do nothing if not implemented, instead of throw exception.
*
     * @param key     the key to represent a configuration
     * @param group   the group where the key belongs to
     * @param timeout timeout value for fetching the target config
     * @return target configuration mapped to the given key and the given group, IllegalStateException will be thrown
     * if timeout exceeds.
     */
    String getConfig(String key, String group, long timeout) throws IllegalStateException;
    /**
     * This method are mostly used to get a compound config file with {@link #getDefaultTimeout() the default timeout},
     * such as a complete dubbo.properties file.
     */
    default String getProperties(String key, String group) throws IllegalStateException {
        return getProperties(key, group, getDefaultTimeout());
    }
    /**
     * This method are mostly used to get a compound config file, such as a complete dubbo.properties file.
     *
     * @revision 2.7.4
     */
    default String getProperties(String key, String group, long timeout) throws IllegalStateException {
        return getConfig(key, group, timeout);
    }
    /**
     * Publish Config mapped to the given key under the {@link #getDefaultGroup() default group}
     *
     * @param key     the key to represent a configuration
     * @param content the content of configuration
     * @return <code>true</code> if success, or <code>false</code>
     * @throws UnsupportedOperationException If the under layer does not support