Faculty of Informatics, University of Lugano School of Information, UC Berkeley

What is REST?

RESTful Web Services:
Principles, Patterns, Emerging Technologies
[./]
Tutorial at WWW2010 [http://www2010.org/] (Raleigh, North Carolina)

Erik Wilde (UC Berkeley School of Information)

April 27, 2010

Creative Commons License [http://creativecommons.org/licenses/by/3.0/]

This work is licensed under a CC
Attribution 3.0 Unported License
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Contents Erik Wilde: What is REST?

Contents

Erik Wilde: What is REST?
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(2) Abstract

Representational State Transfer (REST) is defined as an architectural style, which means that it is not a concrete systems architecture, but instead a set of constraints that are applied when designing a systems architecture. We briefly discuss these constraints, but then focus on explaining how the Web is one such systems architecture that implements REST. In particular, the mechanisms of the Uniform Resource Identifiers (URIs), the Hypertext Transfer Protocol (HTTP), media types, and markup languages such as the Hypertext Markup Language (HTML) and the Extensible Markup Language (XML). We also introduce Atom and the Atom Publishing Protocol (AtomPub) as two established ways on how RESTful services are already provided and used on today's Web.



Abstraction Layers

Outline (Abstraction Layers)

  1. Abstraction Layers [7]
  2. REST: The Definition [6]
  3. Web Architecture [8]
    1. Uniform Resource Identifier (URI) [4]
    2. Hypertext Transfer Protocol (HTTP) [3]
  4. Representations [8]
    1. Structured Documents [6]
    2. Linked Documents [2]
  5. State [6]
Abstraction Layers Erik Wilde: What is REST?
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(4) What is REST?

  1. An architectural style for building loosely coupled systems
    • defined by a set of general constraints (principles)
    • the Web (URI/HTTP/HTML/XML) is an instance of this style
  2. The Web used correctly (i.e., not using the Web as transport)
    • HTTP is built according to RESTful principles
    • services are built on top of Web standards without misusing them
    • most importantly, HTTP is an application protocol (not a transport protocol)
  3. Anything that uses HTTP and XML (XML without SOAP)
    • XML-RPC was the first approach for this
    • violates REST because there is no uniform interface


Abstraction Layers Erik Wilde: What is REST?
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(5) What is Architecture?



Abstraction Layers Erik Wilde: What is REST?
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(6) Architecture Examples

New York City Lübeck


Abstraction Layers Erik Wilde: What is REST?
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(7) Architecture vs. Design

Nice Design, Expensive Architecture

Abstraction Layers Erik Wilde: What is REST?
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(8) Architectural Styles

Louvre Interior
  • Architecture: Louvre [http://en.wikipedia.org/wiki/Louvre]
  • Architectural Style: Baroque [http://en.wikipedia.org/wiki/Baroque_architecture]
Villa Savoye Interior
  • Architecture: Villa Savoye [http://en.wikipedia.org/wiki/Villa_Savoye]
  • Architectural Style: International Style [http://en.wikipedia.org/wiki/International_Style_(architecture)]


Abstraction Layers Erik Wilde: What is REST?
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(9) REST is not an Architecture



Abstraction Layers Erik Wilde: What is REST?
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(10) SOA is not an Architecture



REST: The Definition

Outline (REST: The Definition)

  1. Abstraction Layers [7]
  2. REST: The Definition [6]
  3. Web Architecture [8]
    1. Uniform Resource Identifier (URI) [4]
    2. Hypertext Transfer Protocol (HTTP) [3]
  4. Representations [8]
    1. Structured Documents [6]
    2. Linked Documents [2]
  5. State [6]
REST: The Definition Erik Wilde: What is REST?
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(12) The REST Architectural Style

  1. Resource Identification [Resource Identification (1)]
  2. Uniform Interface [Uniform Interface (1)]
  3. Self-Describing Messages [Self-Describing Messages (1)]
  4. Hypermedia Driving Application State [Hypermedia Driving Application State (1)]
  5. Stateless Interactions [Stateless Interactions (1)]


REST: The Definition Erik Wilde: What is REST?
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(13) Resource Identification



REST: The Definition Erik Wilde: What is REST?
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(14) Uniform Interface



REST: The Definition Erik Wilde: What is REST?
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(15) Self-Describing Messages



REST: The Definition Erik Wilde: What is REST?
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(16) Hypermedia Driving Application State



REST: The Definition Erik Wilde: What is REST?
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(17) Stateless Interactions



Web Architecture

Outline (Web Architecture)

  1. Abstraction Layers [7]
  2. REST: The Definition [6]
  3. Web Architecture [8]
    1. Uniform Resource Identifier (URI) [4]
    2. Hypertext Transfer Protocol (HTTP) [3]
  4. Representations [8]
    1. Structured Documents [6]
    2. Linked Documents [2]
  5. State [6]
Web Architecture Erik Wilde: What is REST?
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(19) What is the Web?



Uniform Resource Identifier (URI)

Outline (Uniform Resource Identifier (URI))

  1. Abstraction Layers [7]
  2. REST: The Definition [6]
  3. Web Architecture [8]
    1. Uniform Resource Identifier (URI) [4]
    2. Hypertext Transfer Protocol (HTTP) [3]
  4. Representations [8]
    1. Structured Documents [6]
    2. Linked Documents [2]
  5. State [6]
Uniform Resource Identifier (URI) Erik Wilde: What is REST?
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(21) Identifying Resources on the Web

  • Essential for implementing a Resource Identification [Resource Identification (1)]
  • URIs are human-readable universal identifiers for stuff
    • many identification schemes are not human-readable (binary or hex strings)
    • many RPC-based systems do not have universally identified objects
  • Making every thing a universally unique identified thing is important
    • it removes the necessity to scope non-universal identifiers
    • it allows to talk about all things in exactly the same way


Uniform Resource Identifier (URI) Erik Wilde: What is REST?
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(22) URI Schemes

URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ]
  • URIs in their general case are very simple
    • the scheme identifies how resources are identified
    • the identification may be hierarchical or non-hierarchical
  • Many URI schemes are hierarchical
    • it is then possible to use relative URIs such as in a href="../"
    • the slash character is not just a character, in URIs it has semantics
[…] the URI syntax is a federated and extensible naming system wherein each scheme's specification may further restrict the syntax and semantics of identifiers using that scheme.

Uniform Resource Identifier (URI): Generic Syntax, RFC 3986, January 2005 [http://dret.net/rfc-index/reference/RFC3986]



Uniform Resource Identifier (URI) Erik Wilde: What is REST?
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(23) Query Information

  • Query components specify additional information
    • it is non-hierarchical information further identifying the resource
    • in most cases, it can be regarded as input to the resource
  • Query components often influence caching
    • successful GET/HEAD requests may be cached
    • only cache query string URIs when explicitly requested (Expires/Cache-Control)
The query component contains non-hierarchical data that, along with data in the path component […], serves to identify a resource within the scope of the URI's scheme and naming authority […].

Uniform Resource Identifier (URI): Generic Syntax, RFC 3986, January 2005 [http://dret.net/rfc-index/reference/RFC3986]



Uniform Resource Identifier (URI) Erik Wilde: What is REST?
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(24) Processing URIs

  • Processing URIs is not as trivial as it may seem
    • escaping and normalization rules are non-trivial
    • many implementations are broken
    • complain about broken implementations
    • even more complicated when processing an Internationalized Resource Identifier (IRI)
  • URIs are not just strings
    • URIs are strings with a considerable set of rules attached to them
    • implementing all these rules is non-trivial
    • implementing all these rules is crucial
    • application development environments provide functions for URI handling


Hypertext Transfer Protocol (HTTP)

Outline (Hypertext Transfer Protocol (HTTP))

  1. Abstraction Layers [7]
  2. REST: The Definition [6]
  3. Web Architecture [8]
    1. Uniform Resource Identifier (URI) [4]
    2. Hypertext Transfer Protocol (HTTP) [3]
  4. Representations [8]
    1. Structured Documents [6]
    2. Linked Documents [2]
  5. State [6]
Hypertext Transfer Protocol (HTTP) Erik Wilde: What is REST?
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(26) How RESTful Applications Talk

  • Essential for implementing a Uniform Interface [Uniform Interface (1)]
    • HTTP defines a small set of methods for acting on URI-identified resources
  • Misusing HTTP turns application into non-RESTful applications
    • they lose the capability to be used just by adhering to REST principles
    • it's a bad sign when you think you need an interface description language
  • Extending HTTP turns applications into more specialized RESTful applications
    • may be appropriate when more operations are required
    • seriously reduces the number of potential clients


Hypertext Transfer Protocol (HTTP) Erik Wilde: What is REST?
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(27) HTTP Methods

  • Safe methods can be ignored or repeated without side-effects
    • arithmetically safe: 41 × 1 × 1 × 1 × 1 …
    • in practice, without side-effects means without relevant side-effects
  • Idempotent methods can be repeated without side-effects
    • arithmetically idempotent: 41 × 0 × 0 × 0 × 0 …
    • in practice, without side-effects means without relevant side-effects
  • Unsafe and non-idempotent methods should be treated with care
  • HTTP has two main safe methods: GET HEAD
  • HTTP has two main idempotent methods: PUT DELETE
  • HTTP has one main overload method: POST


Hypertext Transfer Protocol (HTTP) Erik Wilde: What is REST?
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(28) Cookies

  • Cookies are client site state bound to a domain
    • they are convenient because they work without having to use a representation
    • they are inconvenient because they are not embedded in representations
  • Cookies are managed by the client
    • they are shared across browser tabs
    • they are not shared across browsers used by the same user
    • essentially, the client model of cookies is a bit outdated
  • Two major things to look out for when using cookies:
    1. session IDs are application state (i.e., non-resource state)
    2. cookies break the back button (requests contain a URI/cookie combo)
  • The ideal RESTful cookie is never sent to the server
    • cookies as persistent data storage on the client
    • interactions with the server are only using URIs and representations


Representations

Outline (Representations)

  1. Abstraction Layers [7]
  2. REST: The Definition [6]
  3. Web Architecture [8]
    1. Uniform Resource Identifier (URI) [4]
    2. Hypertext Transfer Protocol (HTTP) [3]
  4. Representations [8]
    1. Structured Documents [6]
    2. Linked Documents [2]
  5. State [6]

Structured Documents

Structured Documents Erik Wilde: What is REST?
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(31) What is identified by a URI?

  • Essential for implementing Self-Describing Messages [Self-Describing Messages (1)]
  • Resource Identification [Resource Identification (1)] only talks about an abstract resource
    • resources are never exchanged or otherwise processed directly
    • all interactions use resource representations
  • Representations depend on various factors
    • the nature of the resource
    • the capabilities of the server
    • the capabilities or the communications medium
    • the capabilities of the client
    • requirements and constraints from the application scenario
    • negotiations to figure out the best representation
  • Representations are identified by media type (sometimes called MIME type) [http://dret.net/lectures/web-fall08/mediatypes]


Structured Documents Erik Wilde: What is REST?
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(32) Extensible Markup Language (XML)

  • The language that started it all
    • created as a streamlined version of SGML
    • took over as the first universal language for structured data
  • XML is a metalanguage (a language for representing languages)
    • many domain-specific languages are defined as XML vocabularies
    • some metalanguages use XML syntax (RDF [Resource Description Framework (RDF) (1)] is a popular example)
  • XML is only syntax and has almost zero semantics
    • very minimal built-in semantics (language identification, IDs, relative URIs)
    • semantics are entirely left to the XML vocabularies
  • XML is built around a tree model
    • each XML document is a tree and thus limited in structure
    • RESTful XML introduces hypermedia to turn XML data into a graph


Structured Documents Erik Wilde: What is REST?
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(33) JavaScript Object Notation (JSON)

  • The XMLHttpRequest API has been built for requesting XML via HTTP
    • this is useful because XML is the most popular data format
    • all requested data has to be processed by using XML access methods in JavaScript
  • JavaScript does not have XML as its internal data model
    • the XML received via XMLHttpRequest has to be parsed into a DOM tree
    • DOM access in JavaScript is inconvenient for complex operations
    • alternatively, the XML can be mapped to JavaScript objects (also requires parsing)
  • JavaScript Object Notation (JSON) encodes data as JavaScript objects
    • more efficient for the consumer if the consumer is written in JavaScript
    • this turns the generally usable XML service into a JavaScript-oriented service
    • for large-scale applications, it might make sense to provide XML and JSON
    • this can be negotiated with HTTP content negotiation


Structured Documents Erik Wilde: What is REST?
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(34) JSON Example

<?xml version="1.0"?>
<menu id="file" value="File">
 <popup>
  <menuitem value="New" onclick="CreateNewDoc()"/>
  <menuitem value="Open" onclick="OpenDoc()"/>
  <menuitem value="Close" onclick="CloseDoc()"/>
 </popup>
</menu>
{ "menu" : {
 "id" : "file",
 "value" : "File",
 "popup" : {
  "menuitem" : [
   { "value" : "New", "onclick" : "CreateNewDoc()" },
   { "value" : "Open", "onclick" : "OpenDoc()" },
   { "value" : "Close", "onclick" : "CloseDoc()" }
  ]
 }
}}


Structured Documents Erik Wilde: What is REST?
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(35) Resource Description Framework (RDF)

  • Developed around the same time as XML was developed
    • based on the idea of machine-readable/understandable semantics
    • builds the Semantic Web as a parallel universe on top of the Web
  • RDF uses URIs for naming things
    • RDF's data model is based on (URI, property, value) triples
    • triples are combined and inference is used to produce a graph
  • RDF is a metalanguage built on the triple-based data model
    • RDF has a number of syntaxes (one of them is XML [Extensible Markup Language (XML) (1)]-based)
    • RDF introduces a number of schema languages (often referred to as ontology languages)


Structured Documents Erik Wilde: What is REST?
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(36) Atom

  • A language for representing syndication feeds
  • Much more modest in its goal than XML [Extensible Markup Language (XML) (1)] or RDF [Resource Description Framework (RDF) (1)]
    • models feeds as a sets of entries with associated metadata
    • uses an XML vocabulary for representing the data model
    • uses links for expressing relationships in the data model
  • Will be discussed in detail as a good foundation for REST [REST in Practice]


State

Outline (State)

  1. Abstraction Layers [7]
  2. REST: The Definition [6]
  3. Web Architecture [8]
    1. Uniform Resource Identifier (URI) [4]
    2. Hypertext Transfer Protocol (HTTP) [3]
  4. Representations [8]
    1. Structured Documents [6]
    2. Linked Documents [2]
  5. State [6]
State Erik Wilde: What is REST?
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(41) State Management on the Web



State Erik Wilde: What is REST?
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(42) State in HTML or HTTP

State in HTML or HTTP

State Erik Wilde: What is REST?
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(43) State in the Server Application

State in the Server Application

State Erik Wilde: What is REST?
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(44) State as a Resource

State as a Resource

State Erik Wilde: What is REST?
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(45) Stateless Shopping



State Erik Wilde: What is REST?
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(46) Reusing Resources

Reusing Resources

Erik Wilde: What is REST?
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(47) Conclusions



April 27, 2010 RESTful Web Services:
Principles, Patterns, Emerging Technologies
[./]
Tutorial at WWW2010 [http://www2010.org/] (Raleigh, North Carolina)