This presentation will discuss how the Representational State Transfer (REST) architectural style can be applied to the design of your web services. You will learn how to use HTTP methods and status codes properly and we will discuss how to use Hypermedia As The Engine Of Application State (HATEOAS). The principles of REST and HATEOAS will be demonstrated through the Atom Publishing Protocol (AtomPub) using the Google Data APIs and other AtomPub implementations as examples.

1. Resource-Oriented
Web Services
Applying the REST Architectural Style

2. This presentation will discuss how the Representational State
Transfer (REST) architectural style can be applied to the design
of your web services.
You will learn how to use HTTP methods and status codes
properly and we will discuss how to use Hypermedia As The
Engine Of Application State (HATEOAS).
The principles of REST and HATEOAS will be demonstrated
through the Atom Publishing Protocol (AtomPub) using the
Google Data APIs and other AtomPub implementations as
examples.

3. HTTP
"Hypertext Transfer Protocol (HTTP) is an
application-level protocol for distributed,
collaborative, hypermedia information systems. Its
use for retrieving inter-linked resources led to the
establishment of the World Wide Web."[1]

4. Methods and Resources
HTTP methods are the actions
that can be performed on resources.

5. Uniform Resource Identi ers (URIs)
Resources are identi ed by a URI.

6. URIs
Example URIs:
http://www.example.org/people
http://www.example.org/people/bradley-holt
Examples of resources could include:
• documents
• people
• places
• things
• abstract concepts (e.g. processes, transactions)

7. Limited Vocabulary
There are only 8 methods: HEAD, GET, POST, PUT,
DELETE, TRACE, OPTIONS, CONNECT (we're only
going to talk about 4 of them); but you get to de ne
your own resources.[2]

8. REST Architecture
• HTTP is just one (very popular) instance of the REST
architecture.
• You can use HTTP correctly and not be RESTful and you
can use REST without HTTP.
• REST is about de ning a uniform interface.
Example of a non-RESTful standard based on HTTP:
• WebDAV

9. What Makes a Service RESTful?
If the HTTP method doesn’t match the method
information, the service isn’t RESTful. If the scoping
information isn’t in the URI, the service isn’t
resource-oriented. These aren’t the only
requirements, but they’re good rules of thumb.
From RESTful Web Services[3]

10. HTTP Methods
How are you manipulating a resource?
We’ll cover four of the eight methods...

11. GET
• GET a representation of a resources.
• Safe: Can't hold the user responsible for side-effects.
• Idempotent: N > 0 identical requests are each the same
as a single request.
• Cacheable.
Example:
GET /people HTTP/1.1
Note that the HTTP request and response examples in this
presentation are meant to be illustrative and are not
always complete. Some HTTP headers may be missing.

12. More on Safety
A hit counter is generally "safe". Yes, it changes state
but the user is not held accountable for that state
transition.
Deleting something is not safe: you've held the user
accountable. For example, Google Web Accelerator
(cache pre-fetching) broke 37signals' Backpack web
application because they were using GET to delete
information[4].

13. POST
• POST a new representation of a resource.
• New resource is subordinate to the requested resource.
• Not safe.
• Not idempotent.
• Can be cached only through the Cache-Control or
Expires header elds.
Example:
POST /people HTTP/1.1
Content-Type: application/x-www-form-urlencoded
fn=Bradley+Holt&url=http%3A%2F%2Fbradley-holt.blogspot.com%2F

14. PUT
• PUT a modi ed representation of a resource.
• Not safe.
• Idempotent: PUTting the same thing multiple times is
the same as doing it once.
• Responses are not cacheable.
Example:
PUT /people/bradley-holt HTTP/1.1
Content-Type: application/x-www-form-urlencoded
fn=Bradley+Holt&url=http%3A%2F%2Fbradley-holt.blogspot.com%2F

15. DELETE
• DELETE a resource.
• Not safe.
• Idempotent. Deleting something multiple times is the
same as doing it once.
• Responses are not cacheable.
Example:
DELETE /people/bradley-holt HTTP/1.1

16. HTTP Status Codes
What was the result of your request?
A few examples...[5]

17. 200 OK
Request:
GET /people/bradley-holt HTTP/1.1
Response:
HTTP/1.1 200 OK
Content-Type: text/html
<!DOCTYPE html>
<html>
<head>
<title>Bradley Holt</title>
</head>
<body>
<div class="vcard">
<a class="url fn" href="http://bradley-holt.blogspot.com/">Bradley Holt</a>
</div>
</body>
</html>

18. 201 Created
Request:
POST /people HTTP/1.1
Content-Type: application/x-www-form-urlencoded
fn=Bradley+Holt&url=http%3A%2F%2Fbradley-holt.blogspot.com%2F
Response:
HTTP/1.1 201 Created
Location: /people/bradley-holt

19. 202 Accepted
Request:
POST /people HTTP/1.1
Content-Type: application/x-www-form-urlencoded
fn=Bradley+Holt&url=http%3A%2F%2Fbradley-holt.blogspot.com%2F
Response:
HTTP/1.1 202 Accepted
Location: /queue/4jn6rk

20. 301 Moved Permanently
Request:
GET /people/bradley-holt HTTP/1.1
Response:
HTTP/1.1 301 Moved Permanently
Location: /people/BradleyHolt

21. 302 Found
Request:
GET /people/bradley-holt HTTP/1.1
Response:
HTTP/1.1 302 Found
Location: /people/BradleyHolt

22. 400 Bad Request
Request:
POST /people HTTP/1.1
Content-Type: application/x-www-form-urlencoded
fn=Bradley+Holt&url=bradley-holt.blogspot.com
Response:
HTTP/1.1 400 Bad Request
Content-Type: text/html
<!DOCTYPE html>
<html>
<head>
<title>Bradley Holt</title>
</head>
<body>
<p>URL is not valid.</p>
</body>
</html>

23. 401 Unauthorized
Request:
GET /people/bradley-holt HTTP/1.1
Response:
HTTP/1.1 401 Unauthorized
WWW-Authenticate: BASIC realm="Area 51"

24. 403 Forbidden
Request:
GET /people/bradley-holt HTTP/1.1
Response:
HTTP/1.1 403 Forbidden

25. 404 Not Found
Request:
GET /people/bradley-holt HTTP/1.1
Response:
HTTP/1.1 404 Not Found

26. 405 Method Not Allowed
Request:
POST /people/bradley-holt HTTP/1.1
Response:
HTTP/1.1 405 Method Not Allowed
Allow: GET, PUT, DELETE

27. 409 Con ict
Request:
PUT /people/bradley-holt HTTP/1.1
Content-Type: application/x-www-form-urlencoded
fn=Bradley+Holt&url=http%3A%2F%www.foundline.com%2F&revision=5
Response:
HTTP/1.1 409 Conflict
Content-Type: text/html
<!DOCTYPE html>
<html>
<head>
<title>Conflict</title>
</head>
<body>
<p>You are editing revision 5 and the latest revision number is 6.</p>
</body>
</html>

28. 418 I’m A Teapot
According to the Hyper Text Coffee Pot
Control Protocol (HTCPCP/1.0)[6]:
Any attempt to brew coffee with a teapot should result
in the error code "418 I'm a teapot". The resulting entity
body MAY be short and stout.
Clipped photo by revolution cycle / CC BY 2.0
http://www. ickr.com/photos/11795120@N06/3832234809/

29. 500 Internal Server Error
Request:
GET /people/bradley-holt HTTP/1.1
Response:
HTTP/1.1 500 Internal Server Error
Content-Type: text/html
<!DOCTYPE html>
<html>
<head>
<title>Internal Server Error</title>
</head>
<body>
<p>Oops, someone broke the application.</p>
</body>
</html>

30. 503 Service Unavailable
Request:
GET /people/bradley-holt HTTP/1.1
Response:
HTTP/1.1 503 Service Unavailable
Retry-After: 120
Content-Type: text/html
<!DOCTYPE html>
<html>
<head>
<title>Service Unavailable</title>
</head>
<body>
<p>Try again in two minutes.</p>
</body>
</html>

31. Uniform Interface
Loose coupling and self-described messages
versus ne-grained functionality.

32. Uniform Interface
• URI identi es the resource.
• HTTP method says how we're manipulating the
resource.
• Entity-header elds and entity-body[7] represent the
resource.
• Requests and responses are self-descriptive and
stateless.

33. Hypermedia As The Engine Of
Application State (HATEOAS)
From Chapter 5 of the Fielding Dissertation[8]:
In order to obtain a uniform interface, multiple architectural constraints are
needed to guide the behavior of components. REST is de ned by four interface
constraints: identi cation of resources; manipulation of resources through
representations; self-descriptive messages; and, hypermedia as the engine of
application state.

34. State of What?
Wait, you just said requests and responses
are "stateless" and now you're talking about
application state?

35. Requests and Responses
• Each request and each response is, itself stateless
(forget about cookies for a minute).
• All relevant state information is included in the request
or response.
• Not just state, but state transitions can be part of a
request: POST, PUT and DELETE can change state on
the server.

36. Hypermedia
• XHTML
• HTML 5
• microformats
• RDFa
• URI Templates
• WADL
• Atom

37. Follow the Hyperlinks
• Link from current resource to another resource
• Form to nd resources (i.e. a "search" form)
• Form to manipulate resource's state
(via POST, PUT or DELETE)

38. RESTful Implementations
REST is just an architectural style.
Implementations can vary in how RESTful they are.

39. AtomPub
• An actual protocol that is RESTful
• Uses XML document hypermedia formats to represent
entities
• Originally designed for publishing blogs
• Used as the base for many RESTful web services
including:
• Google Data APIs (GData)
• Amazon Simple Storage Service (Amazon S3)
• Windows Azure Platform

40. GET Atom Service
Document
Request:
GET / HTTP/1.1
Response:
HTTP/1.1 200 OK
Content-Type: application/atomsvc+xml
<?xml version="1.0" encoding="utf-8"?>
<service xmlns="http://www.w3.org/2007/app"
xmlns:atom="http://www.w3.org/2005/Atom">
<workspace>
<atom:title>Blog</atom:title>
<collection href="http://example.org/blog">
<atom:title>Blog Entries</atom:title>
</collection>
<collection href="http://example.org/media">
<atom:title>Media</atom:title>
<accept>image/png</accept>
<accept>image/jpeg</accept>
<accept>image/gif</accept>
</collection>
</workspace>
</service>
Note: Service Document tells us everything we need to know to get started (loose coupling). For example, it
tells us the URIs of the collections to GET or POST to. URIs are up to the server to decide and should be opaque
to the client.

41. GET Atom Collection
Document
Request:
GET /blog HTTP/1.1
Response:
HTTP/1.1 200 OK
Content-Type: application/atom+xml
<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
<title>Blog Entries</title>
<link rel="self" href="http://example.org/blog"/>
<updated>2009-09-01T20:28:54Z</updated>
<id>urn:uuid:17040b30-9737-11de-8a39-0800200c9a66</id>
</feed>

42. POST Entry to Atom
Collection Document
Request:
POST /blog HTTP/1.1
Slug: =?utf-8?q?blog-entry?=
Content-Type: application/atom+xml
<?xml version="1.0" encoding="utf-8"?>
<entry xmlns="http://www.w3.org/2005/Atom">
<title>A Blog Entry</title>
<updated>2009-09-01T20:32:06Z</updated>
<summary>Summary of my blog entry...</summary>
</entry>
Response:
HTTP/1.1 201 Created
Location: /blog/blog-entry
Content-Type: application/atom+xml
<?xml version="1.0" encoding="utf-8"?>
<entry xmlns="http://www.w3.org/2005/Atom">
<title>A Blog Entry</title>
<link rel="edit" href="http://example.org/blog/blog-entry"/>
<id>urn:uuid:337f26a0-9737-11de-8a39-0800200c9a66</id>
<updated>2009-09-01T20:32:06Z</updated>
<summary>Summary of my blog entry...</summary>
</entry>

43. GET Atom Collection
Document (again)
Request:
GET /blog HTTP/1.1
Response:
HTTP/1.1 200 OK
Content-Type: application/atom+xml
<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
<title>Blog Entries</title>
<link rel="self" href="http://example.org/blog"/>
<updated>2009-09-01T20:32:06Z</updated>
<id>urn:uuid:17040b30-9737-11de-8a39-0800200c9a66</id>
<entry>
<title>A Blog Entry</title>
<link rel="edit" href="http://example.org/blog/blog-entry"/>
<id>urn:uuid:337f26a0-9737-11de-8a39-0800200c9a66</id>
<updated>2009-09-01T20:32:06Z</updated>
<summary>Summary of my blog entry...</summary>
</entry>
</feed>

44. GET Atom Entry
Document
Request:
GET /blog/blog-entry HTTP/1.1
Response:
HTTP/1.1 200 OK
Content-Type: application/atom+xml
<?xml version="1.0" encoding="utf-8"?>
<entry xmlns="http://www.w3.org/2005/Atom">
<title>A Blog Entry</title>
<link rel="edit" href="http://example.org/blog/blog-entry"/>
<id>urn:uuid:337f26a0-9737-11de-8a39-0800200c9a66</id>
<updated>2009-09-01T20:32:06Z</updated>
<summary>Summary of my blog entry...</summary>
</entry>

45. PUT Atom Entry
Document
Request:
PUT /blog/blog-entry HTTP/1.1
Content-Type: application/atom+xml
<?xml version="1.0" encoding="utf-8"?>
<entry xmlns="http://www.w3.org/2005/Atom">
<title>A Blog Entry</title>
<updated>2009-09-01T20:34:51Z</updated>
<summary>Updated summary...</summary>
</entry>
Response:
HTTP/1.1 200 OK
Content-Type: application/atom+xml
<?xml version="1.0" encoding="utf-8"?>
<entry xmlns="http://www.w3.org/2005/Atom">
<title>A Blog Entry</title>
<link rel="edit" href="http://example.org/blog/blog-entry"/>
<id>urn:uuid:337f26a0-9737-11de-8a39-0800200c9a66</id>
<updated>2009-09-01T20:34:51Z</updated>
<summary>Updated summary...</summary>
</entry>

46. POST Atom Media Entry
Document
Request:
POST /media HTTP/1.1
Slug: =?utf-8?q?vacation-photo?=
Content-Type: image/png
...binary data...
Response:
HTTP/1.1 201 Created
Location: /media/vacation-photo.png
Content-Type: application/atom+xml
<?xml version="1.0" encoding="utf-8"?>
<entry xmlns="http://www.w3.org/2005/Atom">
<title>vacation-photo</title>
<link rel="edit-media" href="http://example.org/media/vacation-photo.png"/>
<link rel="edit" href="http://example.org/media/vacation-photo"/>
<id>urn:uuid:00518590-973f-11de-8a39-0800200c9a66</id>
<updated>2009-09-01T20:37:18Z</updated>
</entry>
Note:
• edit-media URI represents the actual media
• edit URI represents the media entry (Atom Entry)

47. DELETE Atom Entry
Document
Request:
DELETE /blog/blog-entry HTTP/1.1
Response:
HTTP/1.1 200 OK

48. Google Calendar API
The Google Calendar Data API allows client
applications to view and update calendar events in
the form of Google Data API feeds.
Your client application can use the Calendar Data
API to create new events, edit or delete existing
events, and query for events that match particular
criteria.
From the Developer's Guide[9]

49. GET Calendar Feed
Request:
GET /calendar/feeds/default/owncalendars/full HTTP/1.1
Response (truncated):
HTTP/1.1 200 OK
Content-Type: application/atom+xml
<feed xmlns='http://www.w3.org/2005/Atom'
xmlns:gCal='http://schemas.google.com/gCal/2005'>
<id>http://www.google.com/calendar/feeds/default/owncalendars/full</id>
<updated>2009-09-01T22:16:48.105Z</updated>
<title type='text'>bradley.holt@gmail.com's Calendar List</title>
<entry>
<id>http://www.google.com/calendar/feeds/default/owncalendars/full/bradley.holt
%40gmail.com</id>
<updated>2009-08-17T06:39:02.000Z</updated>
<title type='text'>Bradley Holt (personal)</title>
<link rel='http://schemas.google.com/gCal/2005#eventFeed'
type='application/atom+xml'
href='http://www.google.com/calendar/feeds/bradley.holt%40gmail.com/
private/full'/>
<gCal:timezone value='America/New_York'/>
</entry>
</feed>

50. GET Event Feed
Request:
GET /calendar/feeds/bradley.holt%40gmail.com/private/full HTTP/1.1
Response (truncated):
HTTP/1.1 200 OK
Content-Type: application/atom+xml
<feed xmlns='http://www.w3.org/2005/Atom'
xmlns:batch='http://schemas.google.com/gdata/batch'
xmlns:gCal='http://schemas.google.com/gCal/2005'
xmlns:gd='http://schemas.google.com/g/2005'>
<id>http://www.google.com/calendar/feeds/bradley.holt%40gmail.com/private/full</id>
<updated>2009-08-17T06:39:02.000Z</updated>
<title type='text'>Bradley Holt (personal)</title>
<gCal:timezone value='America/New_York'/>
<entry>
<id>http://www.google.com/calendar/feeds/bradley.holt%40gmail.com/private/full/t8od3jn8qm65g1bjg78kokukug</id>
<updated>2009-08-15T18:32:09.000Z</updated>
<title type='text'>Biking</title>
<link rel='self' type='application/atom+xml'
href='http://www.google.com/calendar/feeds/bradley.holt%40gmail.com/private/full/t8od3jn8qm65g1bjg78kokukug'/>
<gd:eventStatus value='http://schemas.google.com/g/2005#event.confirmed'/>
<gd:when startTime='2009-08-15T10:00:00.000-04:00'
endTime='2009-08-15T14:00:00.000-04:00'/>
<gd:who rel='http://schemas.google.com/g/2005#event.attendee'
valueString='bradley.holt@gmail.com'
email='bradley.holt@gmail.com'/>
<gd:where valueString=''/>
</entry>
</feed>

51. POST an Event to the
Feed
Request:
POST /calendar/feeds/bradley.holt%40gmail.com/private/full HTTP/1.1
Content-Type: application/atom+xml
<entry xmlns='http://www.w3.org/2005/Atom'
xmlns:gd='http://schemas.google.com/g/2005'>
<updated>2009-09-01T20:48:18.000Z</updated>
<title type='text'>Haircut</title>
<gd:eventStatus value='http://schemas.google.com/g/2005#event.confirmed'/>
<gd:when startTime='2009-09-17T18:00:00.000-04:00'
endTime='2009-09-17T18:30:00.000-04:00'/>
<gd:who rel='http://schemas.google.com/g/2005#event.attendee'
valueString='bradley.holt@gmail.com'
email='bradley.holt@gmail.com'/>
<gd:where valueString='Barber Shop'/>
</entry>
Response (truncated):
HTTP/1.1 201 Created
Content-Type: application/atom+xml
<entry xmlns='http://www.w3.org/2005/Atom'
xmlns:batch='http://schemas.google.com/gdata/batch'
xmlns:gd='http://schemas.google.com/g/2005'>
<id>http://www.google.com/calendar/feeds/bradley.holt%40gmail.com/private/full/xuid3dn5dm65gdbs78k5o3koe</id>
<updated>2009-09-01T20:48:18.000Z</updated>
<title type='text'>Haircut</title>
<link rel='self' type='application/atom+xml'
href='http://www.google.com/calendar/feeds/bradley.holt%40gmail.com/private/full/xuid3dn5dm65gdbs78k5o3koe'/>
<gd:eventStatus value='http://schemas.google.com/g/2005#event.confirmed'/>
<gd:eventStatus value='http://schemas.google.com/g/2005#event.confirmed'/>
<gd:when startTime='2009-09-17T18:00:00.000-04:00'
endTime='2009-09-17T18:30:00.000-04:00'/>
<gd:who rel='http://schemas.google.com/g/2005#event.attendee'
valueString='bradley.holt@gmail.com'
email='bradley.holt@gmail.com'/>
<gd:where valueString='Barber Shop'/>
</entry>

52. Cache-Control
• Allows client and server to control caching of resource.
• An example of why URIs are important (cache only applies
to a given URI).
• Reduces latency.
• Reduces network traffic.
• Cached by: browser, proxy, gateway.
Request:
GET /people/bradley-holt HTTP/1.1
Cache-Control: max-age=1800
Response:
HTTP/1.1 200 OK
Content-Type: text/html
Cache-Control: max-age=3600
...HTML data...

53. Conditional GET
• An entity tag (ETag) allows for a conditional GET.
• An example of why URIs are important (conditional GET
only applies to a given URI).
• Reduces latency.
• Reduces network traffic.

54. Conditional GET (continued)
Request:
GET /people/bradley-holt HTTP/1.1
Response:
HTTP/1.1 200 OK
Content-Type: text/html
ETag: 6f6327696a7c8c6e7e
...HTML data...
Request:
GET /people/bradley-holt HTTP/1.1
If-None-Match: 6f6327696a7c8c6e7e
Response:
HTTP/1.1 304 Not Modified

55. Content Negotiation
• Different representations of a resource served by the same
URI.
Request HTML:
GET /people/bradley-holt HTTP/1.1
Accept: text/html
Response:
HTTP/1.1 200 OK
Content-Type: text/html
<!DOCTYPE html>
<html>
<head>
<title>Bradley Holt</title>
</head>
<body>
<div class="vcard">
<a class="url fn" href="http://bradley-holt.blogspot.com/">Bradley Holt</a>
</div>
</body>
</html>

56. Content Negotiation
(continued)
Request Atom:
GET /people/bradley-holt HTTP/1.1
Accept: application/atom+xml
Response:
HTTP/1.1 200 OK
Content-Type: application/atom+xml
<?xml version="1.0" encoding="utf-8"?>
<entry xmlns="http://www.w3.org/2005/Atom">
<title>Bradley Holt</title>
<link rel="edit" href="http://example.org/people/bradley-holt"/>
<id>urn:uuid:e92dbee0-9d99-11de-8a39-0800200c9a66</id>
<updated>2009-09-09T15:39:08Z</updated>
<summary>Bradley Holt</summary>
</entry>

57. Acceptance
• Accept-Charset
• utf-8
• iso-8859-1
• Accept-Encoding
• compress
• gzip
• 406 Not Acceptable

58. REST is Not...
• XML/JSON over HTTP
• Flickr API
• Twitter API
• If it says, "REST API" there's a good chance it isn't RESTful.

59. Trade-Offs
• No ne-grained function/method calling: course-grained
representation exchange instead.
• Uniform interface and loose coupling over efficiency.

60. Credits
Author: Bradley Holt
Technical Review: Josh Sled
Layout & Design: Jason Pelletier
Photo: Revolution Cycle, Solar Powered Tea Pot, http://www. ickr.com/photos/11795120@N06/3832234809/
This presentation licensed under Creative Commons -- Attribution 3.0 United States License.
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rest_arch_style.htm#sec_5_1_5
[9]: Developer's Guide - Google Calendar APIs and Tools (n.d.). Retrieved September 1, 2009, from Google Data APIs:
http://code.google.com/apis/calendar/docs/2.0/developers_guide.html