Webhooks

Receiving Webhooks

Express (with signature verification)

app.post('/webhooks/stripe', express.raw({ type: 'application/json' }), (req, res) => {
  const signature = req.headers['stripe-signature']!;
  const event = stripe.webhooks.constructEvent(req.body, signature, WEBHOOK_SECRET);

  // Idempotency: check if already processed
  const existing = await db.webhookEvent.findUnique({ where: { eventId: event.id } });
  if (existing) return res.json({ received: true });

  // Process
  switch (event.type) {
    case 'checkout.session.completed':
      await fulfillOrder(event.data.object);
      break;
  }

  // Mark as processed
  await db.webhookEvent.create({ data: { eventId: event.id, type: event.type } });
  res.json({ received: true });
});

Generic HMAC Verification

import { createHmac, timingSafeEqual } from 'crypto';

function verifyWebhookSignature(payload: string, signature: string, secret: string): boolean {
  const expected = createHmac('sha256', secret).update(payload).digest('hex');
  return timingSafeEqual(Buffer.from(signature), Buffer.from(`sha256=${expected}`));
}

app.post('/webhooks/github', express.raw({ type: '*/*' }), (req, res) => {
  const sig = req.headers['x-hub-signature-256'] as string;
  if (!verifyWebhookSignature(req.body.toString(), sig, GITHUB_SECRET)) {
    return res.status(401).json({ error: 'Invalid signature' });
  }
  // Process event...
  res.status(200).json({ received: true });
});

Sending Webhooks

class WebhookDispatcher {
  async dispatch(url: string, event: WebhookEvent, secret: string) {
    const payload = JSON.stringify(event);
    const signature = createHmac('sha256', secret).update(payload).digest('hex');

    const response = await fetch(url, {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
        'X-Webhook-Signature': `sha256=${signature}`,
        'X-Webhook-Id': event.id,
        'X-Webhook-Timestamp': new Date().toISOString(),
      },
      body: payload,
      signal: AbortSignal.timeout(10000),
    });

    if (!response.ok) throw new WebhookDeliveryError(response.status);
  }
}

Retry with Exponential Backoff

// Use job queue for reliable delivery
await webhookQueue.add('deliver', {
  url: subscription.url,
  event: { id: uuid(), type: 'order.created', data: order },
  secret: subscription.secret,
}, {
  attempts: 5,
  backoff: { type: 'exponential', delay: 60000 }, // 1m, 2m, 4m, 8m, 16m
});

Webhook Event Schema

interface WebhookEvent {
  id: string;           // Unique event ID (for idempotency)
  type: string;         // 'order.created', 'user.deleted'
  timestamp: string;    // ISO 8601
  data: unknown;        // Event payload
  version: string;      // API version
}

Anti-Patterns

Anti-Pattern	Fix
No signature verification	Always verify HMAC signature
Processing before responding 200	Respond 200 immediately, process async
No idempotency check	Store processed event IDs
Using parsed body for verification	Use raw body for signature check
No retry on send failures	Use job queue with exponential backoff
Synchronous webhook delivery	Dispatch via background job queue

Production Checklist

• HMAC signature verification on receive
• Raw body parsing (not JSON-parsed) for signature
• Idempotency: deduplicate by event ID
• Respond 200 before processing
• Retry with exponential backoff on send
• Dead letter queue for failed deliveries
• Webhook event log for debugging