When your Rails app begins serving public traffic, your users will make it behave in mysterious ways and find code paths you never knew existed. Understanding, measuring, and troubleshooting its behavior in production is a tough but crucial part of running a successful Rails app. In this talk, you’ll learn how to instrument, debug, and profile your app, using the capabilities of the Rails framework and the Ruby VM. You'll also study techniques for safely instrumenting a live running system, keeping latency to a minimum and avoiding side effects.