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

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/

Michael Feathers, author of the book “Working Effectively with Legacy Code”, gave a presentation at SD West in 2007 about designing an API with consideration for how it will be unit tested. He presents recommendations such as avoiding static methods and avoiding final/non-virtual methods (although I disagree with the latter recommendation in my book, where I argue that being minimally complete is more important). He also presents a golden rule of API Design: “It’s not enough to write tests for an API you develop, you have to write unit tests for code that uses your API.” He also states that you should supply your tests to your users. I could not agree with those statements more!

• http://www.objectmentor.com/resources/articles/as_if_unit_testing_mattered.pdf (PDF)

Most testing efforts are targeted toward testing the actual functionality of an API, i.e., what it is supposed to do. However, there is another class of testing called non-functional testing. This involves testing the quality of the software from the user’s perspective. It can involve things like performance testing, scalability testing, and security testing. The following article gives a comparison between functional and non-functional testing.

• http://www.lessons-from-history.com/node/83

A unit test is a white-box testing technique that a developer performs. White-box means that you write the tests with knowledge of the implementation of the API. There are many frameworks out there that help you write, maintain, and run unit tests for your code. Most of these are based upon the J-Unit style of unit testing. Wikipedia offers a good categorization of unit test frameworks that are available for C and C++. See:

• http://en.wikipedia.org/wiki/List_of_unit_testing_frameworks#C
• http://en.wikipedia.org/wiki/List_of_unit_testing_frameworks#C.2B.2B

This article provides a QA engineer’s perspective on API testing. It pulls out some of different types of testing approaches, such as unit testing and black-box testing, and provides some guidance on writing API tests.

• http://www.scribd.com/doc/9808382/Introduction-to-API-Testing

Note that the term “API Testing” is commonly used in the QA domain to mean programmatic black-box tests that a QA engineer writes.  However, I view the topic of “testing an API” as being wider than this, encompassing unit testing, integration testing, and non-functional testing, and performed by either developers or QA engineers.