Designing Web APIs

Chapter 1. What’s an API?

“What’s an API?” When a new programmer asks this question, they typically get the answer, “an application programming interface.”

But APIs are so much more than their name suggests—and to understand and unleash their value, we must focus on the keyword interface.

An API is the interface that a software program presents to other programs, to humans, and, in the case of web APIs, to the world via the internet. An API’s design belies much about the program behind it—business model, product features, the occasional bug. Although APIs are designed to work with other programs, they’re mostly intended to be understood and used by humans writing those other programs.

APIs are the building blocks that allow interoperability for major business platforms on the web. APIs are how identity is created and maintained across cloud software accounts, from your corporate email address to your collaborative design software to the web applications that help you order pizza delivery. APIs are how weather forecast data is shared from a reputable source like the National Weather Service to hundreds of software apps that specialize in its presentation. APIs process your credit cards and enable companies to seamlessly collect your money without worrying about the minutiae of financial technology and its corresponding laws and regulations.

More and more, APIs are a key component of scalable and successful internet companies like Amazon, Stripe, Google, and Facebook. For companies ...