An Introduction to REST APIs

If you're a developer building for the web, you've undoubtedly encountered the term REST API. REST has become the de facto standard for how different software systems communicate with each other over the internet. But what does it actually mean for an API to be 'RESTful'?

Let's break down the fundamental principles of this important architectural style.

What is an API?

First, let's start with the basics. API stands for Application Programming Interface. It's a set of rules and protocols that allows one software application to interact with another. Think of it as a contract that defines how a client can request information or actions from a server.

What is REST?

REST stands for Representational State Transfer. It's not a protocol or a specific technology, but rather an architectural style for designing networked applications. When an API adheres to the constraints of REST, it is said to be RESTful.

REST was defined by Roy Fielding in his 2000 dissertation, and it's based on the principles that govern the web itself. The goal is to create a system that is simple, scalable, and reliable.

The Core Constraints of REST

For an API to be considered RESTful, it must follow a set of architectural constraints.

1. Client-Server Architecture

The client (who is requesting data) and the server (who is providing it) are separate concerns. The client doesn't need to know anything about the server's implementation, and the server doesn't need to know anything about the client's UI. They communicate through a uniform interface.

2. Statelessness

This is a crucial constraint. Every request from a client to a server must contain all the information the server needs to understand and process the request. The server does not store any information about the client's state between requests. This makes the system highly scalable, as any server can handle any request.

3. Resources

In REST, everything is a resource. A resource is any piece of information that can be named, such as a user, a product, or an order. Each resource is identified by a unique URI (Uniform Resource Identifier).

For example:

• A collection of users: /api/users
• A specific user: /api/users/123

These URIs are the nouns of your API.

4. Uniform Interface

This is the central principle of REST. It simplifies and decouples the architecture. A uniform interface is achieved by using the standard methods of the HTTP protocol to perform actions on resources. These are the verbs of your API.

• GET: Retrieve a resource. (e.g., GET /api/users/123 to get user 123).
• POST: Create a new resource. (e.g., POST /api/users to create a new user).
• PUT: Update an existing resource. (e.g., PUT /api/users/123 to update user 123).
• DELETE: Delete a resource. (e.g., DELETE /api/users/123 to delete user 123).

By using these standard verbs, the API becomes self-describing and predictable.

5. Representation of Resources

When a client interacts with a resource, it does so through a representation of that resource. The server doesn't send its internal database object; it sends a representation of it, most commonly in JSON (JavaScript Object Notation) format.

A GET /api/users/123 request might return a JSON representation like this:

{
  "id": 123,
  "name": "Alice",
  "email": "alice@example.com"
}

Why REST is So Popular

REST became the dominant style for web APIs because it's built on the existing, proven technologies of the web. It's simple, scalable, and easy to understand. By leveraging HTTP and providing a flexible, stateless communication model, REST provides a solid foundation for building the services that power the modern web.