Games in social networks offer interesting scaling challenges, as the user base can grow explosively once they become popular. When wooga started work on the facebook game 'Happy Hospital' in early 2010, we set ourselves an ambitious goal: "Let's try to scale it to a million players per day -- with as few servers as possible." This constraint led us to select Redis as the main backend storage, running on dedicated (but cheap, consumer-grade) hardware. As an in-memory database, Redis offers an order-of-magnitude reduction in query latency, but also introduces new challenges in the area of durability. This case study will recount how our backend architecture has evolved over time, highlight the difficulties we ran into, describe how they shaped our final setup ... and how we came up with a new Redis persistence model (diskstore) on the way. Find out how many machines you *really* need for a high-traffic game backend -- even using a 'slow' language like Ruby.