HATEOAS 101 - Opinionated Introduction to a REST API Style

HATEOAS 101An Opinionated IntroductionBrian Mulloy Apigee@landlessness @apigee

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

What is HATEOAS?

Hypermedia As The Engine Of Application State

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

The Constraints of REST1. Client-server2. Stateless server3. Cache4. Uniform interface a. Identification of resources b. Manipulation of resources through representations c. Self-descriptive messages d. Hypermedia as the engine of application state5. Layered System6. Code-On-Demand (optional)

Let’s break it down.

The first three constraints…

1. Client-server2. Stateless server3. Cache

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

clientcache Each request must contain all information. No stored context on the stateless server. server Client has the right to reuseclient response data.cache

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

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

Each layer Layers can encapsulateprovides services legacy services & protect new services from legacy statelessto 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 theclient to request code from the server & execute it.

Add features to a deployed client, which provides for improved extensibility and configurabilityclient stateless servercode 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 resourceinvolved in an interaction between components.

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

b. Resource Representation represents the state of aresource 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 toyou. 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(){vara=["log","debug","info","warn","error","assert","dir","dirxml","group","groupEnd","time","timeEnd","count","trace","profile","profileEnd"];window.console={};for(varb=0;b<a.length;++b)window.console[a[b]]=function(){}}();</script>…

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

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

d. Hypermedia as the engine of application state

state state transition transition state transition transitionstate 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

States are web pages.

Transitions are hyperlinks.

The key to implementing HATEOAS is pretty simple

In each response message include the links for the nextrequest message.

REST InterfaceApp Browser App UI ServerUser 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 removeblock @jack tweet to @jack Follow @jack @jack new tweet from lists

1 transition in, 32 transitions out

A quick aside

The three greatest inventions of all time are:

Bicycles

The Hyperlink

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

And navigate my way through every state of the entireTwitter application.

It’s amazing!

HATEOAS is not scary.

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

Let’s look at HATEOAS and APIs

With help from the Twitter API.

GET /1/statuses/show/20.jsonHTTP/1.1Host: 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, "created_at": "Tue Mar 21 20:50:14 +0000 2006", "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 HATEOASconstraint?

At least one.

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

A few questions.

Now are we HATEOAS compliant?

If we add another link does it change the consumingapplication’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

When Loren Brichter

Created Tweetie

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

Or would those decisions have been driven by the links inthe 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 appI 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 appuser at runtime.

So the hypermedia in each response message isgenuinely 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 1App API Server AppUser App 2 Developer 2 App App 3 Developer 3

Interface I craft the user experience, I get no aka state diagram. App HATEOAS App 1I 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 interfacebetween them.

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

Instead, the hypermedia are given to the developer atdesign time.

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

We need a Yoda moment.

“ 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 areasking

1. Should we go down the HATEOAS path?

As a practical matter, for many teams that previousquestion 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 aclient app that is also HATEOAS-compliant.

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

But designed to handle generic web APIs.

I guarantee hypermedia is engine of appI 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, shouldyou include links anyway?

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

But I wouldn’t call it HATEOAS because those links areprobably not the engine of application state for the appuser 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

Here’s a call to action

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

We know what SOAP is and isn’t.

But we don’t have an intellectual framework forthe way so many popular apps and web APIs worktoday.

We need a really smart person

Who cares about web APIs

To examine the constraints of REST

The Constraints of REST1. Client-server2. Stateless server3. Cache4. Uniform interface a. Identification of resources b. Manipulation of resources through representations c. Self-descriptive messages d. Hypermedia as the engine of application state5. Layered System6. Code-On-Demand (optional)

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

Interface App App 1 Developer 1App API Server AppUser App 2 Developer 2 App App 3 Developer 3

To give us a new foundation

The Constraints of ____1. ???2. ???3. ???4. ???5. ???6. ???

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

We will be able to communicate more effectively

And we will be able to create more value for theplanet 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

Questions?

THANK YOUSubscribe to API webinars at:youtube.com/apigee

THANK YOUIRC#api-crafton freenode

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

THANK YOUContact me at:@landlessnessbrian@apigee.com

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HATEOAS 101 - Opinionated Introduction to a REST API Style

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