How the interviewer opens it
“Design a video platform like YouTube: creators upload videos, viewers watch them on any device.”
The prompt is vague on purpose. The first few minutes are yours: narrow it to the core features and agree on the numbers before you draw anything.
What to clarify first
- scale
- video size
- time to publish
- devices
- global
The answers stay with the interviewer. In a SysRound round, Alex answers them as an interviewer would.
Scope of a strong answer
Core requirements
- Upload a video
- Process it for many devices/bandwidths
- Watch with smooth playback
Usually out of scope
- Recommendations
- Comments
- Monetization
- Live streaming
What interviewers probe
- Processing pipeline
- Huge files, expensive CPU work, partial failures.
- Delivery and cost
- Egress dominates cost; long-tail content doesn't cache well.
Common mistakes
- Routing upload bytes through API servers
- No adaptive bitrate
- Ignoring CDN cost
- Exact synchronous view counts
The bar by level
- Senior (L5/E5)
- Resumable uploads, segment-level DAG with failure handling, ABR, counters via aggregation, cost awareness.
- Staff (L6+/E6+)
- Cost model across tiers, long-tail strategy, global capacity planning, takedown/compliance propagation.