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Application Programming Interfaces

Summary

Objectives

By the end of this session you should be able to:

  • Explain what an API is.
  • Demonstrate calling an API in the terminal, then demonstrate working with basic APIs in a program.

Key Points

  • An API - Application Programming Interface - is a connection between computer programs.
  • APIs are commonly used to break up services into components which can talk to each other.
  • Web APIs are a useful way to programmatically get data from web services.

Breakdown

APIs

Application Programming Interfaces allow processes to ask each other questions - requests - and receive answers - responses. APIs are a powerful tool on the modern internet, allowing new services to be built on top of existing ones. A common example is price comparison websites, which leverage the APIs of online shops to compare prices on the same product.

info

While we primarily explore Web APIs in this course, not all APIs are Web APIs. APIs can be created between applications running on the same computer - as an example, a time daemon could provide an API for processes to request the time.

Web APIs

There are two popular types of APIs on the modern internet:

  • Simple Object Access Protocol (SOAP) is a protocol that strictly defines APIs, and uses XML (a subset of HTML) to share data.
  • Representational State Transfer (ReST) is a set of principles for building a web API, commonly using JSON to share data.

SOAP is popular in enterprise applications, but most web APIs now use REST (also known as RESTful APIs).

JavaScript Object Notation

JSON is a way of serialising data into a string in a standardised way, such that it can be shared between applications. Although it's got JavaScript in the name it's suitable for use in any programming languages, and most have a built-in library for processing JSON strings. It looks like this:

{
  "firstName": "John",
  "lastName": "Smith",
  "isAlive": true,
  "age": 27,
  "address": {
    "streetAddress": "21 2nd Street",
    "city": "New York",
    "state": "NY",
    "postalCode": "10021-3100"
  },
  "phoneNumbers": [
    {
      "type": "home",
      "number": "212 555-1234"
    },
    {
      "type": "office",
      "number": "646 555-4567"
    }
  ],
  "children": [],
  "spouse": null
}