HATEOAS 101: An Introduction to Hypermedia as the Engine of Application State

HATEOAS 101
An Opinionated Introduction

Brian Mulloy Apigee
@landlessness @apigee

groups.google.com/group/api-craft

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IRC Channel
#api-craft
on freenode

WARNING: The author may betray a bias against
the application of REST constraints to web APIs.

Hypermedia As The Engine Of Application State

Let’s put HATEOAS in the full context of REST.

The Constraints of REST
1. Client-server
2. Stateless server
3. Cache
4. Uniform interface
a. Identification of resources
b. Manipulation of resources through representations
c. Self-descriptive messages
d. Hypermedia as the engine of application state
5. Layered System
6. Code-On-Demand (optional)

The first three constraints…

1. Client-server
2. Stateless server
3. Cache

…give us the client-cache-stateless-server web architecture.

client

cache
Each request
must contain
all information. No stored
context on the stateless
server. server

Client has the
right to reuse
client response data.

cache

(we’ll come back to the 4th constraint)

The 5th constraint, Layered System, lets us add
features like a gateway, load balancer and firewall.

Each layer Layers can encapsulate
provides services legacy services & protect
new services from legacy stateless
to it’s neighbors.
clients. server

load stateless
client firewall gateway
balancer server

Each layer cannot stateless
"see" beyond it’s server
immediate neighbor.

The optional 6th constraint, Code-on-Demand, allows the
client to request code from the server & execute it.

Add features to a
deployed client, which
provides for improved
extensibility and
configurability

client

stateless
server
code

Better user-perceived
performance and
efficiency

Now lets tackle the 4 parts of the 4th constraint,
Uniform Interface

With help from the Twitter UI.

a. Resource Identifier identifies the particular resource
involved in an interaction between components.

http://twitter.com/#!/jack/status/20

b. Resource Representation represents the state of a
resource for transfer between components.

<!DOCTYPE html>
<html data-nav-highlight-class-name="highlight-global-nav-home">
<head>
<title>Twitter</title>
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta charset="utf-8" />
<meta name="description" content="Instantly connect to what's most important to
you. Follow your friends, experts, favorite celebrities, and breaking news." />
<link rel="dns-prefetch" href="http://a0.twimg.com"/>
<link rel="dns-prefetch" href="http://api.twitter.com"/>
<script type="text/javascript" charset="utf-8">
document.domain = 'twitter.com';
// this will be copied to twttr.appStartTime once our JS has started up
document.startTime = new Date().getTime();
var twttr = {};
twttr.versionName = 'phoenix';
twttr.isT1 = true;
twttr.didPingKeynote = false;
twttr.keynoteTTFTPing = function() {
if (!twttr.didPingKeynote && window.location.href.indexOf('keynoteTest') > 0) {
var image = document.createElement('img');
image.src = '/images/keynote.gif';
twttr.didPingKeynote = true;
}
}
window.console||function(){var
a=["log","debug","info","warn","error","assert","dir","dirxml","group","groupEnd","time","t
imeEnd","count","trace","profile","profileEnd"];window.console={};for(var
b=0;b<a.length;++b)window.console[a[b]]=function(){}}();
</script>
…

c. Self-descriptive messages contains all the information
necessary to complete transformations.

GET /#!/jack/status/20 HTTP/1.1
User-Agent: Chrome/18.0.1025.11
Host: twitter.com
Accept: text/html

d. Hypermedia as the engine of application state

state state

transition transition

state

transition transition

state state

“ The name „Representational State Transfer‟ is intended
to evoke an image of how a well-designed Web
application behaves: a network of web pages (a virtual
state-machine), where the user progresses through the
application by selecting links (state transitions),
resulting in the next page (representing the next state of
the application) being transferred to the user and
rendered for their use.
-Roy Fielding
Architectural Styles and the
Design of Network-based Software Architectures
Chapter 6

The key to implementing HATEOAS is pretty simple

In each response message include the links for the next
request message.

REST Interface

App Browser App
UI Server
User App Developer

home connect discover search my profile direct messages

google: lists
jack‟s first favorite about
tweet

help
retweet

reply to keyboard
@jack shortcuts

turn off
retweets settings

report @jack
for spam sign out

add or remove
block @jack tweet to @jack Follow @jack @jack new tweet
from lists

1 transition in, 32 transitions out

The three greatest inventions of all time are:

With a browser I can start at http://twitter.com

And navigate my way through every state of the entire
Twitter application.

HATEOAS is the key constraint that makes surfing the
web with a browser possible.

Let’s look at HATEOAS and APIs

With help from the Twitter API.

GET /1/statuses/show/20.json
HTTP/1.1
Host: api.twitter.com

{
"created_at": "Tue Mar 21 20:50:14 +0000 2006",
"id": 20,
"id_str": "20",

"text": "just setting up my twttr",
"source": "web",
"truncated": false,
"in_reply_to_status_id": null,
"in_reply_to_status_id_str": null,
"in_reply_to_user_id": null,
"in_reply_to_user_id_str": null,
"in_reply_to_screen_name": null,
"user": {
"id": 12,
"id_str": "12",

"name": "Jack Dorsey",
"screen_name": "jack",
"location": "San Francisco",
"description": "Executive Chairman of Twitter, CEO of Square, a founder of both.",
"url": null,
"protected": false,
"followers_count": 1935426,
"friends_count": 1148,
"listed_count": 17312,
"favourites_count": 988,
"utc_offset": -28800,
"time_zone": "Pacific Time (US & Canada)",
"geo_enabled": true,
"verified": true,
"statuses_count": 10894,
"lang": "en",
"contributors_enabled": true,
"is_translator": false,
"profile_background_color": "EBEBEB",
"profile_background_image_url": "http://a0.twimg.com/images/themes/theme7/bg.gif",
"profile_background_image_url_https": "https://si0.twimg.com/images/themes/theme7/bg.gif",
"profile_background_tile": false,
"profile_image_url": "http://a0.twimg.com/profile_images/1563216547/image_normal.jpg",
"profile_image_url_https": "https://si0.twimg.com/profile_images/1563216547/image_normal.jpg",
"profile_link_color": "990000",
"profile_sidebar_border_color": "DFDFDF",
"profile_sidebar_fill_color": "F3F3F3",
"profile_text_color": "333333",
"profile_use_background_image": true,
"show_all_inline_media": true,
"default_profile": false,
"default_profile_image": false,
"following": null,
"follow_request_sent": null,
"notifications": null
},
"geo": null,
"coordinates": null,
"place": null,
"contributors": null,
"retweet_count": 5973,
"favorited": false,
"retweeted": false
}

Uh oh. There are zero hyperlinks.

How many should there be to respect the HATEOAS
constraint?

{
"id": 20,
"id_str": "20",

"text": "just setting up my twttr",
"source": "web",
"truncated": false,
"in_reply_to_status_id": null,
"in_reply_to_status_id_str": null,
"in_reply_to_user_id": null,
"in_reply_to_user_id_str": null,
"in_reply_to_screen_name": null,
"user": {
"id": 12,

"link": ”http://api.twitter.com/1/users/show.json?user_id=12",
"id_str": "12",

"name": "Jack Dorsey",
"screen_name": "jack",
"location": "San Francisco",
"description": "Executive Chairman of Twitter, CEO of Square, a founder of both.",
"url": null,
"protected": false,
"followers_count": 1935426,
"friends_count": 1148,
"listed_count": 17312,
"favourites_count": 988,
"utc_offset": -28800,
"time_zone": "Pacific Time (US & Canada)",
"geo_enabled": true,
"verified": true,
"statuses_count": 10894,
"lang": "en",
"contributors_enabled": true,
"is_translator": false,
"profile_background_color": "EBEBEB",
"profile_background_image_url": "http://a0.twimg.com/images/themes/theme7/bg.gif",
"profile_background_image_url_https": "https://si0.twimg.com/images/themes/theme7/bg.gif",
"profile_background_tile": false,
"profile_image_url": "http://a0.twimg.com/profile_images/1563216547/image_normal.jpg",
"profile_image_url_https": "https://si0.twimg.com/profile_images/1563216547/image_normal.jpg",
"profile_link_color": "990000",
"profile_sidebar_border_color": "DFDFDF",
"profile_sidebar_fill_color": "F3F3F3",
"profile_text_color": "333333",
"profile_use_background_image": true,
"show_all_inline_media": true,
"default_profile": false,
"default_profile_image": false,
"following": null,
"follow_request_sent": null,
"notifications": null
},
"geo": null,
"coordinates": null,
"place": null,
"contributors": null,
"retweet_count": 5973,
"favorited": false,
"retweeted": false
}

If we add another link does it change the consuming
application’s state machine?

Is it ok for the developer to ignore included links?

Can the developer add out-of-band links to her app?

What happens when an app relies on multiple APIs?
Where is the state machine?

If the Twitter API had complied with HATEOAS

Would he have been able to decide which user actions to
include in his design?

Or would those decisions have been driven by the links in
the response from the Twitter API?

Nearly all popular web UIs adhere to HATEOAS.

Nearly all popular web APIs violate HATEOAS.

Let’s examine the two worlds.

I guarantee
hypermedia is
engine of app
I decide where state I craft the user
to click, aka experience, aka
change state. state diagram.
REST Interface

App Browser App
UI Server
User App Developer

The person who crafts the experience (state diagram)
and the app user have the REST interface between them.

And the hypermedia links are given directly to the app
user at runtime.

So the hypermedia in each response message is
genuinely the engine of application state.

This pattern is not limited to user interfaces.

We see the same pattern for syndication feeds.

I guarantee
hypermedia is
engine of app
state I craft stories,
I decide where
categories & related
to click, aka
media, aka state
change state.
diagram.
REST Interface

App Feed Reader Feed Content
User App Server Publisher

But the world of apps and web APIs seems different.

Interface

App
App 1
Developer 1

App API Server
App
User App 2
Developer 2

App
App 3
Developer 3

Interface
I craft the user experience,
I get no
aka state diagram.
App HATEOAS
App 1
I decide where Developer 1 respect.
to click, aka
change state.

App API Server
App
User App 2
Developer 2

App
App 3
Developer 3

The person who crafts the experience (state machine)
and the app user do not have the REST interface
between them.

And the hypermedia links are not given directly to the
app user at runtime.

Instead, the hypermedia are given to the developer at
design time.

And the developer decides which states are possible for
the app user at runtime.

“ You must unlearn what you have learned.
-Yoda

I used to call the world of popular, non-HATEOAS, non-
SOAP web APIs, Pragmatic REST.

How do we answer the questions many API teams are
asking

1. Should we go down the HATEOAS path?

As a practical matter, for many teams that previous
question is the same as the next question.

2. Should we include links in our responses?

But they have different answers.

1. For an API to be HATEOAS-compliant it requires a
client app that is also HATEOAS-compliant.

A user-interface app driven by web APIs would be akin to
a feed reader for syndicated content.

But designed to handle generic web APIs.

I guarantee
hypermedia is
engine of app
I decide where state I craft a system of
to click, aka interrelated resources,
change state. aka state diagram.

REST Interface

App RESTful API API
API Server
User Client App Developer

?

Special thanks to @elasticpath for this metaphor.

There are interesting non-UI applications as well.

“ However, the style does not assume that all applications
are browsers. In fact, the application details are hidden
from the server by the generic connector interface, and
thus a user agent could equally be an automated robot
performing information retrieval for an indexing service,
a personal agent looking for data that matches certain
criteria, or a maintenance spider busy patrolling the
information for broken references or modified content
[39].
-Roy Fielding
Architectural Styles and the
Design of Network-based Software Architectures
Chapter 5

If you’re not going down the HATEOAS client path, should
you include links anyway?

2. If you think including links in the API response will be
helpful for developers at design time, then go for it.

But I wouldn’t call it HATEOAS because those links are
probably not the engine of application state for the app
user at run time.

“ If the engine of application state (and hence the
API) is not being driven by hypertext, then it
cannot be RESTful and cannot be a REST API.
Period. Is there some broken manual
somewhere that needs to be fixed?
-Roy Fielding
“REST APIs must be hypertext-driven”
Untangled: Musings of Roy T. Fielding

We know what REST with the HATEOAS constraint
is and isn’t.

We know what SOAP is and isn’t.

But we don’t have an intellectual framework for
the way so many popular apps and web APIs work
today.

To examine the constraints of REST

While keeping in mind how custom apps are built
by people using web APIs

The Constraints of ____
1. ???
2. ???
3. ???
4. ???
5. ???
6. ???

So that we will have a better shared idea of what
we’re really doing

We will be able to communicate more effectively

And we will be able to create more value for the
planet and the people on it.

But please choose a nice, pronounceable acronym.

Further Exploring
• http://steveklabnik.com/
• http://pinboard.in/u:earth2marsh/t:hateoas/
• http://www.ics.uci.edu/~fielding/pubs/dissert
ation/top.htm
• http://martinfowler.com/articles/richardsonM
aturityModel.html
• http://timelessrepo.com/haters-gonna-
hateoas

THANK YOU
Subscribe to API webinars at:
youtube.com/apigee

THANK YOU
IRC
#api-craft
on freenode

THANK YOU
Questions and ideas to:
groups.google.com/group/api-craft

THANK YOU
Contact me at:

@landlessness
brian@apigee.com

HATEOAS 101: An Introduction to Hypermedia as the Engine of Application State

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