System design interview question
Design a Global Real-Time Feed Ranking System for 2 Billion Users with Sub-100ms Latency
Design a distributed system that ranks and serves personalized content feeds to 2 billion users across 6 continents, with 500 million daily active users generating 50 billion feed impressions per day. The system must deliver ranked feeds in under 100ms at p99, support A/B testing of ranking algorithms, handle regional content moderation policies, minimize operational costs while maintaining 99.99% availability, and gracefully degrade when ranking services fail. Consider the trade-offs between real-time personalization accuracy, infrastructure costs, latency SLAs, and serving stale vs. freshly-ranked content under various failure modes.
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