rack-middleware

Custom Rack middleware extends HTTP request/response cycles by implementing `initialize(app, options)` and `call(env)` methods. Create a class that wraps your app, processes requests before passing to `@app.call(env)`, and modifies responses after. Register it in config.ru using `use MyMiddleware` to integrate into your Rack stack.

Middleware handles cross-cutting concerns like authentication and logging without modifying core logic. Build specialized middleware classes for token validation and request logging, order them in config.ru to ensure security checks run first, and configure skip_paths for routes that bypass certain middleware.

Middleware ordering matters: place security middleware like SSL and authentication near the top, logging and caching in the middle, and compression last. The stack executes top-to-bottom on requests and bottom-to-top on responses. Use conditional middleware to skip processing for specific paths or request types.

Yes, middleware is designed for cross-cutting concerns including caching, compression, and session management. Each concern becomes a separate middleware class that intercepts the request/response cycle. Combine multiple middleware in config.ru to build a complete HTTP processing pipeline for Sinatra and Rails.

Rack ecosystem middleware like rack-attack, rack-cors, rack-ssl, and rack-protection provide rate limiting, CORS handling, SSL enforcement, and security headers. Each follows the standard Rack interface and integrates by adding `use` directives in config.ru, composing them into your application's middleware stack.