Image thumbnail generation
Design a service where users upload a profile photo and the system produces a few resized thumbnails (e.g. 50px, 150px, 300px) for use across the app. Uploads are modest — a few thousand a day — but resizing shouldn't make the upload request slow, and the thumbnails must be available shortly after upload. Cover the components, how resizing happens, and how the app knows when thumbnails are ready.
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.
Clarify3:30 left
Estimate3:30 planned
Design10:00 planned
Deep dive8:00 planned
Failure5:00 planned
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.
Run or narrate your approach, then ask the coach.