Hierarchical IoT pub/sub broker
Design a hierarchical pub/sub broker for an IoT fleet: ~10 million devices publishing to topics like `region/building/floor/device/metric`, and operator dashboards subscribing with wildcards (`region/+/+/+/temperature` or `region/#`). It must support millions of concurrent subscriptions, route a published message to all matching subscribers within ~200ms, and survive a broker node loss without losing in-flight messages for QoS-1 subscribers. Cover topic matching, subscription state, and scale-out.
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.
Clarify5:00 left
Estimate5:00 planned
Design15:00 planned
Deep dive12:00 planned
Failure8:00 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.