code: func (s *Store) expirer() {
  	defer s.wg.Done()
  
  	ticker := time.NewTicker(s.ExpiryCheckInterval)
  	defer ticker.Stop()
  
  	for {
  		// Wait for next check or for close signal.
  		select {
  		case <-s.closing:
  			return
  		case <-ticker.C:
  		}
  
  		// Check all instances for expiration.
  		if err := s.EnforceExpiry(); err != nil && err != raft.ErrNotLeader {
  			s.logger.Printf("enforce expiry: %s", err)
  		}
  	}
  }
docs: // expirer runs in a separate goroutine and checks for instance expiration.