One day, I walked into a supermarket to buy a few things. I paid with my card, but the POS machine froze. The cashier looked at me and said, "Sorry, please try again."
Immediately, one thought came to mind:
"What if the first transaction actually went through?"
Most people would simply retry and hope for the best. But after spending more time understanding fintech systems, I realized this situation has a name. It's one of the reasons idempotency exists.
In simple terms, idempotency ensures that whether a transfer request arrives once or five times, the result remains the same. Duplicate payments are far more expensive than failed payments. That small experience reminded me that some of the concepts we learn as backend engineers aren't just interview topics. They're solving real problems around us every day.
