Exactly-once ledger posting consumer
Design a ledger-posting consumer that reads money-movement commands from a partitioned queue (at-least-once delivery, possible reordering across partitions, occasional duplicates) and applies each as a balance change exactly once. ~15,000 commands/sec. A duplicate must not double-post; an out-of-order pair (capture before its authorize) must not corrupt the balance. How do you make application exactly-once and order-tolerant?
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.