System designHardsd-g006

Subject Pub subLevel Senior–Staff~45 minCommon in Distributed systems interviewsIndustries Technology, Telecom

Question

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.

Learn the concepts

Narrate your design

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Run or narrate your approach, then ask the coach.