Idempotency and deduplication

app.post('/callback', async (req, res) => {
  const entry = req.body.custom_data?.find(
    (item) => item.keyof_customdata === 'transaction_id'
  );
  const transactionId = entry?.valueof_customdata;
  if (!transactionId) return res.sendStatus(400);

  // Insert with unique constraint — fails if already processed
  const inserted = await db.query(
    'INSERT INTO processed_callbacks (transaction_id) VALUES (?) ON CONFLICT DO NOTHING',
    [transactionId]
  );

  if (inserted.rowCount === 0) {
    // Already processed — second request of the LigdiCash duplicate
    return res.sendStatus(200);
  }

  // First reception — business processing
  await processPayment(transactionId);

  res.sendStatus(200);
});

// With a UNIQUE constraint on transaction_id in the database
try {
    $db->insert('processed_callbacks', ['transaction_id' => $transactionId]);
} catch (UniqueConstraintException $e) {
    // Already processed
    http_response_code(200);
    exit;
}

// First reception — business processing
processPayment($transactionId);