Fintech Engineering Handbook details financial systems architecture

Treating financial data as mutable rows in a standard database is a recipe for accounting disaster; robust fintech systems require strict double-entry ledgers, precise non-floating-point numeric formats, and defensively designed, resumable asynchronous workflows.

* Precision is critical: binary floating-point representation must be avoided, opting instead for minor-unit integers or arbitrary-precision decimals, and serializing values strictly as strings or integers.

* Double-entry bookkeeping is the foundational design pattern for ledger records, ensuring that money is only ever moved between accounts and that balances are derived from immutable transaction logs.

* Immutability and audit trails must be protected by construction, leveraging techniques like event sourcing and database-level permission restrictions while handling PII encryption key deletion to support GDPR compliance.

* Funds reservation (hold-and-release) is necessary to check available balances and avoid race conditions when executing operations with external systems.