docs: def _sync(self):
          """
          Synchronize the cached data with the underlyind database.
  
          Uses an internal transaction counter and compares to the checkpoint_operations
          and checkpoint_timeout paramters to determine whether to persist the memory store.
  
          In this implementation, this method wraps calls to C{shelve.Shelf#sync}. 
          """
          if (self._opcount > self.checkpoint_operations or
                  datetime.now() > self._last_sync + self.checkpoint_timeout):
              self.log.debug("Synchronizing queue metadata.")
              self.queue_metadata.sync()
              self._last_sync = datetime.now()
              self._opcount = 0
          else:
              self.log.debug("NOT synchronizing queue metadata.")