Tens of millions of people can download and play the same game thanks to mobile app distribution platforms. Highly scalable and reliable API backends are critical to offering a good game experience. Notable characteristics here is not only the magnitude of the traffic but also the ratio of write traffic. How do you serve millions of database transactions per minute with Rails? The keyword to solve this issue is "multiple databases," especially sharding. From system architecture to coding guidelines, we'd like to share the practical techniques derived from our long-term experience.