System Design

Video Streaming

Design the video playback and delivery system for a streaming service like Netflix or YouTube.

Functional requirements

• Creators/content team can upload source video files (up to 4K), which the system processes and makes available for playback.
• Viewers can stream videos on demand across heterogeneous devices (mobile, web, smart TV) and network conditions, with playback quality adapting automatically.
• Viewers can seek to arbitrary positions in a video and resume playback where they left off across devices.
• The system tracks playback position/heartbeats for resume and basic view accounting.
• Newly uploaded videos become playable within 30 minutes of upload completing (popular-title priority allowed).

Non-functional requirements

• Availability over consistency for the playback path: a viewer should always be able to start a stream, even if metadata (view counts, resume position) is slightly stale.
• Time-to-first-frame under 2 seconds at p95; rebuffer ratio under 0.5% of watch time.
• Source masters are durable (11 nines target); a lost master is unrecoverable, but derived renditions may be regenerated.
• Content must be protected: streams served only to entitled users, with signed/expiring access to media segments; DRM assumed available as a component.
• Transcode pipeline must be reliable: a failed job never silently drops a video; retries and idempotency required.

Scaling & constraints

• 200M MAU, 60M DAU; average viewer watches 90 minutes/day.
• Peak concurrent streams: ~15M globally, heavily skewed to evening hours per region.
• Catalog: 500K titles today, 5K new uploads/day; average source file 20 GB (raw master).
• Popularity is extremely skewed: top 1% of titles account for ~70% of watch hours; long tail is rarely watched but must remain playable.
• Viewers are globally distributed across 6 continents; assume commercial CDN and/or ability to deploy edge caches.
• Read:write is extreme: one upload can serve millions of streams.

Out of scope

• Recommendation, search, and browse/home-page systems.
• Live streaming and real-time broadcast.
• Comments, likes, social features, and creator monetization/analytics.
• Payment, subscription, and account management (assume an auth/entitlement service exists).