The “Black Box” Between the Browser and Your Code When you are learning Rails, you usually think the flow works like this:    User clicks a link.   Magic happens.   Your PostsController runs.

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Rails middleware is the hidden layer between incoming HTTP requests and your controllers, forming an 'onion' stack where each layer can inspect and modify both requests and responses. Running `bin/rails middleware` reveals 20-30 built-in layers handling static files, CORS, cookies, and authentication. A custom middleware follows the Rack protocol: a Ruby class with an `initialize(app)` and a `call(env)` method that wraps the next layer. Practical benefits include performance debugging outside controllers, early request rejection (e.g., IP blocking) before the full Rails stack loads, and centralized API token validation.

The ‘Middle’ of Nowhere: Demystifying Middleware in Rails

The “Black Box” Between the Browser and Your Code

How to Build Your Own (The Rack Protocol)