Design real-time live comments
Company: Meta
Role: Software Engineer
Category: System Design
Difficulty: medium
Interview Round: Onsite
Design a real-time comment system for live video (e.g., Facebook Live comments).
Requirements:
- Viewers post comments; all viewers see new comments quickly.
- High fanout (one live video can have millions of viewers).
- Support moderation (delete/flag), rate limiting, and replay of recent comments.
- Comments should be roughly ordered by time.
Out of scope: full creator tooling, advanced ML moderation.
Overview: This question evaluates the candidate's ability to design scalable, low‑latency real‑time messaging systems, focusing on high‑fanout delivery, approximate time ordering, moderation and rate‑limiting mechanisms, and replay of recent events.