Design a Payment System

Build a payment processing platform that handles transactions, manages wallets, and integrates with external payment providers.

Functional Requirements

• Process credit card and bank transfer payments
• Manage user wallets with balance tracking
• Handle refunds and chargebacks
• Support multiple currencies
• Generate invoices and receipts
• Subscription and recurring payments

Non-Functional Requirements

• Exactly-once payment processing (no double charges)
• PCI DSS compliance
• 99.99% availability for payment processing
• Complete audit trail for all transactions

Questions to Consider

• How do you ensure exactly-once payment semantics?
• What patterns handle distributed transaction consistency?
• How do you design for PCI compliance?