Design a payment ledger that records money movement with exactly-once semantics and no lost cents.

In payments, a duplicate charge or a lost credit is not a bug, it is a financial incident. The design hinges on idempotency keys, double-entry bookkeeping, and an append-only ledger that can be audited and reconciled. Here is how money systems stay correct.

Updated Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.

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No lesson covers this question directly yet. These teach the surrounding topic from the beginning.

Applied AI Engineering·The interviewPremium14mDriving the design conversationA design round is a conversation you are expected to lead, not a question you answer. This lesson is the shape that works, the four moments that decide the outcome, and the two classic ways strong candidates lose one.Applied AI Engineering·The interviewPremium12mTurning this course into a study planA concrete four-week plan mapping the seven modules onto the question bank, plus what to do differently if your interview is next week rather than next month.

UP NEXT ON YOUR JOURNEY

Next in this trackDesign a distributed cache like Redis or Memcached that serves millions of reads per second.Next in this trackDesign a time-series database that ingests millions of metrics per second and answers range queries fast.Next in this trackDesign a data lakehouse pipeline that ingests raw events and serves both analytics and ML features.

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