Detect inventory change

It has been some time ago that I build something for this, I had to listen to multiple observers because they where not all handled by the save of the cataloginventory, I have to following code:

    <events>
        <cataloginventory_stock_item_save_commit_after>
            <observers>
                <genmato_stockupdate>
                    <class>genmato_stockupdate/observer</class>
                    <method>catalogInventorySave</method>
                </genmato_stockupdate>
            </observers>
        </cataloginventory_stock_item_save_commit_after>
        <sales_model_service_quote_submit_before>
            <observers>
                <genmato_stockupdate>
                    <class>genmato_stockupdate/observer</class>
                    <method>subtractQuoteInventory</method>
                </genmato_stockupdate>
            </observers>
        </sales_model_service_quote_submit_before>
        <sales_model_service_quote_submit_failure>
            <observers>
                <genmato_stockupdate>
                    <class>genmato_stockupdate/observer</class>
                    <method>revertQuoteInventory</method>
                </genmato_stockupdate>
            </observers>
        </sales_model_service_quote_submit_failure>
        <sales_order_item_cancel>
            <observers>
                <genmato_stockupdate>
                    <class>genmato_stockupdate/observer</class>
                    <method>cancelOrderItem</method>
                </genmato_stockupdate>
            </observers>
        </sales_order_item_cancel>
        <sales_order_creditmemo_save_after>
            <observers>
                <genmato_stockupdate>
                    <class>genmato_stockupdate/observer</class>
                    <method>refundOrderInventory</method>
                </genmato_stockupdate>
            </observers>
        </sales_order_creditmemo_save_after>
    </events>

And in the observer the following code:

public function catalogInventorySave(Varien_Event_Observer $observer)
{
    if ($this->isEnabled()) {
        $event = $observer->getEvent();
        $_item = $event->getItem();

        if ((int)$_item->getData('qty') != (int)$_item->getOrigData('qty')) {
            $params = array();
            $params['product_id'] = $_item->getProductId();
            $params['qty'] = $_item->getQty();
            $params['qty_change'] = $_item->getQty() - $_item->getOrigData('qty');
        }
    }
}

public function subtractQuoteInventory(Varien_Event_Observer $observer)
{
    if ($this->isEnabled()) {
        $quote = $observer->getEvent()->getQuote();
        foreach ($quote->getAllItems() as $item) {
            $params = array();
            $params['product_id'] = $item->getProductId();
            $params['sku'] = $item->getSku();
            $params['qty'] = $item->getProduct()->getStockItem()->getQty();
            $params['qty_change'] = ($item->getTotalQty() * -1);
        }
    }
}

public function revertQuoteInventory(Varien_Event_Observer $observer)
{
    if ($this->isEnabled()) {
        $quote = $observer->getEvent()->getQuote();
        foreach ($quote->getAllItems() as $item) {
            $params = array();
            $params['product_id'] = $item->getProductId();
            $params['sku'] = $item->getSku();
            $params['qty'] = $item->getProduct()->getStockItem()->getQty();
            $params['qty_change'] = ($item->getTotalQty());
        }
    }
}

public function cancelOrderItem(Varien_Event_Observer $observer)
{
    if ($this->isEnabled()) {
        $item = $observer->getEvent()->getItem();
        $qty = $item->getQtyOrdered() - max($item->getQtyShipped(), $item->getQtyInvoiced()) - $item->getQtyCanceled();
        $params = array();
        $params['product_id'] = $item->getProductId();
        $params['sku'] = $item->getSku();
        $params['qty'] = $item->getProduct()->getStockItem()->getQty();
        $params['qty_change'] = $qty;
    }
}

public function refundOrderInventory(Varien_Event_Observer $observer)
{
    if ($this->isEnabled()) {
        $creditmemo = $observer->getEvent()->getCreditmemo();
        foreach ($creditmemo->getAllItems() as $item) {
            $params = array();
            $params['product_id'] = $item->getProductId();
            $params['sku'] = $item->getSku();
            $params['qty'] = $item->getProduct()->getStockItem()->getQty();
            $params['qty_change'] = ($item->getQty());
       }
    }
}

Hope this is a bit what you are looking for.

You cannot use any event related to the stock item model, because Magento uses an optimized SQL query to decrease stock for all ordered items at once, bypassing the model.

I solved this with a rewrite of Mage_CatalogInventory_Model_Stock where I added an additional event:

<?php
/**
 * Add events to observe stock qty change
 * 
 * @author Fabian Schmengler
 *
 */
class SGH_ShippingExpress_Model_CatalogInventory_Stock
    extends Mage_CatalogInventory_Model_Stock
{
    const EVENT_CORRECT_STOCK_ITEMS_QTY_BEFORE = 'cataloginventory_stock_item_correct_qty_before';
    const EVENT_CORRECT_STOCK_ITEMS_QTY_AFTER = 'cataloginventory_stock_item_correct_qty_after';

    /**
     * (non-PHPdoc)
     * @see Mage_CatalogInventory_Model_Stock::registerProductsSale()
     */
    public function registerProductsSale($items)
    {
        Mage::dispatchEvent(self::EVENT_CORRECT_STOCK_ITEMS_QTY_BEFORE, array(
            'stock'     => $this,
            'items_obj' => (object)array('items' => &$items),
            'operator'  => '-'
        ));
        $result = parent::registerProductsSale($items);
        Mage::dispatchEvent(self::EVENT_CORRECT_STOCK_ITEMS_QTY_AFTER, array(
            'stock'          => $this,
            'items'          => $items,
            'fullsave_items' => $result,
            'operator'       => '-'
        ));
        return $result;
    }
    /**
     * (non-PHPdoc)
     * @see Mage_CatalogInventory_Model_Stock::revertProductsSale()
     */
    public function revertProductsSale($items)
    {
        Mage::dispatchEvent(self::EVENT_CORRECT_STOCK_ITEMS_QTY_BEFORE, array(
            'stock'     => $this,
            'items_obj' => (object)array('items' => &$items),
            'operator'  => '+'
        ));
        $result = parent::revertProductsSale($items);
        Mage::dispatchEvent(self::EVENT_CORRECT_STOCK_ITEMS_QTY_AFTER, array(
            'stock'          => $this,
            'items'          => $items,
            'fullsave_items' => $result,
            'operator'       => '+'
        ));
        return $result;
    }
}

Then the observer for cataloginventory_stock_item_correct_qty_after can look like this:

    /**
     * @var $items array array($productId => array('qty'=>$qty, 'item'=>$stockItem))
     */
    $items = $observer->getItems();
    foreach ($items as $productId => $item) {
        $stockItem = $item['item'];
        $product = $stockItem->getProduct();

        // Do anything you need with $stockItem and $product here

    }

I recommend not to do heavy processing or additional database calls (which are necessary to detect if the product is out of stock for example), but to add the products to a queue that is processed by a cronjob, to minimize additional load time for the user.