Practical Guide to SQL Transaction Isolation(begriffs.com)

    def add_item_to_cart(order, item):
        try:
            lock = acquire_named_lock('order-{}'.format(order.id), timeout=10)
            order.add_item(item)
            order.recalculate_total()
            order.save()
            lock.release()
        except LockTimeoutError:
            raise OrderUpdateError('Timed out trying to add item to order.')


    def checkout(order):
        try:
            lock = acquire_named_lock('order-{}'.format(order.id), timeout=10)
            order.pay()
            order.generate_and_email_invoice()
            order.print_shipping_label()
            order.save()
            lock.release()
        except LockTimeoutError:
            raise OrderUpdateError('Timed out trying to check out.')