What Is an API?

API Example

Two programmers (coders) meet to try to agree on implementing and using the function for a mathematical square root.

One coder will consume (use) the function, and the other will provide the function.

Square Root

The Agreement (Interface)

The two coders agree on the interface (method header) shown.

These coder's are using Java syntax but this applies to any language, or may be language independent as we'll see below with the web API

double sqrt(double x)
/*Where:
	the function receives one number as input "x"
	the function returns the square root (a number)
	double means floating point number*/

Provider

The API provider codes up the logic to satisfy the agreement (e.g. computes the square root of a number).

double sqrt(double x) {
	//The coding magic done by the provider
}

Consumer

The API consumer codes up the function call that will be used to call the API provider.

double result = Math.sqrt(49);

Web API

A Web API follows the same pattern -- only the syntax differs.

https://www.mathy.com/sqrt?x=49

/sqrt -- effectively the function name
x=49 -- is the input