Repair Webhook Replay & Delivery Bugs

I have analyzed the handler. The issue stems from the database insert occurring before the idempotency check is committed, creating a race condition during rapid retries.

Webhook Idempotency Repair

const express = require('express');
const router = express.Router();
const db = require('./db'); // Hypothetical DB client

router.post('/webhook', async (req, res) => {
  const sig = req.headers['stripe-signature'];
  const event = req.body;

  // 1. Verify event identity
  const eventId = event.id;

  try {
    // 2. Use a transaction to ensure atomicity
    await db.transaction(async (tx) => {
      // 3. Check for existing processed event (The Invariant)
      const existing = await tx.query(
        'SELECT id FROM processed_events WHERE id = $1 FOR UPDATE',
        [eventId]
      );

      if (existing.length > 0) {
        console.log(`Event ${eventId} already processed. Skipping.`);
        return;
      }

      // 4. Business logic: Create order
      await tx.query(
        'INSERT INTO orders (user_id, amount, status) VALUES ($1, $2, $3)',
        [event.data.object.metadata.user_id, event.data.object.amount, 'paid']
      );

      // 5. Record event processing
      await tx.query(
        'INSERT INTO processed_events (id, processed_at) VALUES ($1, NOW())',
        [eventId]
      );
    });

    // 6. Acknowledge early only after durability is guaranteed
    res.json({ received: true });
  } catch (err) {
    console.error(`Webhook Error: ${err.message}`);
    // Return 4xx/5xx to trigger provider retry
    res.status(500).send('Webhook handler failed');
  }
});

Deterministic Test Plan

Duplicate Delivery Test: Mock two concurrent requests with the same evt_123 ID. Verify the DB contains exactly one order and one processed_events record.
2.

Partial Failure Test: Force an error during the order insertion. Verify that the processed_events record is NOT created, allowing for a future retry.

Next steps

• Create the processed_events table with a primary key on id.
• Verify your DB driver supports FOR UPDATE or equivalent row-level locking.
• Update the Stripe webhook secret in your environment variables.