Design Twitter/X timeline system for 200M daily active users.

Write path: tweet stored in MongoDB; publish TweetCreated event to Kafka Fanout: for users with < 1K followers: push tweetId to each follower's Redis List Celebrities (> 1K followers): no fanout; pull on read and merge with personal timeline Timeline storage: Redis List per user (LPUSH tweetId; LTRIM 0 999 — keep 1000) Read path: LRANGE timeline:userId 0 49 → fetch tweet details from Redis/MongoDB