app.post('/webhook', express.raw({ type: 'application/json' }), (req, res) => {
  res.status(200).send('OK');           // Acknowledge immediately
  const payload = JSON.parse(req.body); // Process in background
  processEvent(payload);
});

async function handleEvent(payload) {
  const { event_id } = payload;

  // Use a database or Redis — not in-memory sets
  const alreadyProcessed = await redis.get(`webhook:${event_id}`);
  if (alreadyProcessed) return;

  await redis.set(`webhook:${event_id}`, '1', 'EX', 86400); // TTL: 24h
  // Process...
}

# Check session status (Sessions API only)
GET /api/v1/partner/payments/sessions/{session_id}

switch (payload.event_type) {
  case 'payment.succeeded':
    handlePaymentSucceeded(payload.data);
    break;
  case 'payment.refunded':
    handlePaymentRefunded(payload.data);
    break;
  // ... other known events
  default:
    console.log(`Unhandled event type: ${payload.event_type}`);
}