System design interview question
"Design live commenting for a livestream. While a video is streaming, viewers post short comments and everyone watching sees them appear in near real time. A popular stream can have 2 million concurrent viewers and 10,000 comments per second."
connection-oriented fan-out at burst scale. Unlike a feed, there is no "pull later" — the data is worthless 10 seconds after it is written. The candidate must reason about persistent connections, per-room broadcast, and — the crux — that at 10,000 comments/sec no human can read them, so sampling is a feature, not a compromise.
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