Idempotency keys for payment APIs in ASP.NET Core
An idempotency key turns "the client retried" from a duplicate payment into a harmless replay. Here is a small, runnable ASP.NET Core implementation and the trade-offs behind it.
#payments
Automated nightly reconciliation: finding mismatches before users do
Every night, compare what your ledger says happened with what the payment gateway says happened. A small, testable reconciler plus a BackgroundService turns days of manual checks into one log line.