High-Level Design

Design the home feed for a social product. When a user opens the app, they see a ranked, near-real-time stream of posts from the accounts they follow. The scope here is producing posts, assembling each reader's timeline, following and unfollowing, and recording engagement. Profile timelines, notifications, search, ads, comment threads, and stories are separate problems and out of scope.

Two facts make this hard. Producers vary enormously: an account may have three followers or a hundred million. Consumers vary too: a reader may follow ten accounts or ten thousand. The feed must feel instant for all of them, and it must stay instant when the producers include celebrities and the consumers include power users.