WebSocket price-tick service
Design the WebSocket layer for a live stock-and-crypto price service that pushes tick updates to ~2 million concurrent browser/mobile clients, where each client subscribes to a personal watchlist of symbols and must see updates within ~250ms. Cover how you scale connections across many gateway nodes, how a client's subscriptions get the right ticks, what happens on a flaky-network reconnect, and how you avoid hammering slow clients.
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.