What is the usage of itemId inside Update inventory REST API?

Question

According to the swagger, the REST API PUT /V1/products/{productSku}/stockItems/{itemId} is used to update the product inventory.

However, for the attribute value itemId, I found no differences no matter I entered which values. So, the following request URLs can update the same product without any errors AND return the same integer value:

http://<magento_url>/rest/V1/products/{productSku}/stockItems/1
http://<magento_url>/rest/V1/products/{productSku}/stockItems/{correctItemId}
http://<magento_url>/rest/V1/products/{productSku}/stockItems/999999

Given that the {productSku} is a simple product, my question is: What is the exact usage of itemId in the request URL?

Answer 1

If you see the method declaration in StockRegistryInterface.php

/**
 * @param string $productSku
 * @param \Magento\CatalogInventory\Api\Data\StockItemInterface $stockItem
 * @return int
 * @throws \Magento\Framework\Exception\NoSuchEntityException
 */
public function updateStockItemBySku($productSku, \Magento\CatalogInventory\Api\Data\StockItemInterface $stockItem);

With help of this itemId magento instantiates the object of

 \Magento\CatalogInventory\Api\Data\StockItemInterface $stockItem

then passing this object as parameter in updateStockItemBySku() method.

If we provide wrong ItemId, the inventory will be updated it default website only. If we look at the method updateStockItemBySku() in Magento\CatalogInventory\Model\StockRegistry, the code

$websiteId = $stockItem->getWebsiteId() ?: null;

So, itemId make significance to define the scope of website to update inventory.