AppliedAIPrep logoAppliedAI/Prep
← System Design for AI in Production / 100
hardStripeAmazonUber

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.

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.

Unlock the other 750 answers · ₹2,000 / $25Your progress and mastery stay saved · 6 months · one payment · no auto-renew
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.
DISCUSSION · 0

No comments yet — be the first to share your approach.