Introduction à Ring et aux bases de HTTP

Welcome to web development with Clojure! In this lesson, we'll dive into the basics of how web applications work and introduce Ring, Clojure's standard interface for building web apps.

Understanding these fundamentals is key to creating powerful web services!

The internet relies on a protocol called HTTP (Hypertext Transfer Protocol). It's the language web browsers and servers use to talk to each other.

Think of it as a conversation: your browser sends a request, and the server sends back a response.

An HTTP request is what your browser sends to a server. It typically includes:

• Method: The action you want to perform (e.g., GET, POST).
• URI: The specific resource you're asking for (e.g., /users/123).
• Headers: Extra info about the request (e.g., browser type, accepted formats).
• Body: Data being sent to the server (e.g., form submissions, JSON data).

Two very common HTTP methods are:

• GET: Used to retrieve data from the server. Like asking for a webpage or an image. GET requests typically don't have a body.
• POST: Used to send data to the server to create or update a resource. Like submitting a login form or uploading a file. POST requests usually have a body.

After receiving a request, the server processes it and sends back an HTTP response. This response contains:

• Status Code: A number indicating the outcome (e.g., 200 for success, 404 for not found).
• Headers: Metadata about the response (e.g., content type, caching instructions).
• Body: The actual content being sent back (e.g., HTML, JSON, an image).

Status codes tell the browser how the request went. Some common ones:

• 200 OK: The request was successful!
• 404 Not Found: The requested resource doesn't exist.
• 500 Internal Server Error: Something went wrong on the server's side.
• 302 Found: The resource is temporarily at a different location (redirection).

Building web apps in Clojure often starts with Ring. Ring isn't a web framework itself, but a simple specification.

It defines a minimal interface for HTTP handlers, making it easy to swap out web servers and build modular applications.

Ring takes an incoming HTTP request and translates it into a standard Clojure map. This request map contains all the details:

{:request-method :get
 :uri "/users/1"
 :headers {"host" "localhost:3000"}
 :body ...
 ; and many more keys
}

Your web application then works with this map.

Similarly, when your application wants to send a response, it returns a response map that Ring converts back into an HTTP response. A typical response map looks like this:

{:status 200
 :headers {"Content-Type" "text/plain"}
 :body "Hello, World!"
}

These three keys are the most important.

A Ring handler is simply a Clojure function that takes a request map as its argument and returns a response map.

Try running this example to see how a handler processes a mock request and produces a response:

Let's check your understanding of Ring handlers.

Great job! You've learned the fundamental building blocks of web development in Clojure:

• HTTP: The communication protocol for the web (requests & responses).
• HTTP Requests: Method, URI, Headers, Body.
• HTTP Responses: Status, Headers, Body.
• Ring: Clojure's simple, powerful spec for web handlers.
• Ring Handlers: Functions that transform a request map into a response map.

Next, we'll explore how to handle different routes with Compojure!