Upload integrity and malware scanning
Design the integrity-and-safety gate for a file-upload service where every uploaded file (docs, images, archives, executables) must be checksum-verified end-to-end and scanned for malware BEFORE it becomes downloadable to other users, at 30K uploads/sec. Scanning is slow and bursty (some files take seconds, archives must be unpacked and scanned recursively). The uploader should get a fast 'received' ack, but the file must not be shareable until it clears, and a verdict can change later when virus signatures update.
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.
Clarify4:30 left
Estimate4:30 planned
Design13:30 planned
Deep dive10:30 planned
Failure7:00 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.