How Twitter's Architecture Handles 500 Million Tweets and Celebrity Fan-Out

A system design breakdown of Twitter-scale infrastructure explains how the platform manages 500 million daily tweets using Cassandra for write-heavy, time-series storage partitioned by user. Timelines are served through a hybrid approach: pre-computed tweet ID lists cached in Redis for regular users, combined with real-time fetches for high-follower celebrity accounts at read time. This hybrid fan-out model solves the celebrity problem, where a pure write fan-out to 100 million followers simultaneously is computationally impossible. Media uploads are handled via a dedicated service that stores originals in S3, asynchronously generates multiple resolutions, and serves all assets through CloudFront CDN. The overall design prioritises high availability and low latency, accepting eventual consistency of one to two seconds on timelines as an acceptable trade-off.