func (self *NsqdCoordinator) updateChannelOffsetOnSlave(tc *coordData, channelName string, offset ChannelConsumerOffset) *CoordErr {
	topicName := tc.topicInfo.Name
	partition := tc.topicInfo.Partition

	if !tc.IsMineISR(self.myNode.GetID()) {
		return ErrTopicWriteOnNonISR
	}

	if coordLog.Level() >= levellogger.LOG_DETAIL {
		coordLog.Debugf("got update channel(%v) offset on slave : %v", channelName, offset)
	}
	coord, coordErr := self.getTopicCoord(topicName, partition)
	if coordErr != nil {
		return ErrMissingTopicCoord
	}

	topic, localErr := self.localNsqd.GetExistingTopic(topicName, partition)
	if localErr != nil {
		coordLog.Warningf("slave missing topic : %v", topicName)
		// TODO: leave the isr and try re-sync with leader
		return &CoordErr{localErr.Error(), RpcCommonErr, CoordSlaveErr}
	}

	if topic.GetTopicPart() != partition {
		coordLog.Errorf("topic on slave has different partition : %v vs %v", topic.GetTopicPart(), partition)
		return ErrLocalMissingTopic
	}
	var ch *nsqd.Channel
	ch, localErr = topic.GetExistingChannel(channelName)
	// if a new channel on slave, we should set the consume offset by force
	if localErr != nil {
		offset.AllowBackward = true
		ch = topic.GetChannel(channelName)
		coordLog.Infof("slave init the channel : %v, %v, offset: %v", topic.GetTopicName(), channelName, ch.GetConfirmed())
	}
	if ch.IsEphemeral() {
		coordLog.Errorf("ephemeral channel %v should not be synced on slave", channelName)
	}
	currentEnd := ch.GetChannelEnd()
	if nsqd.BackendOffset(offset.VOffset) > currentEnd.Offset() {
		coordLog.Debugf("update channel(%v) consume offset exceed end %v on slave : %v", channelName, offset, currentEnd)
		// cache the offset (using map?) to reduce the slave channel flush.
		coord.consumeMgr.Lock()
		cur, ok := coord.consumeMgr.channelConsumeOffset[channelName]
		if !ok || cur.VOffset < offset.VOffset {
			coord.consumeMgr.channelConsumeOffset[channelName] = offset
		}
		coord.consumeMgr.Unlock()

		if offset.Flush {
			topic.ForceFlush()
			currentEnd = ch.GetChannelEnd()
			if nsqd.BackendOffset(offset.VOffset) > currentEnd.Offset() {
				offset.VOffset = int64(currentEnd.Offset())
				offset.VCnt = currentEnd.TotalMsgCnt()
			}
		} else {
			return nil
		}
	}
	err := ch.ConfirmBackendQueueOnSlave(nsqd.BackendOffset(offset.VOffset), offset.VCnt, offset.AllowBackward)
	if err != nil {
		coordLog.Warningf("update local channel(%v) offset %v failed: %v, current channel end: %v, topic end: %v",
			channelName, offset, err, currentEnd, topic.TotalDataSize())
		if err == nsqd.ErrExiting {
			return &CoordErr{err.Error(), RpcNoErr, CoordTmpErr}
		}
		return &CoordErr{err.Error(), RpcCommonErr, CoordSlaveErr}
	}
	return nil
}