Scaling

Design a system that moves money: a payer is charged, a payee is credited, and the balances are always right — even though every step runs over networks that fail, and an external payment provider (a card network, a bank) that cannot participate in your database transaction.

In scope: initiating a payment, executing it through an external provider, recording every money movement in an auditable ledger, exposing balances and payment status, and refunding or voiding a payment. Fraud detection, the provider's own internals, and pricing/fee calculation are out of scope.