Leaderboard Service

Design a scalable Leaderboard Service for Pinterest that can handle hundreds of millions of users and billions of scores. The service must support real-time score updates, per-category leaderboards (e.g., global, sports, tech), and fast queries for top-K users and a user’s own rank. Scores are computed from engagement metrics (likes, comments, shares) with a time-decay factor so fresh content rises and old content falls. The system should be globally available with sub-100 ms latency for reads and 5:1 read:write ratio. You must choose storage and caching strategies that scale horizontally, prevent hotspots, and tolerate failures.