Advanced Code Debugging

Additional advanced technical scenarios

These examples describe engineering approaches and code patterns in my areas of specialisation. Integration-specific helpers and environment setup are identified where needed.

$order = wc_get_order($order_id);
$order->payment_complete($transaction_id);
send_to_warehouse($order);

// Payment handler: transaction ID must be unique.
// Acquire a DB-level lock or unique event row first.
if (!claim_event_once($gateway_event_id)) {
    return new WP_REST_Response(['ok' => true], 200);
}
$order = wc_get_order($order_id);
if (!$order || !hash_equals(
    (string) $order->get_transaction_id(),
    (string) $transaction_id
)) {
    // Validate gateway signature + transaction separately.
    release_event_claim($gateway_event_id);
    return new WP_Error('invalid_payment', 'Mismatch', ['status' => 400]);
}
queue_fulfilment_once($order_id);

<span>{{ product.price | money }}</span>
<button>Add to cart</button>

<span id="variant-price">
  {{ product.selected_or_first_available_variant.price | money }}
</span>
<button id="add-to-cart"
  {% unless product.selected_or_first_available_variant.available %}disabled{% endunless %}>
  Add to cart
</button>
<script>
// In theme variant-change handler:
function syncVariant(variant) {
  document.querySelector('#variant-price').textContent =
    Shopify.formatMoney(variant.price, window.themeMoneyFormat);
  document.querySelector('#add-to-cart').disabled = !variant.available;
}
</script>

for (const item of products) {
  await graphql(mutation, { item });
}

// Pseudocode: adapt to Admin GraphQL API version.
for (const batch of remainingBatches(cursor)) {
  const response = await shopifyGraphQL(batch);
  if (response.errors?.some(isThrottleError)) {
    await sleep(retryDelay(response));
    retrySameBatch();
    continue;
  }
  assertNoUserErrors(response.data);
  persistCheckpoint(batch.cursor);
}

register_rest_route('studio/v1', '/rebuild', [
  'methods' => 'POST',
  'callback' => 'studio_rebuild_index',
]);

add_action('rest_api_init', function () {
  register_rest_route('studio/v1', '/rebuild', [
    'methods' => WP_REST_Server::CREATABLE,
    'callback' => 'studio_rebuild_index',
    'permission_callback' => function () {
      return current_user_can('manage_options');
    },
  ]);
});

$stock = getStock($sku);
if ($stock > 0) {
  setStock($sku, $stock - 1);
}

// InnoDB / PDO; inside an existing transaction:
$pdo->beginTransaction();
$stmt = $pdo->prepare(
  'UPDATE inventory SET stock = stock - 1 '.
  'WHERE sku = ? AND stock >= 1'
);
$stmt->execute([$sku]);
if ($stmt->rowCount() !== 1) {
  $pdo->rollBack();
  throw new RuntimeException('Out of stock');
}
$pdo->commit();

const total = cart.reduce((s, x) => s + x.price, 0);
await fetch('/checkout', {
  method: 'POST', body: JSON.stringify({ cart, total })
});

// Browser sends only product IDs and quantities.
await fetch('/checkout', {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify({
    lines: cart.map(x => ({ id: x.id, qty: x.qty }))
  })
});
// Server: authenticate, validate quantities,
// reprice from trusted catalogue and verify discounts.

for row in fetch_all():
    insert(row)

# SQLite illustration: upsert stable external IDs.
with sqlite3.connect('items.db') as db:
    db.executemany(
        "INSERT INTO items(id, payload) VALUES(?, ?) "
        "ON CONFLICT(id) DO UPDATE SET payload=excluded.payload",
        [(r['id'], json.dumps(r)) for r in page]
    )
# Save source cursor only after successful commit.
# Resume from the last committed cursor.

await fetch('/api/invoices', {
  method: 'POST', body: JSON.stringify(invoice)
});

const key = invoice.requestId; // stable across retries
const res = await fetch('/api/invoices', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
    'Idempotency-Key': key
  },
  body: JSON.stringify(invoice)
});
if (!res.ok) throw new Error(`HTTP ${res.status}`);
// Server must enforce uniqueness of key + payload hash.

SELECT id, created_at FROM orders
ORDER BY created_at DESC
LIMIT 50 OFFSET 5000;

-- Keyset pagination: cursor is (created_at, id).
SELECT id, created_at FROM orders
WHERE created_at < :cursor_time
   OR (created_at = :cursor_time AND id < :cursor_id)
ORDER BY created_at DESC, id DESC
LIMIT 50;
-- Index: (created_at DESC, id DESC)

$rates = wp_remote_get($api_url);
$price = json_decode(wp_remote_retrieve_body($rates), true);

$rates = get_transient('studio_rates_v1');
if ($rates === false) {
  $res = wp_remote_get($api_url, ['timeout' => 3]);
  if (!is_wp_error($res) &&
      wp_remote_retrieve_response_code($res) === 200) {
    $rates = json_decode(wp_remote_retrieve_body($res), true);
    if (is_array($rates)) {
      set_transient('studio_rates_v1', $rates, 300);
    }
  }
}
// On failure, use a defined safe fallback policy.

add_action('woocommerce_payment_complete', function ($id) {
  warehouse_api_ship($id);
});

add_action('woocommerce_payment_complete', function ($id) {
  if (!as_next_scheduled_action(
      'studio_send_warehouse', ['order_id' => $id], 'studio')) {
    as_enqueue_async_action(
      'studio_send_warehouse', ['order_id' => $id], 'studio', true);
  }
});
// Worker must verify order payment and use a
// durable fulfilment idempotency key.
add_action('studio_send_warehouse', 'studio_worker', 10, 1);

const event = JSON.parse(req.body);
processEvent(event);

// Node.js: req.rawBody must be the exact received bytes.
const expected = crypto.createHmac('sha256', secret)
  .update(req.rawBody).digest('hex');
const received = req.headers['x-signature'];
if (!/^[a-f0-9]{64}$/i.test(received || ''))
  return res.sendStatus(401);
if (!crypto.timingSafeEqual(
  Buffer.from(expected, 'hex'), Buffer.from(received, 'hex')))
  return res.sendStatus(401);
// Also enforce timestamp window + unique event ID.
processVerifiedEvent(req.rawBody);