Welcome to the Fintech Engineering Handbook. This resource aims to describe the most important patterns used in software engineering, where money is the primary focus of the system. It can be read in full to get a comprehensive understanding or in parts when dealing with a particular problem. For whom? People joining fintech. To get familiar with the domain and the patterns that make money systems trustworthy. People already in fintech. As a reference to reach for when facing a particular problem, and a shared vocabulary to point colleagues at. People outside fintech. To understand how building for money differs from what they’re used to, and why. It’s meant as a living document and contributions are welcomed. Principles Everything you will read below is a way to adhere to the three principles: No invented data. Money can’t be created out of nowhere, so we can’t tolerate duplicates or arbitrary balance updates. We enforce this with idempotency, deduplication and reconciliation. No lost data.…