An API is a promise. The difference between an ordinary shared library, OS feature, or RESTful service and advertising the same code as an API is fundamentally

An API is a promise. The difference between an ordinary shared library, OS feature, or RESTful service and advertising the same code as an API is fundamentally a promise. If you usr my library by calling functions foo() and bar() as described in my API documentation, I promise your program should continue to work with future versions of my library that support the same API. The technical specifics of any API are important, but the promise is what matters. Without that promise, using a "RESTful API" is just screen scraping; it might work today, but it could break unexpectedly anytime in the future.