Design a News Feed System (Twitter / Facebook / LinkedIn)

A News Feed system aggregates, ranks, and serves a personalized stream of content (posts, photos, status updates) generated by the accounts, friends, and entities a user follows.

Functional Requirements

1. Feed Publishing: A user can publish a text/image post.
2. Feed Generation & Retrieval: A user can fetch a paginated list of the most recent and relevant posts from all accounts they follow.
3. Follow Graph: Users can follow and unfollow other accounts.
4. Ranking & Chronology: Support chronological ordering (Reverse-Chronological) and ML-ranked scoring (Engagement/Relevance).

Non-Functional Requirements

• Ultra-Fast Read Latency: Feed retrieval must return the first page of 20 posts in < 20 ms (p99).
• Scale: Support 500 million daily active users (DAU) generating 100 million posts/day and 10 billion feed page views/day.
• Handling Celebrity Hotspots (The "Justin Bieber / Elon Musk Problem"): An account with 100M+ followers must be able to post without stalling the platform or creating multi-minute write latency lag.