Build a Social Graph Service at 500M Users Free
Design a graph service that answers 'does user A follow user B?' in under 1ms, supports mutual friend queries, and stays consistent as billions of follow/unfollow events happen every day.
Build Twitter's At-Mention Fan-Out for Celebrities Premium
Design a fan-out system that notifies the followers of a celebrity with 100 million followers when they are mentioned, without triggering a write storm that takes down the database.
Build Uber's Real-Time Driver-Rider Matching System Free
Design a matching system that pairs riders with the nearest available driver in dense urban areas within 2 seconds, handles thousands of concurrent match requests, and degrades gracefully during demand spikes.
Build Uber's Surge Pricing Engine Free
Design a dynamic pricing system that detects supply-demand imbalance in each geographic zone, computes surge multipliers in under a second, and communicates price changes to riders and drivers without race conditions.
Build a Wallet Ledger System at Scale Premium
Design a digital wallet system that records every credit and debit as an immutable ledger entry, enforces double-entry bookkeeping, and provides accurate balance reads under thousands of concurrent transactions per second.
Build a WebSocket-Based Presence System Premium
Design an online presence and typing indicator system that tracks 10 million concurrent users, propagates status changes to relevant subscribers in under 200ms, and handles connection drops gracefully.