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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.
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Applying the REST Architectural Style
Resource-Oriented Web Services
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.
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]
Methods and ResourcesHTTP methods are the actions that can be performed on resources.
Uniform Resource Identi!ers (URIs)Resources are identi!ed by a URI.
Example URIs:http://www.example.org/peoplehttp://www.example.org/people/bradley-holt
Examples of resources could include:• documents• people• places• things• abstract concepts (e.g. processes, transactions)
URIs
Limited VocabularyThere 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]
• 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
REST Architecture
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]
HTTP MethodsHow are you manipulating a resource?We’ll cover four of the eight methods...
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.
More on SafetyA 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].
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/1.1Content-Type: application/x-www-form-urlencoded
fn=Bradley+Holt&url=http%3A%2F%2Fbradley-holt.blogspot.com%2F
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/1.1Content-Type: application/x-www-form-urlencoded
fn=Bradley+Holt&url=http%3A%2F%2Fbradley-holt.blogspot.com%2F
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
HTTP Status CodesWhat was the result of your request? A few examples...[5]
Request:GET /people/bradley-holt HTTP/1.1
Response:HTTP/1.1 200 OKContent-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>
200 OK
201 CreatedRequest:POST /people HTTP/1.1Content-Type: application/x-www-form-urlencoded
fn=Bradley+Holt&url=http%3A%2F%2Fbradley-holt.blogspot.com%2F
Response:HTTP/1.1 201 CreatedLocation: /people/bradley-holt
202 AcceptedRequest:POST /people HTTP/1.1Content-Type: application/x-www-form-urlencoded
fn=Bradley+Holt&url=http%3A%2F%2Fbradley-holt.blogspot.com%2F
Response:HTTP/1.1 202 AcceptedLocation: /queue/4jn6rk
301 Moved PermanentlyRequest:GET /people/bradley-holt HTTP/1.1
Response:HTTP/1.1 301 Moved PermanentlyLocation: /people/BradleyHolt
302 FoundRequest:GET /people/bradley-holt HTTP/1.1
Response:HTTP/1.1 302 FoundLocation: /people/BradleyHolt
400 Bad RequestRequest:POST /people HTTP/1.1Content-Type: application/x-www-form-urlencoded
fn=Bradley+Holt&url=bradley-holt.blogspot.com
Response:HTTP/1.1 400 Bad RequestContent-Type: text/html
<!DOCTYPE html><html><head> <title>Bradley Holt</title></head><body><p>URL is not valid.</p></body></html>
401 UnauthorizedRequest:GET /people/bradley-holt HTTP/1.1
Response:HTTP/1.1 401 UnauthorizedWWW-Authenticate: BASIC realm="Area 51"
403 ForbiddenRequest:GET /people/bradley-holt HTTP/1.1
Response:HTTP/1.1 403 Forbidden
404 Not FoundRequest:GET /people/bradley-holt HTTP/1.1
Response:HTTP/1.1 404 Not Found
405 Method Not AllowedRequest:POST /people/bradley-holt HTTP/1.1
Response:HTTP/1.1 405 Method Not AllowedAllow: GET, PUT, DELETE
409 Con!ictRequest:PUT /people/bradley-holt HTTP/1.1Content-Type: application/x-www-form-urlencoded
fn=Bradley+Holt&url=http%3A%2F%www.foundline.com%2F&revision=5
Response:HTTP/1.1 409 ConflictContent-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>
418 I’m A TeapotAccording 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 2.0http://www.#ickr.com/photos/11795120@N06/3832234809/
500 Internal Server ErrorRequest:GET /people/bradley-holt HTTP/1.1
Response:HTTP/1.1 500 Internal Server ErrorContent-Type: text/html
<!DOCTYPE html><html><head> <title>Internal Server Error</title></head><body><p>Oops, someone broke the application.</p></body></html>
503 Service UnavailableRequest:GET /people/bradley-holt HTTP/1.1
Response:HTTP/1.1 503 Service UnavailableRetry-After: 120Content-Type: text/html
<!DOCTYPE html><html><head> <title>Service Unavailable</title></head><body><p>Try again in two minutes.</p></body></html>
Uniform InterfaceLoose coupling and self-described messages versus !ne-grained functionality.
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.
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.
State of What?Wait, you just said requests and responses are "stateless" and now you're talking about application state?
• 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.
Requests and Responses
Hypermedia• XHTML• HTML 5• microformats• RDFa• URI Templates• WADL• Atom
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)
RESTful ImplementationsREST is just an architectural style. Implementations can vary in how RESTful they are.
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
GET Atom Service Document
Request:GET / HTTP/1.1
Response:HTTP/1.1 200 OKContent-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.
GET Atom Collection Document
Request:GET /blog HTTP/1.1
Response:HTTP/1.1 200 OKContent-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>
POST Entry to Atom Collection Document
Request:POST /blog HTTP/1.1Slug: =?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 CreatedLocation: /blog/blog-entryContent-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>
GET Atom Collection Document (again)
Request:GET /blog HTTP/1.1
Response:HTTP/1.1 200 OKContent-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>
GET Atom Entry Document
Request:GET /blog/blog-entry HTTP/1.1
Response:HTTP/1.1 200 OKContent-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>
PUT Atom Entry Document
Request:PUT /blog/blog-entry HTTP/1.1Content-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 OKContent-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>
POST Atom Media Entry Document
Request:POST /media HTTP/1.1Slug: =?utf-8?q?vacation-photo?=Content-Type: image/png
...binary data...
Response:HTTP/1.1 201 CreatedLocation: /media/vacation-photo.pngContent-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)
DELETE Atom Entry Document
Request:DELETE /blog/blog-entry HTTP/1.1
Response:HTTP/1.1 200 OK
Google Calendar APIThe 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]
GET Calendar FeedRequest:GET /calendar/feeds/default/owncalendars/full HTTP/1.1
Response (truncated):HTTP/1.1 200 OKContent-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'>[email protected]'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>
GET Event FeedRequest:GET /calendar/feeds/bradley.holt%40gmail.com/private/full HTTP/1.1
Response (truncated):HTTP/1.1 200 OKContent-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='[email protected]' email='[email protected]'/> <gd:where valueString=''/> </entry></feed>
POST an Event to the Feed
Request:POST /calendar/feeds/bradley.holt%40gmail.com/private/full HTTP/1.1Content-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='[email protected]' email='[email protected]'/> <gd:where valueString='Barber Shop'/></entry>
Response (truncated):HTTP/1.1 201 CreatedContent-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='[email protected]' email='[email protected]'/> <gd:where valueString='Barber Shop'/></entry>
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/1.1Cache-Control: max-age=1800
Response:HTTP/1.1 200 OKContent-Type: text/htmlCache-Control: max-age=3600
...HTML data...
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.
Conditional GET (continued)
Request:GET /people/bradley-holt HTTP/1.1
Response:HTTP/1.1 200 OKContent-Type: text/htmlETag: 6f6327696a7c8c6e7e
...HTML data...
Request:GET /people/bradley-holt HTTP/1.1If-None-Match: 6f6327696a7c8c6e7e
Response:HTTP/1.1 304 Not Modified
Content Negotiation• Different representations of a resource served by the same
URI.
Request HTML:GET /people/bradley-holt HTTP/1.1Accept: text/html
Response:HTTP/1.1 200 OKContent-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>
Content Negotiation (continued)
Request Atom:GET /people/bradley-holt HTTP/1.1Accept: application/atom+xml
Response:HTTP/1.1 200 OKContent-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>
Acceptance• Accept-Charset
• utf-8• iso-8859-1
• Accept-Encoding• compress• gzip
• 406 Not Acceptable
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.
Trade-Offs• No !ne-grained function/method calling: course-grained
representation exchange instead.• Uniform interface and loose coupling over efficiency.
CreditsAuthor: Bradley HoltTechnical Review: Josh SledLayout & Design: Jason PelletierPhoto: 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.
[1]: Hypertext Transfer Protocol. (2009, August 25). In Wikipedia, The Free Encyclopedia. Retrieved August 25, 2009, from http://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol[2]: HTTP/1.1: Method De!nitions. (n.d.). Retrieved August 26, 2009, from World Wide Web Consortium - Web Standards: http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html[3]: Richardson, L., & Ruby, S. (2007). RESTful Web Services. Sebastopol, CA: O’Reilly Media, Inc.[4]: Google Web Accelerator: Hey, not so fast - an alert for web app designers. (2005, May 6). Retrieved September 8, 2009, from Signal vs. Noise: http://37signals.com/svn/archives2/google_web_accelerator_hey_not_so_fast_an_alert_for_web_app_designers.php[5]: HTTP/1.1: Status Code De!nitions. (n.d.). Retrieved August 27, 2009, from World Wide Web Consortium - Web Standards: http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html[6]: Hyper Text Coffee Pot Control Protocol (HTCPCP/1.0) (1998, April 1). Retrieved August 31, 2009, from The Internet Engineering Task Force (IETF): http://www.ietf.org/rfc/rfc2324.txt[7]: HTTP/1.1: Entity (n.d.). Retrieved September 1, 2009, from World Wide Web Consortium - Web Standards: http://www.w3.org/Protocols/rfc2616/rfc2616-sec7.html[8]: Fielding Dissertation: CHAPTER 5: Representational State Transfer (REST) (2000). Retrieved September 1, 2009, from Architectural Styles and the Design of Network-based Software Architectures: http://www.ics.uci.edu/~!elding/pubs/dissertation/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
