Delayed message delivery
Design a delayed/scheduled message service that lets producers enqueue a message to be delivered at an arbitrary future timestamp (from seconds to 1 year out). Scale: 500M scheduled messages in flight, 50K new schedules/sec, delivery accuracy within ±1s of the target time, at-least-once delivery to downstream consumers. Schedules can be cancelled or rescheduled before they fire.
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.