milvus/internal/querycoord/query_coord.go

// Licensed to the LF AI & Data foundation under one
// or more contributor license agreements. See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership. The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License. You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package querycoord

import (
	"context"
	"errors"
	"fmt"
	"math"
	"math/rand"
	"sort"
	"strconv"
	"sync"
	"sync/atomic"
	"syscall"
	"time"

	"github.com/golang/protobuf/proto"
	"go.etcd.io/etcd/api/v3/mvccpb"
	"go.uber.org/zap"

	"github.com/milvus-io/milvus/internal/allocator"
	etcdkv "github.com/milvus-io/milvus/internal/kv/etcd"
	"github.com/milvus-io/milvus/internal/log"
	"github.com/milvus-io/milvus/internal/msgstream"
	"github.com/milvus-io/milvus/internal/proto/commonpb"
	"github.com/milvus-io/milvus/internal/proto/internalpb"
	"github.com/milvus-io/milvus/internal/proto/querypb"
	"github.com/milvus-io/milvus/internal/types"
	"github.com/milvus-io/milvus/internal/util/metricsinfo"
	"github.com/milvus-io/milvus/internal/util/retry"
	"github.com/milvus-io/milvus/internal/util/sessionutil"
	"github.com/milvus-io/milvus/internal/util/tsoutil"
	"github.com/milvus-io/milvus/internal/util/typeutil"
)

const (
	handoffSegmentPrefix = "querycoord-handoff"
)

// Timestamp is an alias for the Int64 type
type Timestamp = typeutil.Timestamp

type queryChannelInfo struct {
	requestChannel  string
	responseChannel string
}

// QueryCoord is the coordinator of queryNodes
type QueryCoord struct {
	loopCtx    context.Context
	loopCancel context.CancelFunc
	loopWg     sync.WaitGroup
	kvClient   *etcdkv.EtcdKV

	initOnce sync.Once

	queryCoordID uint64
	meta         Meta
	cluster      Cluster
	newNodeFn    newQueryNodeFn
	scheduler    *TaskScheduler
	idAllocator  func() (UniqueID, error)
	indexChecker *IndexChecker

	metricsCacheManager *metricsinfo.MetricsCacheManager

	dataCoordClient  types.DataCoord
	rootCoordClient  types.RootCoord
	indexCoordClient types.IndexCoord

	session   *sessionutil.Session
	eventChan <-chan *sessionutil.SessionEvent

	stateCode  atomic.Value
	enableGrpc bool

	msFactory msgstream.Factory
}

// Register register query service at etcd
func (qc *QueryCoord) Register() error {
	qc.session.Register()
	go qc.session.LivenessCheck(qc.loopCtx, func() {
		log.Error("Query Coord disconnected from etcd, process will exit", zap.Int64("Server Id", qc.session.ServerID))
		if err := qc.Stop(); err != nil {
			log.Fatal("failed to stop server", zap.Error(err))
		}
		// manually send signal to starter goroutine
		syscall.Kill(syscall.Getpid(), syscall.SIGINT)
	})
	return nil
}

func (qc *QueryCoord) initSession() error {
	qc.session = sessionutil.NewSession(qc.loopCtx, Params.MetaRootPath, Params.EtcdEndpoints)
	if qc.session == nil {
		return fmt.Errorf("session is nil, the etcd client connection may have failed")
	}
	qc.session.Init(typeutil.QueryCoordRole, Params.Address, true)
	Params.NodeID = uint64(qc.session.ServerID)
	Params.SetLogger(qc.session.ServerID)
	return nil
}

// Init function initializes the queryCoord's meta, cluster, etcdKV and task scheduler
func (qc *QueryCoord) Init() error {
	log.Debug("query coord session info", zap.String("metaPath", Params.MetaRootPath), zap.Strings("etcdEndPoints", Params.EtcdEndpoints), zap.String("address", Params.Address))
	log.Debug("query coordinator start init")
	//connect etcd
	connectEtcdFn := func() error {
		etcdKV, err := etcdkv.NewEtcdKV(Params.EtcdEndpoints, Params.MetaRootPath)
		if err != nil {
			return err
		}
		qc.kvClient = etcdKV
		return nil
	}
	var initError error
	qc.initOnce.Do(func() {
		err := qc.initSession()
		if err != nil {
			log.Error("QueryCoord init session failed", zap.Error(err))
			initError = err
			return
		}
		log.Debug("query coordinator try to connect etcd")
		initError = retry.Do(qc.loopCtx, connectEtcdFn, retry.Attempts(300))
		if initError != nil {
			log.Debug("query coordinator try to connect etcd failed", zap.Error(initError))
			return
		}
		log.Debug("query coordinator try to connect etcd success")

		// init id allocator
		var idAllocatorKV *etcdkv.EtcdKV
		idAllocatorKV, initError = tsoutil.NewTSOKVBase(Params.EtcdEndpoints, Params.KvRootPath, "queryCoordTaskID")
		if initError != nil {
			return
		}
		idAllocator := allocator.NewGlobalIDAllocator("idTimestamp", idAllocatorKV)
		initError = idAllocator.Initialize()
		if initError != nil {
			log.Debug("query coordinator idAllocator initialize failed", zap.Error(initError))
			return
		}
		qc.idAllocator = func() (UniqueID, error) {
			return idAllocator.AllocOne()
		}

		// init meta
		qc.meta, initError = newMeta(qc.loopCtx, qc.kvClient, qc.msFactory, qc.idAllocator)
		if initError != nil {
			log.Error("query coordinator init meta failed", zap.Error(initError))
			return
		}

		// init cluster
		qc.cluster, initError = newQueryNodeCluster(qc.loopCtx, qc.meta, qc.kvClient, qc.newNodeFn, qc.session)
		if initError != nil {
			log.Error("query coordinator init cluster failed", zap.Error(initError))
			return
		}

		// init task scheduler
		qc.scheduler, initError = NewTaskScheduler(qc.loopCtx, qc.meta, qc.cluster, qc.kvClient, qc.rootCoordClient, qc.dataCoordClient, qc.indexCoordClient, qc.idAllocator)
		if initError != nil {
			log.Error("query coordinator init task scheduler failed", zap.Error(initError))
			return
		}

		// init index checker
		qc.indexChecker, initError = newIndexChecker(qc.loopCtx, qc.kvClient, qc.meta, qc.cluster, qc.scheduler, qc.rootCoordClient, qc.indexCoordClient, qc.dataCoordClient)
		if initError != nil {
			log.Error("query coordinator init index checker failed", zap.Error(initError))
			return
		}

		qc.metricsCacheManager = metricsinfo.NewMetricsCacheManager()
	})
	log.Debug("query coordinator init success")
	return initError
}

// Start function starts the goroutines to watch the meta and node updates
func (qc *QueryCoord) Start() error {
	m := map[string]interface{}{
		"PulsarAddress":  Params.PulsarAddress,
		"ReceiveBufSize": 1024,
		"PulsarBufSize":  1024}
	err := qc.msFactory.SetParams(m)
	if err != nil {
		return err
	}
	qc.scheduler.Start()
	log.Debug("start scheduler ...")

	qc.indexChecker.start()
	log.Debug("start index checker ...")

	Params.CreatedTime = time.Now()
	Params.UpdatedTime = time.Now()

	qc.loopWg.Add(1)
	go qc.watchNodeLoop()

	qc.loopWg.Add(1)
	go qc.watchHandoffSegmentLoop()

	if Params.AutoBalance {
		qc.loopWg.Add(1)
		go qc.loadBalanceSegmentLoop()
	}

	qc.UpdateStateCode(internalpb.StateCode_Healthy)

	return nil
}

// Stop function stops watching the meta and node updates
func (qc *QueryCoord) Stop() error {
	qc.UpdateStateCode(internalpb.StateCode_Abnormal)

	qc.scheduler.Close()
	log.Debug("close scheduler ...")
	qc.indexChecker.close()
	log.Debug("close index checker ...")
	qc.loopCancel()

	qc.loopWg.Wait()
	qc.session.Revoke(time.Second)
	return nil
}

// UpdateStateCode updates the status of the coord, including healthy, unhealthy
func (qc *QueryCoord) UpdateStateCode(code internalpb.StateCode) {
	qc.stateCode.Store(code)
}

// NewQueryCoord creates a QueryCoord object.
func NewQueryCoord(ctx context.Context, factory msgstream.Factory) (*QueryCoord, error) {
	rand.Seed(time.Now().UnixNano())
	queryChannels := make([]*queryChannelInfo, 0)
	channelID := len(queryChannels)
	searchPrefix := Params.SearchChannelPrefix
	searchResultPrefix := Params.SearchResultChannelPrefix
	allocatedQueryChannel := searchPrefix + "-" + strconv.FormatInt(int64(channelID), 10)
	allocatedQueryResultChannel := searchResultPrefix + "-" + strconv.FormatInt(int64(channelID), 10)

	queryChannels = append(queryChannels, &queryChannelInfo{
		requestChannel:  allocatedQueryChannel,
		responseChannel: allocatedQueryResultChannel,
	})

	ctx1, cancel := context.WithCancel(ctx)
	service := &QueryCoord{
		loopCtx:    ctx1,
		loopCancel: cancel,
		msFactory:  factory,
		newNodeFn:  newQueryNode,
	}

	service.UpdateStateCode(internalpb.StateCode_Abnormal)
	log.Debug("query coordinator", zap.Any("queryChannels", queryChannels))
	return service, nil
}

// SetRootCoord sets root coordinator's client
func (qc *QueryCoord) SetRootCoord(rootCoord types.RootCoord) error {
	if rootCoord == nil {
		return errors.New("null RootCoord interface")
	}

	qc.rootCoordClient = rootCoord
	return nil
}

// SetDataCoord sets data coordinator's client
func (qc *QueryCoord) SetDataCoord(dataCoord types.DataCoord) error {
	if dataCoord == nil {
		return errors.New("null DataCoord interface")
	}

	qc.dataCoordClient = dataCoord
	return nil
}

func (qc *QueryCoord) SetIndexCoord(indexCoord types.IndexCoord) error {
	if indexCoord == nil {
		return errors.New("null IndexCoord interface")
	}

	qc.indexCoordClient = indexCoord
	return nil
}

func (qc *QueryCoord) watchNodeLoop() {
	ctx, cancel := context.WithCancel(qc.loopCtx)
	defer cancel()
	defer qc.loopWg.Done()
	log.Debug("QueryCoord start watch node loop")

	offlineNodes, err := qc.cluster.offlineNodes()
	if err == nil {
		offlineNodeIDs := make([]int64, 0)
		for id := range offlineNodes {
			offlineNodeIDs = append(offlineNodeIDs, id)
		}
		loadBalanceSegment := &querypb.LoadBalanceRequest{
			Base: &commonpb.MsgBase{
				MsgType:  commonpb.MsgType_LoadBalanceSegments,
				SourceID: qc.session.ServerID,
			},
			SourceNodeIDs: offlineNodeIDs,
		}

		baseTask := newBaseTask(qc.loopCtx, querypb.TriggerCondition_NodeDown)
		loadBalanceTask := &loadBalanceTask{
			baseTask:           baseTask,
			LoadBalanceRequest: loadBalanceSegment,
			rootCoord:          qc.rootCoordClient,
			dataCoord:          qc.dataCoordClient,
			indexCoord:         qc.indexCoordClient,
			cluster:            qc.cluster,
			meta:               qc.meta,
		}
		//TODO::deal enqueue error
		qc.scheduler.Enqueue(loadBalanceTask)
		log.Debug("start a loadBalance task", zap.Any("task", loadBalanceTask))
	}

	qc.eventChan = qc.session.WatchServices(typeutil.QueryNodeRole, qc.cluster.getSessionVersion()+1, nil)
	for {
		select {
		case <-ctx.Done():
			return
		case event, ok := <-qc.eventChan:
			if !ok {
				return
			}
			switch event.EventType {
			case sessionutil.SessionAddEvent:
				serverID := event.Session.ServerID
				log.Debug("start add a QueryNode to cluster", zap.Any("nodeID", serverID))
				err := qc.cluster.registerNode(ctx, event.Session, serverID, disConnect)
				if err != nil {
					log.Error("QueryCoord failed to register a QueryNode", zap.Int64("nodeID", serverID), zap.String("error info", err.Error()))
				}
				qc.metricsCacheManager.InvalidateSystemInfoMetrics()
			case sessionutil.SessionDelEvent:
				serverID := event.Session.ServerID
				log.Debug("get a del event after QueryNode down", zap.Int64("nodeID", serverID))
				nodeExist := qc.cluster.hasNode(serverID)
				if !nodeExist {
					log.Error("QueryNode not exist", zap.Int64("nodeID", serverID))
					continue
				}

				qc.cluster.stopNode(serverID)
				loadBalanceSegment := &querypb.LoadBalanceRequest{
					Base: &commonpb.MsgBase{
						MsgType:  commonpb.MsgType_LoadBalanceSegments,
						SourceID: qc.session.ServerID,
					},
					SourceNodeIDs: []int64{serverID},
					BalanceReason: querypb.TriggerCondition_NodeDown,
				}

				baseTask := newBaseTask(qc.loopCtx, querypb.TriggerCondition_NodeDown)
				loadBalanceTask := &loadBalanceTask{
					baseTask:           baseTask,
					LoadBalanceRequest: loadBalanceSegment,
					rootCoord:          qc.rootCoordClient,
					dataCoord:          qc.dataCoordClient,
					indexCoord:         qc.indexCoordClient,
					cluster:            qc.cluster,
					meta:               qc.meta,
				}
				qc.metricsCacheManager.InvalidateSystemInfoMetrics()
				//TODO:: deal enqueue error
				qc.scheduler.Enqueue(loadBalanceTask)
				log.Debug("start a loadBalance task", zap.Any("task", loadBalanceTask))
			}
		}
	}
}

func (qc *QueryCoord) watchHandoffSegmentLoop() {
	ctx, cancel := context.WithCancel(qc.loopCtx)

	defer cancel()
	defer qc.loopWg.Done()
	log.Debug("QueryCoord start watch segment loop")

	watchChan := qc.kvClient.WatchWithRevision(handoffSegmentPrefix, qc.indexChecker.revision+1)

	for {
		select {
		case <-ctx.Done():
			return
		case resp := <-watchChan:
			for _, event := range resp.Events {
				segmentInfo := &querypb.SegmentInfo{}
				err := proto.Unmarshal(event.Kv.Value, segmentInfo)
				if err != nil {
					log.Error("watchHandoffSegmentLoop: unmarshal failed", zap.Any("error", err.Error()))
					continue
				}
				switch event.Type {
				case mvccpb.PUT:
					if Params.AutoHandoff && qc.indexChecker.verifyHandoffReqValid(segmentInfo) {
						qc.indexChecker.enqueueHandoffReq(segmentInfo)
						log.Debug("watchHandoffSegmentLoop: enqueue a handoff request to index checker", zap.Any("segment info", segmentInfo))
					} else {
						log.Debug("watchHandoffSegmentLoop: collection/partition has not been loaded or autoHandoff equal to false, remove req from etcd", zap.Any("segmentInfo", segmentInfo))
						buildQuerySegmentPath := fmt.Sprintf("%s/%d/%d/%d", handoffSegmentPrefix, segmentInfo.CollectionID, segmentInfo.PartitionID, segmentInfo.SegmentID)
						err = qc.kvClient.Remove(buildQuerySegmentPath)
						if err != nil {
							log.Error("watchHandoffSegmentLoop: remove handoff segment from etcd failed", zap.Error(err))
							panic(err)
						}
					}
				default:
					// do nothing
				}
			}
		}
	}
}

func (qc *QueryCoord) loadBalanceSegmentLoop() {
	ctx, cancel := context.WithCancel(qc.loopCtx)
	defer cancel()
	defer qc.loopWg.Done()
	log.Debug("QueryCoord start load balance segment loop")

	timer := time.NewTicker(time.Duration(Params.BalanceIntervalSeconds) * time.Second)

	for {
		select {
		case <-ctx.Done():
			return
		case <-timer.C:
			onlineNodes, err := qc.cluster.onlineNodes()
			if err != nil {
				log.Warn("loadBalanceSegmentLoop: there are no online QueryNode to balance")
				continue
			}
			// get mem info of online nodes from cluster
			nodeID2MemUsageRate := make(map[int64]float64)
			nodeID2MemUsage := make(map[int64]uint64)
			nodeID2TotalMem := make(map[int64]uint64)
			nodeID2SegmentInfos := make(map[int64]map[UniqueID]*querypb.SegmentInfo)
			onlineNodeIDs := make([]int64, 0)
			for nodeID := range onlineNodes {
				nodeInfo, err := qc.cluster.getNodeInfoByID(nodeID)
				if err != nil {
					log.Warn("loadBalanceSegmentLoop: get node info from query node failed", zap.Int64("nodeID", nodeID), zap.Error(err))
					delete(onlineNodes, nodeID)
					continue
				}

				updateSegmentInfoDone := true
				leastSegmentInfos := make(map[UniqueID]*querypb.SegmentInfo)
				segmentInfos := qc.meta.getSegmentInfosByNode(nodeID)
				for _, segmentInfo := range segmentInfos {
					leastInfo, err := qc.cluster.getSegmentInfoByID(ctx, segmentInfo.SegmentID)
					if err != nil {
						log.Warn("loadBalanceSegmentLoop: get segment info from query node failed", zap.Int64("nodeID", nodeID), zap.Error(err))
						delete(onlineNodes, nodeID)
						updateSegmentInfoDone = false
						break
					}
					leastSegmentInfos[segmentInfo.SegmentID] = leastInfo
				}
				if updateSegmentInfoDone {
					nodeID2MemUsageRate[nodeID] = nodeInfo.(*queryNode).memUsageRate
					nodeID2MemUsage[nodeID] = nodeInfo.(*queryNode).memUsage
					nodeID2TotalMem[nodeID] = nodeInfo.(*queryNode).totalMem
					onlineNodeIDs = append(onlineNodeIDs, nodeID)
					nodeID2SegmentInfos[nodeID] = leastSegmentInfos
				}
			}
			log.Debug("loadBalanceSegmentLoop: memory usage rate of all online query node", zap.Any("mem rate", nodeID2MemUsageRate))
			if len(onlineNodeIDs) <= 1 {
				log.Warn("loadBalanceSegmentLoop: there are too few online query nodes to balance", zap.Int64s("onlineNodeIDs", onlineNodeIDs))
				continue
			}

			// check which nodes need balance and determine which segments on these nodes need to be migrated to other nodes
			memoryInsufficient := false
			loadBalanceTasks := make([]*loadBalanceTask, 0)
			for {
				var selectedSegmentInfo *querypb.SegmentInfo
				sort.Slice(onlineNodeIDs, func(i, j int) bool {
					return nodeID2MemUsageRate[onlineNodeIDs[i]] > nodeID2MemUsageRate[onlineNodeIDs[j]]
				})

				// the memoryUsageRate of the sourceNode is higher than other query node
				sourceNodeID := onlineNodeIDs[0]
				dstNodeID := onlineNodeIDs[len(onlineNodeIDs)-1]
				memUsageRateDiff := nodeID2MemUsageRate[sourceNodeID] - nodeID2MemUsageRate[dstNodeID]
				// if memoryUsageRate of source node is greater then 90%, and the max memUsageDiff is greater than 30%
				// then migrate the segments on source node to other query nodes
				if nodeID2MemUsageRate[sourceNodeID] > Params.OverloadedMemoryThresholdPercentage ||
					memUsageRateDiff > Params.MemoryUsageMaxDifferencePercentage {
					segmentInfos := nodeID2SegmentInfos[sourceNodeID]
					// select the segment that needs balance on the source node
					selectedSegmentInfo, err = chooseSegmentToBalance(sourceNodeID, dstNodeID, segmentInfos, nodeID2MemUsage, nodeID2TotalMem, nodeID2MemUsageRate)
					if err == nil && selectedSegmentInfo != nil {
						req := &querypb.LoadBalanceRequest{
							Base: &commonpb.MsgBase{
								MsgType: commonpb.MsgType_LoadBalanceSegments,
							},
							BalanceReason:    querypb.TriggerCondition_LoadBalance,
							SourceNodeIDs:    []UniqueID{sourceNodeID},
							DstNodeIDs:       []UniqueID{dstNodeID},
							SealedSegmentIDs: []UniqueID{selectedSegmentInfo.SegmentID},
						}
						baseTask := newBaseTask(qc.loopCtx, querypb.TriggerCondition_LoadBalance)
						balanceTask := &loadBalanceTask{
							baseTask:           baseTask,
							LoadBalanceRequest: req,
							rootCoord:          qc.rootCoordClient,
							dataCoord:          qc.dataCoordClient,
							indexCoord:         qc.indexCoordClient,
							cluster:            qc.cluster,
							meta:               qc.meta,
						}
						loadBalanceTasks = append(loadBalanceTasks, balanceTask)
						nodeID2MemUsage[sourceNodeID] -= uint64(selectedSegmentInfo.MemSize)
						nodeID2MemUsage[dstNodeID] += uint64(selectedSegmentInfo.MemSize)
						nodeID2MemUsageRate[sourceNodeID] = float64(nodeID2MemUsage[sourceNodeID]) / float64(nodeID2TotalMem[sourceNodeID])
						nodeID2MemUsageRate[dstNodeID] = float64(nodeID2MemUsage[dstNodeID]) / float64(nodeID2TotalMem[dstNodeID])
						delete(nodeID2SegmentInfos[sourceNodeID], selectedSegmentInfo.SegmentID)
						nodeID2SegmentInfos[dstNodeID][selectedSegmentInfo.SegmentID] = selectedSegmentInfo
						continue
					}
				}
				if err != nil {
					// no enough memory on query nodes to balance, then notify proxy to stop insert
					memoryInsufficient = true
				}
				// if memoryInsufficient == false
				// all query node's memoryUsageRate is less than 90%, and the max memUsageDiff is less than 30%
				// this balance loop is done
				break
			}
			if !memoryInsufficient {
				for _, t := range loadBalanceTasks {
					qc.scheduler.Enqueue(t)
					log.Debug("loadBalanceSegmentLoop: enqueue a loadBalance task", zap.Any("task", t))
					err = t.waitToFinish()
					if err != nil {
						// if failed, wait for next balance loop
						// it may be that the collection/partition of the balanced segment has been released
						// it also may be other abnormal errors
						log.Error("loadBalanceSegmentLoop: balance task execute failed", zap.Any("task", t))
					} else {
						log.Debug("loadBalanceSegmentLoop: balance task execute success", zap.Any("task", t))
					}
				}
				log.Debug("loadBalanceSegmentLoop: load balance Done in this loop", zap.Any("tasks", loadBalanceTasks))
			} else {
				// no enough memory on query nodes to balance, then notify proxy to stop insert
				//TODO:: xige-16
				log.Error("loadBalanceSegmentLoop: query node has insufficient memory, stop inserting data")
			}
		}
	}
}

func chooseSegmentToBalance(sourceNodeID int64, dstNodeID int64,
	segmentInfos map[UniqueID]*querypb.SegmentInfo,
	nodeID2MemUsage map[int64]uint64,
	nodeID2TotalMem map[int64]uint64,
	nodeID2MemUsageRate map[int64]float64) (*querypb.SegmentInfo, error) {
	memoryInsufficient := true
	minMemDiffPercentage := 1.0
	var selectedSegmentInfo *querypb.SegmentInfo
	for _, info := range segmentInfos {
		dstNodeMemUsageAfterBalance := nodeID2MemUsage[dstNodeID] + uint64(info.MemSize)
		dstNodeMemUsageRateAfterBalance := float64(dstNodeMemUsageAfterBalance) / float64(nodeID2TotalMem[dstNodeID])
		// if memUsageRate of dstNode is greater than OverloadedMemoryThresholdPercentage after balance, than can't balance
		if dstNodeMemUsageRateAfterBalance < Params.OverloadedMemoryThresholdPercentage {
			memoryInsufficient = false
			sourceNodeMemUsageAfterBalance := nodeID2MemUsage[sourceNodeID] - uint64(info.MemSize)
			sourceNodeMemUsageRateAfterBalance := float64(sourceNodeMemUsageAfterBalance) / float64(nodeID2TotalMem[sourceNodeID])
			// assume all query node has same memory capacity
			// if the memUsageRateDiff between the two nodes does not become smaller after balance, there is no need for balance
			diffBeforBalance := nodeID2MemUsageRate[sourceNodeID] - nodeID2MemUsageRate[dstNodeID]
			diffAfterBalance := dstNodeMemUsageRateAfterBalance - sourceNodeMemUsageRateAfterBalance
			if diffAfterBalance < diffBeforBalance {
				if math.Abs(diffAfterBalance) < minMemDiffPercentage {
					selectedSegmentInfo = info
				}
			}
		}
	}

	if memoryInsufficient {
		return nil, errors.New("all QueryNode has insufficient memory")
	}

	return selectedSegmentInfo, nil
}