code: def flush(self):
          
          if self._cache is None:
              return []
          with self._cache as c, self._out as out:
              c.expire()
              now = self._timer()
              for item in c.values():
                  if (not self._in_flush_all) and (not self._should_expire(item)):
                      if (not item.is_in_flight) and item._op_aggregator is not None:
                          item.is_in_flight = True
                          item.last_refresh_timestamp = now
                          out.append(item.extract_request())  # pylint: disable=no-member
              flushed_items = list(out)
              out.clear()  # pylint: disable=no-member
              for req in flushed_items:
                  assert isinstance(req, sc_messages.ServicecontrolServicesAllocateQuotaRequest)
              return flushed_items
docs: Flushes this instance's cache.
  
          The driver of this instance should call this method every
          `flush_interval`.
  
          Returns:
            list['ServicecontrolServicesAllocateQuotaRequest']: corresponding
            to AllocateQuotaRequests that were pending