Introduce Stock quantity calculation

app/code/Magento/Inventory/Model/Reservation.php

@@ -39,6 +39,15 @@ class Reservation implements ReservationInterface
      */
     private $metadata;
 
+    /**
+     * Reservation constructor.
+     * 
+     * @param int|null $reservationId
+     * @param int $stockId
+     * @param string $sku
+     * @param float $quantity
+     * @param null $metadata
+     */
     public function __construct(
         $reservationId,
         int $stockId,

app/code/Magento/Inventory/Model/Reservation/ReservationBuilder.php

@@ -109,6 +109,7 @@ public function setMetadata(string $metadata = null): ReservationBuilderInterfac
     public function build(): ReservationInterface
     {
         $data = [
+            ReservationInterface::RESERVATION_ID => null,
             ReservationInterface::STOCK_ID => $this->stockId,
             ReservationInterface::SKU => $this->sku,
             ReservationInterface::QUANTITY => $this->quantity,

app/code/Magento/Inventory/Model/Stock/Command/GetProductQuantityInStock.php

@@ -0,0 +1,125 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+namespace Magento\Inventory\Model\Stock\Command;
+
+use Magento\Framework\App\ResourceConnection;
+use Magento\Inventory\Indexer\Alias;
+use Magento\Inventory\Indexer\IndexNameBuilder;
+use Magento\Inventory\Indexer\IndexNameResolverInterface;
+use Magento\Inventory\Indexer\StockItem\IndexStructure as StockItemIndex;
+use Magento\Inventory\Indexer\StockItemIndexerInterface;
+use Magento\Inventory\Setup\Operation\CreateReservationTable;
+use Magento\InventoryApi\Api\Data\ReservationInterface;
+use Magento\InventoryApi\Api\GetProductQuantityInStockInterface;
+
+/**
+ * Return Quantity of products available to be sold by Product SKU and Stock Id
+ *
+ * @see \Magento\InventoryApi\Api\GetProductQuantityInStockInterface
+ * @api
+ */
+class GetProductQuantityInStock implements GetProductQuantityInStockInterface
+{
+    /**
+     * @var ResourceConnection
+     */
+    private $resource;
+
+    /**
+     * @var IndexNameBuilder
+     */
+    private $indexNameBuilder;
+
+    /**
+     * @var IndexNameResolverInterface
+     */
+    private $indexNameResolver;
+
+    /**
+     * GetProductQuantityInStock constructor.
+     *
+     * @param ResourceConnection $resource
+     * @param IndexNameBuilder $indexNameBuilder
+     * @param IndexNameResolverInterface $indexNameResolver
+     */
+    public function __construct(
+        ResourceConnection $resource,
+        IndexNameBuilder $indexNameBuilder,
+        IndexNameResolverInterface $indexNameResolver
+    ) {
+        $this->resource = $resource;
+        $this->indexNameBuilder = $indexNameBuilder;
+        $this->indexNameResolver = $indexNameResolver;
+    }
+
+    /**
+     * @inheritdoc
+     */
+    public function execute(string $sku, int $stockId): float
+    {
+        $productQtyInStock = $this->getStockItemQty($sku, $stockId) - $this->getReservationQty($sku, $stockId);
+        return (float) $productQtyInStock;
+    }
+
+    /**
+     * Return product quantity in stock.
+     *
+     * @param string $sku
+     * @param int $stockId
+     * @return float
+     */
+    private function getStockItemQty(string $sku, int $stockId): float
+    {
+        $indexName = $this->indexNameBuilder
+            ->setIndexId(StockItemIndexerInterface::INDEXER_ID)
+            ->addDimension('stock_', $stockId)
+            ->setAlias(Alias::ALIAS_MAIN)
+            ->create();
+
+        $stockItemTableName = $this->indexNameResolver->resolveName($indexName);
+
+        $connection = $this->resource->getConnection();
+
+        $select = $connection->select()
+            ->from($stockItemTableName, [StockItemIndex::QUANTITY])
+            ->where(StockItemIndex::SKU . '=?', $sku)
+            ->limit(1);
+
+        $stockItemQty = $connection->fetchOne($select);
+        if (false === $stockItemQty) {
+            $stockItemQty = 0;
+        }
+
+        return (float) $stockItemQty;
+    }
+
+    /**
+     * Return the sum of all product reservations in stock.
+     *
+     * @param string $sku
+     * @param int $stockId
+     * @return float
+     */
+    private function getReservationQty(string $sku, int $stockId): float
+    {
+        $connection = $this->resource->getConnection();
+
+        $reservationTableName = $connection->getTableName(CreateReservationTable::TABLE_NAME_RESERVATION);
+
+        $select = $connection->select()
+            ->from($reservationTableName, [ReservationInterface::QUANTITY => 'sum(' . ReservationInterface::QUANTITY . ')'])
+            ->where(ReservationInterface::SKU . '=?', $sku)
+            ->where(ReservationInterface::STOCK_ID . '=?', $stockId)
+            ->limit(1);
+
+        $reservationQty = $connection->fetchOne($select);
+        if (false === $reservationQty) {
+            $reservationQty = 0;
+        }
+
+        return (float) $reservationQty;
+    }
+}

app/code/Magento/InventoryApi/Api/GetProductQuantityInStockInterface.php

@@ -0,0 +1,23 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+namespace Magento\InventoryApi\Api;
+
+/**
+ * Service which returns Quantity of products available to be sold by Product SKU and Stock Id
+ *
+ * @api
+ */
+interface GetProductQuantityInStockInterface
+{
+    /**
+     * Get Product Quantity for given SKU in a given Stock
+     *
+     * @param string $sku
+     * @param int $stockId
+     * @return float
+     */
+    public function execute(string $sku, int $stockId): float;
+}