protected function retrieveItemIndex()
    {
        $this->itemIndex = new QtiTestCompilerIndex();
        try {
            $directories = $this->getCompilationDirectory();
            $data = $directories['private']->read(taoQtiTest_models_classes_QtiTestService::TEST_COMPILED_INDEX);
            if ($data) {
                $this->itemIndex->unserialize($data);
            }
        } catch(\Exception $e) {
            \common_Logger::d('Ignoring file not found exception for Items Index');
        }
    }