4. Implement Idempotency

Prevent duplicate payments for the same order:

async function getOrCreatePayment(orderId, paymentDetails) {
  const order = await db.orders.get(orderId);

  // Return existing payment if already created
  if (order.paymentReference) {
    return client.getPayment(order.paymentReference);
  }

  // Create new payment
  const payment = await client.createPayment(paymentDetails);

  // Store reference immediately
  await db.orders.update(orderId, {
    paymentReference: payment.payment.reference,
  });

  return payment;
}