/** The key of the configuration. e.g. click_count */
    String INDEX_FIELD_click_count = "index.field.click_count";

    /** The key of the configuration. e.g. config_id */
    String INDEX_FIELD_config_id = "index.field.config_id";

    /** The key of the configuration. e.g. expires */
    String INDEX_FIELD_EXPIRES = "index.field.expires";

    /** The key of the configuration. e.g. url */