System designMediumsd-g262

Subject Thumbnail generationLevel Mid–Senior~35 minCommon in Distributed systems interviewsIndustries Technology, Software development

Question

Design the video-scrubbing-preview (storyboard / sprite-sheet) and poster-frame system for a VOD platform. When a user hovers/drags the seek bar, the player must instantly show a thumbnail of the frame at that position across the whole timeline, for videos from 30s clips to 6-hour streams, on bandwidth-constrained clients. The platform also wants a good auto-selected poster frame per video. Design how previews are generated, packaged, and delivered, and how you keep client bandwidth and request count low.

What a strong answer looks like

Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts — data model, bottlenecks, consistency, failure modes — and name the trade-offs you are making.

Learn the concepts

Narrate your design

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Run or narrate your approach, then ask the coach.