Fintech Engineering Handbook

A free handbook of the unglamorous fundamentals of building financial software: representing money (precision, rounding, currency, FX), double-entry ledgers, value time versus booking time versus settlement time, immutability and audit trails (and how that squares with GDPR), idempotency and full resumability of money flows, reconciliation, and controls like four-eyes and segregation of duties. Appendices walk end-to-end flows such as a crypto withdrawal. The kind of domain knowledge that usually lives only in the heads of people who’ve been burned.

In the HN discussion, a fintech veteran vouches that it matches five years of hard lessons (answering the now-obligatory “is this AI slop?” question), and the sharpest thread warns against using minor-units integer amounts as an interchange format, however clever it feels.