The problem

Black Friday, 11 p.m. A customer taps Pay for a 240 € coffee machine. The spinner turns for 30 seconds, the page says "Something went wrong", and she taps Pay again.

The next morning her bank app shows two charges of 240 €. The first request had reached the payment provider and charged her card. Only the answer was lost on its way back to the shop.

The shop’s app sends a card charge to its provider with no memory of earlier attempts. Your job is a checkout where a lost answer or a second tap never charges a card twice.