How often do you have to leave your comfort zone? How often does it happen that you need to change the fundamentals of your application? Well… it has happened to my team recently. We had to abandon our way of thinking about Ruby on Rails applications. Single-threaded servers were not a choice anymore, neither was JRuby nor Event Machine. And we had to handle up to 200k requests per minute. This talk is a case study about how, step by step, we got to multi threaded RoR application with MRI 1.9, MySQL, DataMapper. I want to present how we came to the solution, what did we have to do to make our application thread safe, how did we manage to use threads in MRI and what we learned while working on the big feature, that made us change the fundamentals of our application. Although the application in this case was built with Ruby on Rails, the talk won't be specific to this framework. I want to talk about threading and parallelism in Ruby and its open source libraries, as well as the process of making deep changes in the application that handles pretty big traffic.