Handling Personal Data the Right Way in .NET
A few years ago I inherited a payments service that stored full customer date-of-birth, national ID, and unmasked PAN fragments in the same table it used for...
9 min428Read →
Tagged #fintech
36 articles tagged #fintech.
Why Your Retries Need Idempotency Keys
A few years ago I got paged at 2am because a merchant had been charged three times for the same settlement batch. Not a huge sum, about 4,200 euros total, but...
9 min645Read →
Building a Feature Store for Fraud Models
The first fraud model I ever put into production was wrong about a third of the time, and it took us two weeks to figure out why. The model was fine. The...
10 min962Read →
The Anatomy of a Payment Authorization
A customer taps their card at a coffee shop, and roughly 300 milliseconds later a little screen says "Approved." In that blink, a message crossed four or five...
9 min579Read →
Designing for Chargebacks and Disputes
The first serious chargeback I ever dealt with cost us about £40,000 and, worse, taught the whole company that our dispute process was held together with a...
11 min339Read →
How to Test Code That Talks to a Bank
The first time one of my payment services took real money out of a real account during a test run, I was standing at a whiteboard explaining to a new hire why...
11 min857Read →
Building an Idempotent Refund Flow
The first time a refund double-fired in production, it cost us about 4,200 euros before anyone noticed. A merchant's support agent clicked the refund button,...
10 min814Read →
The Saga Pattern for Distributed Transactions
A few years ago I watched a payout run leave forty-two merchants with money debited from our settlement account but never credited to their bank. The culprit...
9 min815Read →
Handling Partial Failures in Distributed Systems
A payment either happens or it doesn't. That's the story we tell customers, and it's the story regulators want to hear. But underneath, a single card...
10 min342Read →
Why We Chose PostgreSQL Over a NoSQL Store
The meeting that decided this ran about forty minutes. We had a greenfield ledger service to build, a payments platform that would eventually touch a few...
10 min585Read →
Designing a Ledger-Backed Wallet System
The first wallet system I inherited stored balances as a single column on the user row. A background job updated it. When two payments landed in the same 200...
9 min771Read →
Database Transactions and Isolation Levels, Explained
A few years ago we shipped a payment reconciliation job that, under load, occasionally credited a merchant twice for the same settlement. Not often. Maybe one...
10 min847Read →