public function get_requirements_bin() {
        $requires = 0;
        if ($this->require_data_guarantee()) {
            $requires += cache_store::SUPPORTS_DATA_GUARANTEE;
        }
        if ($this->require_multiple_identifiers()) {
            $requires += cache_store::SUPPORTS_MULTIPLE_IDENTIFIERS;
        }
        if ($this->require_searchable()) {
            $requires += cache_store::IS_SEARCHABLE;
        }
        return $requires;
    }