Hypermedia

APIs of the Web

Sébastien Cevey

@theefer

APIDays / APIStrat

Berlin, 24th April 2015

About me

Sébastien Cevey

@theefer

Hypermedia

APIs

REST

REpresentational State Transfer”

 

Architectural style inspired by the design of the Web

(Roy Fielding's dissertation, 2000)

RESTful API

REST applied to design scalable APIs

Pick a baseline network protocol

0. HTTP protocol*

POST /getImage
imageId = 123

client

server

{ ... }

* not necessarily HTTP, but usually is

Model “things” as Resources

Give Resources a Uniform Resource Identifier

1. URIs

GET

client

server

{ ... }
/images/123

Map actions to HTTP methods

2. Verbs

GET
POST
PUT
PATCH
DELETE

retrieve
create/update
replace
partial update
delete

PUT
{ ... }

client

server

{ ... }
/images/123

No, we’re not done yet.

Hypermedia

Key constraint of REST and the Web

“the simultaneous presentation of information and controls such that the information becomes the affordance through which the user obtains choices and selects actions”

(Fielding, 2008)

Hypermedia API ≈ strict REST API

3. Hypermedia Controls

<article class="image">
  <img src="http://example.io/cat.jpg" width="800" height="600">

  <a href="/images/123/crops">crops</a>

  <form action="/images/123" method="POST">
    <div>Resize dimensions</div>
    <label><input name="width"  type="number"> width</label>
    <label><input name="height" type="number"> height</label>
    <input type="submit" value="Update">
  </form>
</article>

Embed controls in API response:

  • Links to related resources
  • Forms to describe actions

API optimised

XML or JSON

+

Hypermedia semantics

Media-Types

{
  "properties": {
    "type":   "image",
    "url":    "http://example.io/cat.jpg",
    "width":  800,
    "height": 600,
  },
  "links": [
    { "rel": ["crops"], "href": "/images/123/crops" }
  ],
  "actions": [{
     "name": "resize",
     "title": "Resize dimensions",
     "href": "/images/123",
     "method": "POST",
     "fields": [
       { "name": "width",  "type": "number" },
       { "name": "height", "type": "number" }
     ]
  }]
}
Content-Type: application/vnd.siren+json

Formats

HAL, Siren, JSON-LD, Uber, Argo

“HATEOAS”

Hypermedia As The Engine Of Application State

GET

client

server

/
/
/
client.resource('https://api.media.gu.com');

theseus (JavaScript library for argo APIs)

Follow link

GET

client

server

/images/123
/images/123
/images/123
/
resource.follow('image', {id: '123'});
{
  "data": {
    "description": "This is the Media API"
  },
  "links": [
    {"rel":"image", "href": "/images/{id}"},
    {"rel":"cropper","href":"https://cropper.media.gu.com"},
    ...
  ]
}
/
PATCH
/images/123
{width: 300,
 height: 400}

client

server

/images/123
/images/123

Update state

resource.perform('resize', {
  width:  300,
  height: 400
});
{
  "data": {
    "src":    "http://example.io/cat.jpg",
    "width":  600,
    "height": 800,
    "credit": "Graham"
  },
  "actions": [{
    "name": "resize",
    "title": "Resize dimensions",
    "method": "PATCH",
    "href": "/images/123",
    "fields": [
      { "name": "width",  "type": "number" },
      { "name": "height", "type": "number" }
    ]
  }]
}
/images/123
/images/123
/images/123

State transfer

Resources as state

Model state on client and server

Hypermedia as affordances

Links to other resources

Forms to describe available actions on resources

Verbs to transfer

Synchronise state between client and server

Apply changes to both

Benefits of Hypermedia APIs

“The best network-based

application performance is obtained by not using the network”

Cacheable

client

server

/images/123

browser cache

CDN edge cache

caching proxy

client

server

app cache

server cache

Hypermedia

RPC

HTTP POST, TCP

Cacheable

0

1

Uniform HTTP interface, standard semantics

Resource URI as cache key

Verb semantics: GET is safe & idempotent

2

Layer standard HTTP components

CDN, proxy, server, etc.

 

Control caching from the server

Headers: Cache-Control, Vary, E-Tag, etc.

Transparently respected downstream

Cache stories

theguardian.com

Edge cache JSON

Control TTL with headers

zik.ca

Serve expensive responses

from file on disk

Split out and link to

dynamic part of response

Discoverable

URI as explicit reference (unlike IDs)

Links and forms self-documenting (in-band)

1

3

Web-like

Browse API from the root or any resource

Submit forms while exploring in browser

Resource address returns its data

Discoverable

API federation

Links to Media API from our Content API

Self-service

Give root of the Media API
to other teams

Service index in Media API

Discoverable

API as UI

Use content negociation

JSON ⇒ HTML

URI templates ⇒ forms

API as REPL

Dove client

Works with any Argo APIs

Evolveable

Persistent URIs

Clients rely on links, not out-of-band config

Extensible

Add data properties

Add links & form fields

Ignored if unsupported

Server configures clients

Only config is API root

No URL construction

Change link targets transparently

1

3

Composeable

Add new services via links and forms

Shared media-type helps

Avoids monolithic library

Simple

Plain HTTP

JSON or XML + Hypermedia layer

Visibility

Send URL around

Server request logs

Browser network panel

Wireshark

Scalable

cURL

Bookmarklets

Apple Scripts

Small web clients

Mobile apps

Large single-page apps

0

3

1

2

Authoritative

Conditional links/forms drive the UI

Based on permissions, business logic

Avoids hardcoding rules in both server & client

Server rules can change, apply to all clients

if valid, expose link
if invalid, omit link

Requirement:
can only crop valid images

Links & forms as affordances

3

Forms drive client-side validation

Expose constraints in form fields

Apply to HTML UI form

Avoids hardcoding rules in both server & client

R&D

e.g. required, min/max length, “email”, etc

Authoritative

Limitations

Performance

Serialisation/Deserialisation

Payload size


Plain JSON: 23.0KB
Argo+JSON:  37.4KB
              +40%
gzipped
  4.4KB
  5.1KB
   +15%

No data schema

Versioning

State transfer model

Avoid side-effects

Poll, no PUSH

Weak tooling

References

RESTful Web APIs
Leonard Richardson, Mike Amundsen

Hypermedia

Uniform HTTP interface

Cacheable for performance

Simple to integrate and debug

Resources model state
Verbs transfer state
Hypermedia controls as affordances

Discoverable for developers

Dynamic link structure allows evolveability

Authoritative source of client actions through forms

Thank you!

Sébastien Cevey

@theefer

Hypermedia: APIs of the Web [APIDays/APIStrat Berlin '15]

By Sébastien Cevey

Hypermedia: APIs of the Web [APIDays/APIStrat Berlin '15]

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