DataSource::process

public function process($data = NULL, Timer $timer = NULL)
{
	$apiHelper = new ApiHelper($this->exportId);
	
	$productsFromVk = $apiHelper->getProductsFromVk($this->vkGroupId);
	$productsMapped = VkMap::getMappedProducts($this->exportId);

//		remove from mapping products which not exist in VK
	$productsMappedToRemove = array();
	foreach ($productsMapped as $key => $productMapped)
	{
		if (!isset($productsFromVk[$productMapped["VK_ID"]]))
		{
			$productsMappedToRemove[] = array("VALUE_EXTERNAL" => $productMapped["VK_ID"]);
			unset($productsMapped[$key]);
		}
	}
//		remove not exists in VK items
	if (!empty($productsMappedToRemove))
		VkMap::removeProductMapping($productsMappedToRemove, $this->exportId);

//		In delete procedure we not need http file upload.
// 		It means that we can not limit max items by settings and using max possible count.
	$productsMapped = array_chunk($productsMapped, VkVk::MAX_EXECUTION_ITEMS);    // max 25 items in execute()
	foreach ($productsMapped as $chunk)
	{
		$resDelete = $this->executer->executeMarketProductDelete(array(
			"owner_id" => $this->vkGroupId,
			"data" => $chunk,
			"count" => count($chunk),
		));

		foreach ($resDelete as $res)
		{
			if ($res["flag_product_delete_result"])
				$productsMappedToRemove[] = array("VALUE_EXTERNAL" => $res["VK_ID"]);
		}

		// remove success deleted items
		if (!empty($productsMappedToRemove))
			VkMap::removeProductMapping($productsMappedToRemove, $this->exportId);

//			abstract start position - only for continue export, not for rewind to position
		if ($timer !== NULL && !$timer->check())
			throw new TimeIsOverException("Timelimit for export is over", '1');
	}

//		remove products from cache
	$vkExportedData = new VkVkExportedData($this->exportId, 'PRODUCTS');
	$vkExportedData->removeData();

	return true;
}