System Design · Karat · Medium
An arcade-style business sells prepaid game cards to its players. To play, a player taps the card on a game machine; before the game is allowed to begin, the machine contacts the company's central account server and asks it to subtract the required credits from that card's balance. Card balances can be topped up in two ways: by paying cash, or by linking a credit card to the game card so that the balance is refilled automatically. The fleet consists of roughly 125,000…