How do you hold a large payment for a second pair of eyes without ever letting the unapproved money touch a balance, and how do you stream that decision to the outside world without standing up a broker?
How do you show the total under a parent account when the whole system refuses to store a balance? A recursive query, a trigger that refuses to draw a circle, and a rule about what actually has to sum to zero.
Every time I wanted to change an FX rate I had to edit a file on the server and restart the app. So I moved rates and markup into a live admin API, and then audited it hard enough to find the bug that quietly undid the whole thing.
A double-entry ledger that passed every test still had a bug that turned 100 dollars into 15 dollars, a chain that forked the moment you ran two copies, and no way to ever forget a customer. Here is the teardown, and the fix.
You have tapped Pay twice when the app froze and then held your breath. This week I built the two things that mean you never get charged twice, and that anyone can prove it later.
The running-balance line in your banking app looks like a stored number, but building it taught me more about pagination and window functions than any tutorial ever did
