What is an API?

A 1969 NASA publication entitled Computer Program Abstracts (which can be found at the following link: https://tinyurl.com/y52x4aqy) contains a summary of a real-time display control program sold by IBM (only $310! Plus $36 if you want the documentation). This program is said to have been designed as an operator-application programming interface, in other words, an API.

Application Programming Interfaces, or APIs, have been around for about as long as computer code has. Conceptually, it is just a way for two different pieces of code (or a human and some code) to interface with each other. A class that provides certain public methods that other code can call has an API. A script that accepts certain kinds of inputs has an API. A driver on your computer that requires programs to call it in a certain way has an API.

However, as the internet grew, the term API narrowed in focus. Almost always now, when someone is talking about an API, they are talking about a web API. That is the context I will use in this book. A web API takes the concept of an interface between two things and applies it to the client/server relationship that the internet is built on. In a web API, a client is on one side of the interface and sends requests, while a server (or servers) is on the other side of the interface and responds to the request. 

Over time, the internet has changed and evolved, and web APIs have changed and evolved along with it. Many early web APIs were built for corporate use cases with strict rules in place as to how the two sides of the interface could interact with each other. The SOAP protocol was developed for this purpose. However, in the early 2000s, the web started to shift toward becoming a more consumer-based place. Some of the e-commerce sites, such as eBay and Amazon, started to publish APIs that were more public and flexible. This was followed by many of the social sites, including Twitter, Facebook and others. Many of these APIs were built using the REST protocol, which was more flexible and is built directly on the underlying protocols of the internet. 

The internet continued to change though, and as mobile applications and sites grew in popularity, so did the importance of APIs. Some companies faced challenges with the amount of data they wanted to transfer on mobile devices, and so Facebook created GraphQL. This query language helps to reduce the amount of data that gets transferred while introducing a slightly more rigid structure to the API. Each of the different API types work well in some scenarios, and I will explain more about what they are later in the chapter. However, before I get into the differences between them, I want to talk about some common terms that all web APIs share.