System design interview question
Design a Real-time Feed Ranking System for a Social Network (like Twitter/X Home Timeline)
Design a distributed system that powers the home feed ranking for a social network with 500M monthly active users and 50M daily active users. The system must rank and serve personalized feeds with sub-500ms latency at the p99, handling 200K feed requests per second globally across multiple regions. Users follow up to 10K accounts, and the platform generates 1M new posts per minute. The feed must balance relevance (engagement signals, recency, social graph), freshness (updates within 30 seconds), and consistency (same user seeing different orders across devices is tolerable, but egregious inconsistencies harm UX). You must support real-time notifications when a followed account posts, handle feed ranking model updates without downtime, and gracefully degrade when ranking services are unavailable.
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