Idempotency everywhere: payments, wallet posts, and claim-once webhooks
There's a moment every billing engineer eventually has. A customer emails: "I was charged twice." You open the logs, and the request looks fine — one checkout, one order. Then you find the second event. Same payment, redelivered by the provider three minutes later because our 200 got lost on the wire. We processed it twice, minted the currency twice. The math was, briefly, generous to exactly the wrong person.
The lesson isn't "be more careful" — careful doesn't scale. It's that at-least-once delivery is the only honest contract a distributed system can offer, and once you accept that, exactly-once effects become something you build, on purpose, at every layer that touches money. Here's how we do it at Fibe — across wallet posts, the Mana-to-Sparks conversion, daily funding, provider webhooks, and the keys our SDK ships.