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If you want to quickly try out some Rails feature or code in the browser without spinning up a whole new controller and a view, simply map the incoming request to a lambda Rack endpoint, i.e. a lambda that returns the status, headers, and response body.

Concerns are an important concept in Rails that can be confusing to understand for those new to Rails as well as seasoned practitioners. This post explains what concerns are, how they work, and how & when you should use them to simplify your code, with practical, real-world examples.