Ruby

Charging a card once in Rails: the guard I shipped, and the gaps I didn't close

A payments flow is 'at least once' at every layer — retries, double-clicks, replayed webhooks. Here's the guard that stops the common cases, an honest accounting of the ones it doesn't, and how I'd close them.

Building an invoice tax engine in Rails that accountants trust

Tax-inclusive and tax-exclusive pricing round differently, and one cent of drift makes an invoice look broken. How I built a tax engine with the Strategy pattern, BigDecimal, and spreadsheet-verified golden-master tests.