Ferenc Mihaly maintains a blog call the Amiable API where he posts various articles on API design. As part of this blog, he provides a checklist of dos and don’t in API design. The list is focused on the Java programming language, however, most of what Ferenc presents it applicable to other languages too. I really like the bullet list format and the use of verbs to produce a concise yet extensive set of general rules to consider when developing an interface.

• http://theamiableapi.com/2012/01/16/java-api-design-checklist/

Martin Fowler wrote about a way to approach RESTful Web APIs developed by Leonard Richardson. This presents 4 successive levels of concepts, from the simple “plain old XML” layer, through the notion of resources, http verbs, and hypermedia controls. Martin does a great job of explaining each of the layers through the use of figures and example XML data.

• http://martinfowler.com/articles/richardsonMaturityModel.html

Pamela Fox wrote about some great process that AT&T implemented to test the usability of their APIs. They invited several developers to come in and test out their APIs and give feedback on ways they can improve their developer experience. This is a great idea and fantastic way to improve an API based upon actual customer experience:

• http://blog.pamelafox.org/2012/03/api-usability-testing.html

The folk at apigee have distilled everything they’ve learned about Web API design over the last few years into a new free eBook called “Web API Design: Crafting Interfaces that Developers Love.” This collates many of the details from their various blog posts into a single reference, covering topics such as REST, handling errors, versioning, authentication, complementary SDKs, and many others. If you’re writing a Web API for your platform this is a great read.

• http://blog.apigee.com/detail/announcement_new_ebook_on_web_api_design

A new book on developing Web-based APIs is about to be published called “APIs: A Strategy Guide: Creating Channels with Application Programming Interfaces.” The book is written by Dan Jacobson, Greg Brail, and Dan Woods, who between them have an impressive background in developing real-world Web APIs. The book aims to educate executives and business development teams on the value and strategy for building Web APIs.

• http://shop.oreilly.com/product/0636920021223.do

Brian Mulloy at Apigee wrote a short article on versioning for Web APIs where he contrasts three different methods of including versioning information in a URL. These include using a timestamp or version number in the URL path, or accepting a version number as an optional parameter. Brian suggests using a mandatory version number as far left in the URL as possible, and using a whole number instead of dot notation for your versions.

• http://blog.apigee.com/detail/restful_api_design_tips_for_versioning/

Bj Rollison wrote an article on API testing in which he talks about the differences between unit, API, and GUI testing. He goes on to describe some of the key benefits of API testing, including: reduced testing costs, improved productivity, and higher functional quality. Bj concludes by talking about the relationship between developers and testers.

• http://www.testingmentor.com/imtesty/2011/12/01/api-testinghow-can-it-help/

If you want to read something a little longer than a blog article on how to build REST Web APIs then there are a few books out there that may be of interest to you:

• REST API Design Rulebook
• Restful Web Services
• RESTful Web Services Cookbook
• REST in Practice

In this Twilio Engineering blog article, Frank Stratton makes the point that computers don’t care about API design. APIs should be designed for human beings first and computers second. Using example Web APIs developed at Twilio, he talks about the need for having many eyes and minds see the design first, simplifying the surface area of your API, and how it is easy to add new calls but hard to remove existing ones.

• http://www.twilio.com/engineering/2011/10/31/apis-are-for-human-beings/

Noah at 37 Signals wrote about API design for Web APIs. The key point he makes is about the need for strong documentation and having that documentation in an easy-to-search format. He then talks about authentication, advocating that an API offers multiple authentication schemes. Finally, he talks about building REST-like interfaces.

• http://37signals.com/svn/posts/3018-api-design-for-humans