IoT device pub-sub and commands
Design the cloud ingestion and command pub-sub layer for an IoT fleet: 20M devices on constrained networks (intermittent cellular, low bandwidth, sleepy devices) publishing telemetry and subscribing to commands. Requirements: efficient on the wire, support at-most-once/at-least-once delivery levels, deliver commands to devices that were offline when the command was issued, and authenticate per-device. Bursts when a firmware push wakes the whole fleet.
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.