System designMediumsd-g383

Subject IdempotencyLevel Mid–Senior~35 minCommon in Networking & APIs interviewsIndustries Technology, Software development

Question

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.

Learn the concepts

Narrate your design

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Run or narrate your approach, then ask the coach.