'Starts reading our descriptor files.
:raises: **ValueError** if we\'re already reading the descriptor files'

def start(self):

    with self._reader_thread_lock:
        if self._reader_thread:
            raise ValueError('Already    running,    you    need    to    call    stop()    first')
        else:
            self._is_stopped.clear()
            self._reader_thread = threading.Thread(target=self._read_descriptor_files, name='Descriptor    Reader')
            self._reader_thread.setDaemon(True)
            self._reader_thread.start()
