Instant payments changed the rules. SEPA Instant (SCT Inst) doesn't give a bank 24 hours to settle a transfer — it gives 10 seconds, end to end, 24/7/365. That single regulatory constraint cascades into almost every architectural decision covered in this series: how services communicate, how fraud gets caught without adding latency, how a database is sharded, and how a team responds when something breaks at 3 AM.
This series is a field guide to building that kind of system, written as I build one. It's organized as twelve episodes, each tackling one layer of the problem — starting with the regulatory and business constraints, moving through architecture, performance, fraud detection, resilience, compliance, infrastructure and security, and closing with the human side of shipping and sustaining a system like this.
New episodes are added as they're written. Jump into whichever one is most relevant to what you're working on, or start from Episode 1 if you want the full arc.