System Design Article

Design TikTok (Short-Form Video)

Difficulty: Hard

Design TikTok with 1.5B monthly active users, 100M short videos uploaded daily, and the For You Page that decides which video plays next for every viewer in under 100 ms. Unlike Instagram and Twitter, TikTok has no follower-driven feed - the For You Page is pure ML recommendation from a global pool. The interview centerpiece is the recommendation system architecture: candidate retrieval, two-tower models, online ranking with engagement signals, and how to keep video pre-loaded so the next swipe is instant. We also cover content moderation at scale, edge caching for the long-form-of-short-form access pattern, and why TikTok's product choice eliminated the celebrity fan-out problem entirely.