Archie — the systems that move moneyA software architect writing about fintech systems: payments, ledgers, real-time rails, and the trade-offs behind them.https://hiarchie.com/Why I write about money movementhttps://hiarchie.com/journal/2026-07-04-why-money-movement/https://hiarchie.com/journal/2026-07-04-why-money-movement/Most software can afford to lose a request. Money can't. That single constraint is why these systems are worth writing about.Sat, 04 Jul 2026 00:00:00 GMTJournalfintechsystemsmetaDouble-entry is 500 years old and still the right data modelhttps://hiarchie.com/thoughts/double-entry-ledger/https://hiarchie.com/thoughts/double-entry-ledger/The obvious way to store balances — a column you update — is the one that will eventually betray you. Here's why the accountants had it right.Thu, 02 Jul 2026 00:00:00 GMTThoughtsledgersdata-modelingcorrectnessThe most important header in paymentshttps://hiarchie.com/journal/2026-06-30-idempotency-key/https://hiarchie.com/journal/2026-06-30-idempotency-key/An idempotency key turns a terrifying question — did that charge go through? — into a safe retry.Tue, 30 Jun 2026 00:00:00 GMTJournalpaymentsidempotencyapisExactly-once is a lie you tell yourselfhttps://hiarchie.com/thoughts/exactly-once-is-a-lie/https://hiarchie.com/thoughts/exactly-once-is-a-lie/You can't deliver a message exactly once across a network. What you can do is process at-least-once and make duplicates harmless — which is the real job.Thu, 25 Jun 2026 00:00:00 GMTThoughtsdistributed-systemspaymentsidempotencyISO 20022 was never really about the schemahttps://hiarchie.com/trends/iso-20022-migration/https://hiarchie.com/trends/iso-20022-migration/The big payments-messaging migration is mostly done. The format change was the easy part; what banks do with the richer data is the real story.Sat, 20 Jun 2026 00:00:00 GMTTrendspaymentsiso-20022standards