<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _ensure_worker(self):
 """Ensure there are enough workers available"""

        while len(self._workers) < self._min_workers or len(self._workers) < self._queue.qsize() < self._max_workers:
            worker = threading.Thread(
                target=self._execute_futures,
                name=self.identifier + '_%d' % time.time(),
            )
            worker.daemon = True
            self._workers.add(worker)
            worker.start()