Idempotency

1. The Incident

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

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