What is middleware used for in applications?

Middleware in web applications is an intermediary software that processes HTTP requests and responses. It sits between the web server and the main application logic.

Main tasks of middleware:

• Pre-processing the request:
  • User authentication and authorization.
  • Parsing incoming request data (JSON, XML, etc.).
  • Validating request data.
  • Logging requests.
  • Adding headers to the request.
• Post-processing the response:
  • Modifying the response before sending it to the client (e.g., adding headers).
  • Compressing the response.
  • Error handling (exceptions).
• Flow control: Middleware can interrupt request processing and return a response early (e.g., when access rights are missing).

Advantages of using middleware:

• Modularity: Separating tasks into individual, reusable components.
• Flexibility: Easily adding or removing functionality without changing the main application logic.
• Code readability: Improving application structure by highlighting cross-cutting concerns.
• Testability: Individual middleware are easier to test in isolation.

In PHP frameworks (such as Laravel, Symfony, Slim), middleware is often implemented as classes that implement a specific interface.

<?php

// Example middleware structure (interface)
interface RequestHandlerInterface {
    public function handle(RequestInterface $request): ResponseInterface;
}

// Example middleware implementation (authentication)
class AuthenticationMiddleware implements RequestHandlerInterface {
    public function handle(RequestInterface $request): ResponseInterface {
        // Authentication check
        if (!isAuthenticated()) {
            // If the user is not authenticated, interrupt and return an error
            return new Response(401, [], 'Unauthorized');
        }

        // If everything is fine, pass the request to the next handler (or main application logic)
        return $this->nextHandler->handle($request);
    }
}