links": [
    {
      "href": "/authors",
      "rel": ["collection", "section"],
      "title": "Authors"
    },
    {
      "href": "/books",
      "rel": ["collection", "section"],
      "title": "Books"
    },
    {
      "href": "/genres",
      "rel": ["collection", "section"],
      "title": "Genres"
    },
    {
      "href": "/",
      "rel": ["root", "self"],
      "title": "My Library API"
    }
  ],
  "properties": {
    "title": "My Library API"
  }
}

Conclusion

Hypermedia does not remove work; however, it will shift where the work is and what the nature of the work will be.