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Money Never Lies
Designing Payment Systems That Survive Production
This book is 100% completeLast updated on 2026-10-13
Retries, timeouts, webhooks and month-end reconciliation: the parts tutorials skip. A practical book for backend engineers who build or own payment systems, from cards to real-time rails like Pix, UPI and QRIS.
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About
About the Book
A customer is charged twice. A webhook arrives before your API call returns. Finance finds money nobody can explain. Every service did its job, and the money is still wrong.
Money Never Lies is a practical book for backend engineers who build or own payment systems. It follows one payment from the moment it is created to the day it matches the bank statement, and shows the design that keeps it correct at every step.
What you will learn
- Store money without losing a single unit, and round in the right place
- Model payments as state machines that never move backwards
- Make every retry safe with idempotency keys, on your API and towards your providers
- Trust webhooks only after you verify, store and process them idempotently
- Treat timeouts as UNKNOWN, not failed, and resolve them without double charges
- Derive balances from a double-entry ledger instead of a column you update
- Reconcile your records against provider reports and the bank statement, every day
- Pay out sellers safely, handle refunds and chargebacks, and stop common fraud
- Answer "design a payment system" in a 45-minute interview
Beyond cards. The book also covers real-time push payments (virtual accounts, QR codes, e-wallets, Pix, UPI, QRIS, FedNow, SEPA Instant), where customers pay late, pay twice and pay the wrong amount.
What is inside: 17 chapters in four parts, 7 appendices, 19 figures, an Interview Corner, production checklist and TL;DR in every chapter, and companion Go code with tests. PDF (231 pages) and EPUB.
Who it is for: backend engineers new to payments or fintech, engineers preparing for system design interviews, and founders or tech leads adding payments to a product.
Who it is not for: beginners learning to program, accountants looking for accounting standards, compliance teams looking for country-by-country regulation.
All incidents in the book are composites. Not legal or financial advice.
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