It was built as a monolith using Ruby on Rails. At the time, our chat system ran on a set of 8 machines, with chat channels randomly distributed across those machines. Having chat working well for large streams was a priority for Twitch. Curiously enough, unlike the backend, the frontend became a successful monolith (see Guiding A Monolith With A Gentle Touch). Breaking the Monolith at Twitch: Part 2This is just part of the story.