YouTube — Mock Interview

Hard

A UGC video platform: ingest and transcode uploads at scale, then stream them worldwide from the CDN edge with adaptive bitrate.

Interviewer

You're designing YouTube for a live system-design interview. Creators upload raw video; the platform processes it and streams it back to viewers on every device and network. The defining challenge lives between upload and playback: turning ~500 hours of new video per minute into a full ladder of renditions, then delivering billions of views a day at hundreds of Tbps.

Scale assumptions:

~1B DAU, each watching ~5 videos/day → ~5B views/day (~50k/s avg, ~250k/s peak).
Ingest ~500 hours of video/minute → ~5M uploads/day; ~10 PB of new media/day.
Peak CDN egress in the hundreds of Tbps; media accumulates to exabytes/year.
Targets: playback start < ~2 s p99, 99.99%+ watch availability, 11-nines media durability.

I'll walk you through requirements, estimation, architecture, the transcoding/CDN deep dive, and trade-offs. Think out loud and state your assumptions.

The interviewer grades each stage against a hidden rubric and asks a probing follow-up, mirroring a real system design round. Sessions are capped at 30 turns.