Home arrow Web Services arrow Page 3 - Safety, Idempotence, and the Resource-Oriented Architecture
WEB SERVICES

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.

Author Info:
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!

print this article
SEARCH DEVARTICLES

Safety, Idempotence, and the Resource-Oriented Architecture - Why safety and idempotence matter
(Page 3 of 5 )

Safety and idempotence let a client make reliable HTTP requests over an unreliable network. If you make a GET request and never get a response, just make another one. It’s safe: even if your earlier request went through, it didn’t have any real effect on the server. If you make a PUT request and never get a response, just make another one. If your earlier request got through, your second request will have no additional effect.

POST is neither safe nor idempotent. Making two identical POST requests to a “factory” resource will probably result in two subordinate resources containing the same information. With overloaded POST, all bets are off.

The most common misuse of the uniform interface is to expose unsafe operations through GET. The del.icio.us and Flickr APIs both do this. When you GET https:// api.del.icio.us/posts/delete, you’re not fetching a representation: you’re modifying the del.icio.us data set.

Why is this bad? Well, here’s a story. In 2005 Google released a client-side caching tool called Web Accelerator. It runs in conjunction with your web browser and “pre-fetches” the pages linked to from whatever page you’re viewing. If you happen to click one of those links, the page on the other side will load faster, because your computer has already fetched it.

Web Accelerator was a disaster. Not because of any problem in the software itself, but because the Web is full of applications that misuse GET. Web Accelerator assumed that GET operations were safe, that clients could make them ahead of time just in case a human being wanted to see the corresponding representations. But when it made those GET requests to real URIs, it changed the data sets. People lost data.

There’s plenty of blame to go around: programmers shouldn’t expose unsafe actions through GET, and Google shouldn’t have released a real-world tool that didn’t work with the real-world web. The current version of Web Accelerator ignores all URIs that contain query variables. This solves part of the problem, but it also prevents many resources that are safe to use through GET (such as Google web searches) from being pre-fetched.

Multiply the examples if you like. Many web services and web applications use URIs as input, and the first thing they do is send a GET request to fetch a representation of a resource. These services don’t mean to trigger catastrophic side effects, but it’s not up to them. It’s up to the service to handle a GET request in a way that complies with the HTTP standard.


blog comments powered by Disqus
WEB SERVICES ARTICLES

- Dealing with Loose Coupling in a Service-Ori...
- Loose Coupling in a Service-Oriented Archite...
- Safety, Idempotence, and the Resource-Orient...
- The Resource-Oriented Architecture in Action
- Features of the Resource-Oriented Architectu...
- The Resource-Oriented Architecture
- Getting Started with Flex
- Automated Billing and Faxing for the Web
- An Introduction to Web Services
- The Foundations of Web Services: From Novice...
- Web Services Reengineering: Finishing Touches
- Fault Handling with Web Services
- Flow and Web Services
- Process Lifecycles and Web Services
- Business Processes and Web Services

Watch our Tech Videos 
Dev Articles Forums 
 RSS  Articles
 RSS  Forums
 RSS  All Feeds
Write For Us 
Weekly Newsletter
 
Developer Updates  
Free Website Content 
Contact Us 
Site Map 
Privacy Policy 
Support 

Developer Shed Affiliates

 




© 2003-2017 by Developer Shed. All rights reserved. DS Cluster - Follow our Sitemap
Popular Web Development Topics
All Web Development Tutorials