fintech

Fintech Engineering Handbook

3 months ago

Introduces the Fintech Engineering Handbook, a guide on patterns for building money-handling software.
Targets fintech newcomers, practitioners, and outsiders to explain trustworthiness in money systems.
Core principles: No invented data, no lost data, no trust.
Money representation: avoid floating-point; use arbitrary precision, minor-units, or rational numbers.
Rounding strategies: explicit, business-decided, minimal; track residuals to avoid losing or inventing money.
Currency handling: pair amount with currency; validate codes; treat crypto distinctly; pack in a Money type.
FX rates: directional, time-sensitive; distinguish transactional vs reference rates; track source.
Ledger uses double-entry bookkeeping: immutable entries, balances derived, accounts typed (assets, liabilities, equity).
Timestamps: value time, booking time, settlement time; record all to preserve information.
Audit trails: capture what, when, who, why; event sourcing as a systematic approach; immutability required.
Reversals/corrections: post compensating entries; link to originals; consider reporting periods.
Invariants enforced by construction, runtime checks, and post-factum verification.
Funds reservation prevents double-spending; distinguish total vs available balance; requires strong consistency.
Overdrafts: intentional (credit product) vs unintentional; handle explicitly, don't clamp to zero.
Idempotency: use explicit keys; handle errors; validate payloads; ensure atomicity; test retries.
Full resumability: persist flow state; resume stalled flows; each step idempotent; roll forward or compensate.
External APIs: don't trust schema; expect failures; store requests/responses; consider provider redundancy.
Webhooks: don't assume ordering, validity, or single delivery; acknowledge fast; verify caller; persist raw payload.
Reliable notifications: use outbox, CDC, listen-to-yourself, or event sourcing; ensure at-least-once delivery.
Reconciliation: compare systems; handle missing/different data; match heuristically; fix discrepancies properly.
Controls: segregation of duties, four-eyes; record approvals; enforce least privilege; audit access changes.
Change trail: use source control; enforce reviews and pipelines; trace deployments.
Testing: property-based, invariant checks, generative idempotency, crash injection, round-trip, golden, backward-compatibility, production testing.
Appendix A: Glossary of fintech terms (accounting, money/FX, transactions, payments, trading, custody, compliance).
Appendix B: Example flows: crypto withdrawal, card deposit, in-app conversion with cashback.
Appendix C: Author background, writing process, and intent as a living document.

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Fintech Engineering Handbook