#idempotency

Polling a resource once a minute costs 1,440 requests a day, almost all of them empty. A webhook fixes that by inverting the call, and in exchange hands you signature verification, duplicate events, out-of-order events, and retry storms. Here is what each one actually costs.

The design detail that surprises people: Stripe caches the result of the first request under a key even when that result was a 500, and replays the failure to every retry. That is correct, and once you see why, the rest of the design follows, including where most homegrown implementations go wrong.