Magento – Products (seemingly) randomly disappearing until reindex

This works fine in Magento CE 1.6 and newer:

$event = Mage::getSingleton('index/indexer')->logEvent(
    $product,
    $product->getResource()->getType(),
    Mage_Index_Model_Event::TYPE_SAVE,
    false
);
Mage::getSingleton('index/indexer')
    ->getProcessByCode('catalog_url') // Adjust the indexer process code as needed
    ->setMode(Mage_Index_Model_Process::MODE_REAL_TIME)
    ->processEvent($event);

The available indexer codes can be viewed using the query:

SELECT indexer_code FROM index_process;

In a native Magento 1.7 there are:

+---------------------------+
| indexer_code              |
+---------------------------+
| cataloginventory_stock    |
| catalogsearch_fulltext    |
| catalog_category_flat     |
| catalog_category_product  |
| catalog_product_attribute |
| catalog_product_flat      |
| catalog_product_price     |
| catalog_url               |
| groupscatalog2_category   |
| groupscatalog2_product    |
| tag_summary               |
+---------------------------+

In Magento EE 1.13 it's different, there the indexer automatically picks up changed entities on each cron run (every minute).

UPDATE

The above answer is 100% correct anyway I think the below information can add something more.

• If you need to change just few attributes values in a product and automatically update the relative index table you can use this function: Mage::getSingleton('catalog/product_action')->updateAttributes();

• if you want to manage the reindex by your own use the resourse model instead: Mage::getResourceSingleton('catalog/product_action')->updateAttributes();

For example I use the following function to fast update only certain attributes in a product.

 public function updateProductAttribute($product_id, $arrayChanges, $reindex = true)
{
    if (!is_array($product_id)) {
        $product_id = array($product_id);
    }

    // ths should trigger all required reindex
    $update = Mage::getSingleton('catalog/product_action');
    // Update value
    if (!$reindex) {
        $update = Mage::getResourceSingleton('catalog/product_action');
    }

    $update->updateAttributes($product_id, $arrayChanges, 0);
}

Note:

If you need to change the same attribute/values couple in a group of products you can pass the whole array of product_ids

Checking the block class outputting the recently viewed products Mage_Reports_Block_Product_Viewed you can see the _toHtml method uses the getItemsCollection method from Mage_Reports_Block_Product_Abstract.

In this class the collection is actually retrieved and on line 124 the customer ID is added to the collection as a filtering.

So a good place to start would be to rewrite the Mage_Reports_Block_Product_Abstract to a custom extension and modify the collection retrieval excluding the customer filter.

Optionally you could copy this file to local and modify it but that might lead to issues when upgrading to a newer version of Magento (and is also just a bad practice in general)