select {
		case resp := <-con.responsesChan:
			// TODO: separate upstream response handling from requests sending, which are both time costly
			proxyLog.WithLabels(
				"id", con.conID,
				"type", model.GetShortType(resp.TypeUrl),
				"resources", len(resp.Resources),
			).Debugf("upstream response")
			metrics.XdsProxyResponses.Increment()
			if h, f := p.handlers[resp.TypeUrl]; f {
				if len(resp.Resources) == 0 {