Design real-time live comments
Quick 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.
Meta
Feb 25, 2026, 12:00 AM
Software Engineer
Onsite
System Design
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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.
Solution
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