Link unfurling service
Design a link-unfurling / URL-preview service for a chat app (Slack-like) that, when a user pastes a URL, fetches the page and shows a rich preview (title, description, image). Scale: ~30K URLs/sec at peak, many duplicate URLs (news links shared widely), target preview render <300ms when cached. Requirements: don't fetch the same URL repeatedly, handle slow/huge/malicious target sites without hanging or being abused as a proxy, and degrade gracefully when a site has no metadata.
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.
Clarify4:00 left
Estimate4:00 planned
Design11:30 planned
Deep dive9:30 planned
Failure6:00 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.