Groupon’s entire U.S. web frontend has been a single Rails codebase from its inception. The frontend codebase quickly grew large, which made it difficult to maintain and challenging to ship new features. As a solution to this gigantic monolith, we decided to re-architect the frontend by splitting it into small, independent and more manageable pieces. At the center of this project, we rebuilt each major section of the website as an independent Node.js application. We also rebuilt the infrastructure to make all the independent apps work together. Interaction Tier (I-Tier) was the result.