# Middleware

Middleware implements Psr\Http\Server\MiddlewareInterface. It receives a server request and a request handler, may inspect or decorate the request, and either returns a response or delegates to the next handler.

use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Server\MiddlewareInterface;
use Psr\Http\Server\RequestHandlerInterface;

final class ExampleMiddleware implements MiddlewareInterface
{
    public function process(
        ServerRequestInterface $request,
        RequestHandlerInterface $handler
    ): ResponseInterface {
        $request = $request->withAttribute('processed_by_example', true);
        return $handler->handle($request);
    }
}

# Middleware stack

The framework MiddlewareStack executes registered middleware in order and can limit a middleware entry to a URI prefix. Application bootstrap is responsible for registering middleware and any required services. For example, StartSessionMiddleware starts the session before downstream handlers run; ExceptionMiddleware converts unhandled exceptions into HTTP responses; CorsMiddleware handles cross-origin policy.

Keep middleware focused on cross-cutting HTTP concerns. Put business operations in services and endpoint-specific orchestration in controllers. Middleware that terminates a request should return a response without calling the next handler.