System design interview question
Design a Real-Time Activity Feed System for a Social Network (Instagram/Twitter Scale)
Design a system that powers real-time activity feeds for 500M monthly active users, where each user follows 0-10K accounts and generates ~50 updates per day (posts, likes, comments). The system must deliver personalized feeds to users with <500ms latency for 99th percentile, handle 2M concurrent active users at peak, and support 100K writes/second. Users expect eventual consistency within 5 seconds for their own actions but can tolerate longer delays for other users' content. The system operates globally across multiple regions with users in Asia, Europe, and North America. Design the entire pipeline from content ingestion through feed ranking, retrieval, and serving, including handling of high-follower accounts (celebrities with 50M+ followers) without creating systemic bottlenecks.
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