Idempotency

1. The Incident

In the previous lesson you added retries, and 500 customers a day stopped seeing error screens. Now comes the cost.

A customer clicks Pay on a $499 course bundle. Your service calls the payment provider. The provider charges the card successfully, and then the response is lost on the way back. One dropped packet, on the return trip. Your retry logic sees a timeout, waits 100 ms, and sends the request again. The provider sees what looks like a brand-new request and charges the card again