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Safety, Idempotence, and the Resource-Oriented Architecture


In this conclusion to a four-part series on the resource-oriented architecture, you will learn (among other things) about safety and idempotence, and why they matter. This article is excerpted from chapter four of the book RESTful Web Services, written by Leonard Richardson and Sam Ruby (O'Reilly, 2008; ISBN: 0596529260). Copyright © 2008 O'Reilly Media, Inc. All rights reserved. Used with permission from the publisher. Available from booksellers or direct from O'Reilly Media.

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By: O'Reilly Media
Rating: 5 stars5 stars5 stars5 stars5 stars / 3
February 20, 2009
TABLE OF CONTENTS:
  1. · Safety, Idempotence, and the Resource-Oriented Architecture
  2. · Safety and Idempotence
  3. · Why safety and idempotence matter
  4. · Why the Uniform Interface Matters
  5. · That’s It!

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Safety, Idempotence, and the Resource-Oriented Architecture - That’s It!
(Page 5 of 5 )

 

That’s the Resource-Oriented Architecture. It’s just four concepts:

  1. Resources
  2. Their names (URIs)
  3. Their representations
  4. The links between them

and four properties:

  1. Addressability
  2. Statelessness
  3. Connectedness
  4. A uniform interface

Of course, there are still a lot of open questions. How should a real data set be split into resources, and how should the resources be laid out? What should go into the actual HTTP requests and responses? I’m going to spend much of the rest of the book exploring issues like these.

 


 

* The earliest instance of “resource-oriented” I’ve found is a 2004 IBM developerWorks article by James Snell: “Resource-oriented vs. activity-oriented Web services” (http://www128.ibm.com/developerworks/xml/
library/ws-restvsoap/). Alex Bunardzic used “Resource-Oriented Architecture” in August 2006, before this book was announced: http://jooto.com/blog/index.php/2006/08/08/replacing-service-oriented-architecture-with
-
resource-oriented-architecture/
. I don’t agree with everything in those articles, but I do acknowledge their priority in terminology.

† “The Architecture of the World Wide Web” (http://www.w3.org/TR/2004/REC-webarch-20041215/#p39), which is full of good quotes, incidentally: “Software developers should expect that sharing URIs across applications will be useful, even if that utility is not initially evident.” This could be the battle cry of the ROA.

‡ Compare the Ajax interface against the more addressable version of Gmail you get by starting off at the URI
https://mail.google.com/mail/?ui=html. If you use this plain HTML interface, the resource “email messages about jellyfish” is addressable.

§ Other consumers of this web service include the libgmail library for Python (http://libgmail.sourceforge.net/).

‖ This idea is based on the CMU Coke machine (http://www.cs.cmu.edu/%7Ecoke/), which for many years was observed by instruments and whose current state was accessible through the Finger protocol. The machine is still around, though at the time of writing its state was not accessible online.

#Multiplying a number by one is both safe and idempotent: 4 ×1 ×1 ×1 is the same as 4 ×1, which is the same as 4. Multiplication by zero is not safe, because 4 ×0 is not the same as 4. Multiplying by any other number
is neither safe nor idempotent.


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