code: func (do *Domain) Close() {
  	if do.ddl != nil {
  		terror.Log(do.ddl.Stop())
  	}
  	if do.info != nil {
  		do.info.RemoveServerInfo()
  	}
  	close(do.exit)
  	if do.etcdClient != nil {
  		terror.Log(errors.Trace(do.etcdClient.Close()))
  	}
  	do.sysSessionPool.Close()
  	do.slowQuery.Close()
  	do.wg.Wait()
  	logutil.Logger(context.Background()).Info("domain closed")
  }
docs: // Close closes the Domain and release its resource.