Real-Time Leaderboard

Leaderboards appear everywhere — gaming platforms, sales dashboards, competitive coding sites — and they present a deceptively complex scaling challenge. A naive approach using SQL ORDER BY collapses under millions of concurrent score updates. In this challenge, you will design a real-time leaderboard that supports instant rank queries, score updates, and paginated browsing for millions of entries. You will implement a sorted set architecture using Amazon ElastiCache for Redis, which provides O(log N) rank lookups via ZRANGEBYSCORE and ZRANK commands. The design includes a write-behind pattern where score updates hit Redis first for speed, then asynchronously persist to DynamoDB for durability. You will handle edge cases like tie-breaking strategies, regional leaderboards using partition keys, and time-windowed leaderboards (daily, weekly, all-time) using separate sorted sets with Lambda-based rotation. The architecture also covers fan-out for real-time WebSocket notifications when players enter the top 100, using API Gateway WebSocket APIs and SNS for pub/sub distribution. This challenge teaches sorted data structures at scale, write-behind caching, and event-driven notification patterns.