server.go

// Copyright 2015 The etcd Authors
//
// Licensed 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 etcdserver

import (
	"encoding/json"
	"expvar"
	"fmt"
	"math"
	"math/rand"
	"net/http"
	"os"
	"path"
	"path/filepath"
	"regexp"
	"sync"
	"sync/atomic"
	"time"

	"github.com/coreos/etcd/alarm"
	"github.com/coreos/etcd/auth"
	"github.com/coreos/etcd/compactor"
	"github.com/coreos/etcd/discovery"
	"github.com/coreos/etcd/etcdserver/api"
	"github.com/coreos/etcd/etcdserver/api/v2http/httptypes"
	pb "github.com/coreos/etcd/etcdserver/etcdserverpb"
	"github.com/coreos/etcd/etcdserver/membership"
	"github.com/coreos/etcd/etcdserver/stats"
	"github.com/coreos/etcd/lease"
	"github.com/coreos/etcd/mvcc"
	"github.com/coreos/etcd/mvcc/backend"
	"github.com/coreos/etcd/pkg/contention"
	"github.com/coreos/etcd/pkg/fileutil"
	"github.com/coreos/etcd/pkg/idutil"
	"github.com/coreos/etcd/pkg/pbutil"
	"github.com/coreos/etcd/pkg/runtime"
	"github.com/coreos/etcd/pkg/schedule"
	"github.com/coreos/etcd/pkg/types"
	"github.com/coreos/etcd/pkg/wait"
	"github.com/coreos/etcd/raft"
	"github.com/coreos/etcd/raft/raftpb"
	"github.com/coreos/etcd/rafthttp"
	"github.com/coreos/etcd/snap"
	"github.com/coreos/etcd/store"
	"github.com/coreos/etcd/version"
	"github.com/coreos/etcd/wal"
	"github.com/coreos/go-semver/semver"
	"github.com/coreos/pkg/capnslog"
	"golang.org/x/net/context"
)

const (
	DefaultSnapCount = 10000

	StoreClusterPrefix = "/0"
	StoreKeysPrefix    = "/1"

	// HealthInterval is the minimum time the cluster should be healthy
	// before accepting add member requests.
	HealthInterval = 5 * time.Second

	purgeFileInterval = 30 * time.Second
	// monitorVersionInterval should be smaller than the timeout
	// on the connection. Or we will not be able to reuse the connection
	// (since it will timeout).
	monitorVersionInterval = rafthttp.ConnWriteTimeout - time.Second

	databaseFilename = "db"
	// max number of in-flight snapshot messages etcdserver allows to have
	// This number is more than enough for most clusters with 5 machines.
	maxInFlightMsgSnap = 16

	releaseDelayAfterSnapshot = 30 * time.Second

	// maxPendingRevokes is the maximum number of outstanding expired lease revocations.
	maxPendingRevokes = 16
)

var (
	plog = capnslog.NewPackageLogger("github.com/coreos/etcd", "etcdserver")

	storeMemberAttributeRegexp = regexp.MustCompile(path.Join(membership.StoreMembersPrefix, "[[:xdigit:]]{1,16}", "attributes"))
)

func init() {
	rand.Seed(time.Now().UnixNano())

	expvar.Publish(
		"file_descriptor_limit",
		expvar.Func(
			func() interface{} {
				n, _ := runtime.FDLimit()
				return n
			},
		),
	)
}

type Response struct {
	Event   *store.Event
	Watcher store.Watcher
	err     error
}

type Server interface {
	// Start performs any initialization of the Server necessary for it to
	// begin serving requests. It must be called before Do or Process.
	// Start must be non-blocking; any long-running server functionality
	// should be implemented in goroutines.
	Start()
	// Stop terminates the Server and performs any necessary finalization.
	// Do and Process cannot be called after Stop has been invoked.
	Stop()
	// ID returns the ID of the Server.
	ID() types.ID
	// Leader returns the ID of the leader Server.
	Leader() types.ID
	// Do takes a request and attempts to fulfill it, returning a Response.
	Do(ctx context.Context, r pb.Request) (Response, error)
	// Process takes a raft message and applies it to the server's raft state
	// machine, respecting any timeout of the given context.
	Process(ctx context.Context, m raftpb.Message) error
	// AddMember attempts to add a member into the cluster. It will return
	// ErrIDRemoved if member ID is removed from the cluster, or return
	// ErrIDExists if member ID exists in the cluster.
	AddMember(ctx context.Context, memb membership.Member) error
	// RemoveMember attempts to remove a member from the cluster. It will
	// return ErrIDRemoved if member ID is removed from the cluster, or return
	// ErrIDNotFound if member ID is not in the cluster.
	RemoveMember(ctx context.Context, id uint64) error

	// UpdateMember attempts to update an existing member in the cluster. It will
	// return ErrIDNotFound if the member ID does not exist.
	UpdateMember(ctx context.Context, updateMemb membership.Member) error

	// ClusterVersion is the cluster-wide minimum major.minor version.
	// Cluster version is set to the min version that an etcd member is
	// compatible with when first bootstrap.
	//
	// ClusterVersion is nil until the cluster is bootstrapped (has a quorum).
	//
	// During a rolling upgrades, the ClusterVersion will be updated
	// automatically after a sync. (5 second by default)
	//
	// The API/raft component can utilize ClusterVersion to determine if
	// it can accept a client request or a raft RPC.
	// NOTE: ClusterVersion might be nil when etcd 2.1 works with etcd 2.0 and
	// the leader is etcd 2.0. etcd 2.0 leader will not update clusterVersion since
	// this feature is introduced post 2.0.
	ClusterVersion() *semver.Version
}

// EtcdServer is the production implementation of the Server interface
type EtcdServer struct {
	// inflightSnapshots holds count the number of snapshots currently inflight.
	inflightSnapshots int64  // must use atomic operations to access; keep 64-bit aligned.
	appliedIndex      uint64 // must use atomic operations to access; keep 64-bit aligned.
	committedIndex    uint64 // must use atomic operations to access; keep 64-bit aligned.
	// consistIndex used to hold the offset of current executing entry
	// It is initialized to 0 before executing any entry.
	consistIndex consistentIndex // must use atomic operations to access; keep 64-bit aligned.
	Cfg          *ServerConfig

	readych chan struct{}
	r       raftNode

	snapCount uint64

	w wait.Wait

	readMu sync.RWMutex
	// read routine notifies etcd server that it waits for reading by sending an empty struct to
	// readwaitC
	readwaitc chan struct{}
	// readNotifier is used to notify the read routine that it can process the request
	// when there is no error
	readNotifier *notifier

	// stop signals the run goroutine should shutdown.
	stop chan struct{}
	// stopping is closed by run goroutine on shutdown.
	stopping chan struct{}
	// done is closed when all goroutines from start() complete.
	done chan struct{}

	errorc     chan error
	id         types.ID
	attributes membership.Attributes

	cluster *membership.RaftCluster

	store store.Store

	applyV2 ApplierV2

	// applyV3 is the applier with auth and quotas
	applyV3 applierV3
	// applyV3Base is the core applier without auth or quotas
	applyV3Base applierV3
	applyWait   wait.WaitTime

	kv         mvcc.ConsistentWatchableKV
	lessor     lease.Lessor
	bemu       sync.Mutex
	be         backend.Backend
	authStore  auth.AuthStore
	alarmStore *alarm.AlarmStore

	stats  *stats.ServerStats
	lstats *stats.LeaderStats

	SyncTicker <-chan time.Time
	// compactor is used to auto-compact the KV.
	compactor *compactor.Periodic

	// peerRt used to send requests (version, lease) to peers.
	peerRt   http.RoundTripper
	reqIDGen *idutil.Generator

	// forceVersionC is used to force the version monitor loop
	// to detect the cluster version immediately.
	forceVersionC chan struct{}

	// wgMu blocks concurrent waitgroup mutation while server stopping
	wgMu sync.RWMutex
	// wg is used to wait for the go routines that depends on the server state
	// to exit when stopping the server.
	wg sync.WaitGroup
}

// NewServer creates a new EtcdServer from the supplied configuration. The
// configuration is considered static for the lifetime of the EtcdServer.
func NewServer(cfg *ServerConfig) (srv *EtcdServer, err error) {
	st := store.New(StoreClusterPrefix, StoreKeysPrefix)

	var (
		w  *wal.WAL
		n  raft.Node
		s  *raft.MemoryStorage
		id types.ID
		cl *membership.RaftCluster
	)

	if terr := fileutil.TouchDirAll(cfg.DataDir); terr != nil {
		return nil, fmt.Errorf("cannot access data directory: %v", terr)
	}

	haveWAL := wal.Exist(cfg.WALDir())

	if err = fileutil.TouchDirAll(cfg.SnapDir()); err != nil {
		plog.Fatalf("create snapshot directory error: %v", err)
	}
	ss := snap.New(cfg.SnapDir())

	bepath := filepath.Join(cfg.SnapDir(), databaseFilename)
	beExist := fileutil.Exist(bepath)

	var be backend.Backend
	beOpened := make(chan struct{})
	go func() {
		be = backend.NewDefaultBackend(bepath)
		beOpened <- struct{}{}
	}()

	select {
	case <-beOpened:
	case <-time.After(time.Second):
		plog.Warningf("another etcd process is running with the same data dir and holding the file lock.")
		plog.Warningf("waiting for it to exit before starting...")
		<-beOpened
	}

	defer func() {
		if err != nil {
			be.Close()
		}
	}()

	prt, err := rafthttp.NewRoundTripper(cfg.PeerTLSInfo, cfg.peerDialTimeout())
	if err != nil {
		return nil, err
	}
	var (
		remotes  []*membership.Member
		snapshot *raftpb.Snapshot
	)

	switch {
	case !haveWAL && !cfg.NewCluster:
		if err = cfg.VerifyJoinExisting(); err != nil {
			return nil, err
		}
		cl, err = membership.NewClusterFromURLsMap(cfg.InitialClusterToken, cfg.InitialPeerURLsMap)
		if err != nil {
			return nil, err
		}
		existingCluster, gerr := GetClusterFromRemotePeers(getRemotePeerURLs(cl, cfg.Name), prt)
		if gerr != nil {
			return nil, fmt.Errorf("cannot fetch cluster info from peer urls: %v", gerr)
		}
		if err = membership.ValidateClusterAndAssignIDs(cl, existingCluster); err != nil {
			return nil, fmt.Errorf("error validating peerURLs %s: %v", existingCluster, err)
		}
		if !isCompatibleWithCluster(cl, cl.MemberByName(cfg.Name).ID, prt) {
			return nil, fmt.Errorf("incompatible with current running cluster")
		}

		remotes = existingCluster.Members()
		cl.SetID(existingCluster.ID())
		cl.SetStore(st)
		cl.SetBackend(be)
		cfg.Print()
		id, n, s, w = startNode(cfg, cl, nil)
	case !haveWAL && cfg.NewCluster:
		if err = cfg.VerifyBootstrap(); err != nil {
			return nil, err
		}
		cl, err = membership.NewClusterFromURLsMap(cfg.InitialClusterToken, cfg.InitialPeerURLsMap)
		if err != nil {
			return nil, err
		}
		m := cl.MemberByName(cfg.Name)
		if isMemberBootstrapped(cl, cfg.Name, prt, cfg.bootstrapTimeout()) {
			return nil, fmt.Errorf("member %s has already been bootstrapped", m.ID)
		}
		if cfg.ShouldDiscover() {
			var str string
			str, err = discovery.JoinCluster(cfg.DiscoveryURL, cfg.DiscoveryProxy, m.ID, cfg.InitialPeerURLsMap.String())
			if err != nil {
				return nil, &DiscoveryError{Op: "join", Err: err}
			}
			var urlsmap types.URLsMap
			urlsmap, err = types.NewURLsMap(str)
			if err != nil {
				return nil, err
			}
			if checkDuplicateURL(urlsmap) {
				return nil, fmt.Errorf("discovery cluster %s has duplicate url", urlsmap)
			}
			if cl, err = membership.NewClusterFromURLsMap(cfg.InitialClusterToken, urlsmap); err != nil {
				return nil, err
			}
		}
		cl.SetStore(st)
		cl.SetBackend(be)
		cfg.PrintWithInitial()
		id, n, s, w = startNode(cfg, cl, cl.MemberIDs())
	case haveWAL:
		if err = fileutil.IsDirWriteable(cfg.MemberDir()); err != nil {
			return nil, fmt.Errorf("cannot write to member directory: %v", err)
		}

		if err = fileutil.IsDirWriteable(cfg.WALDir()); err != nil {
			return nil, fmt.Errorf("cannot write to WAL directory: %v", err)
		}

		if cfg.ShouldDiscover() {
			plog.Warningf("discovery token ignored since a cluster has already been initialized. Valid log found at %q", cfg.WALDir())
		}
		snapshot, err = ss.Load()
		if err != nil && err != snap.ErrNoSnapshot {
			return nil, err
		}
		if snapshot != nil {
			if err = st.Recovery(snapshot.Data); err != nil {
				plog.Panicf("recovered store from snapshot error: %v", err)
			}
			plog.Infof("recovered store from snapshot at index %d", snapshot.Metadata.Index)
		}
		cfg.Print()
		if !cfg.ForceNewCluster {
			id, cl, n, s, w = restartNode(cfg, snapshot)
		} else {
			id, cl, n, s, w = restartAsStandaloneNode(cfg, snapshot)
		}
		cl.SetStore(st)
		cl.SetBackend(be)
		cl.Recover(api.UpdateCapability)
		if cl.Version() != nil && !cl.Version().LessThan(semver.Version{Major: 3}) && !beExist {
			os.RemoveAll(bepath)
			return nil, fmt.Errorf("database file (%v) of the backend is missing", bepath)
		}
	default:
		return nil, fmt.Errorf("unsupported bootstrap config")
	}

	if terr := fileutil.TouchDirAll(cfg.MemberDir()); terr != nil {
		return nil, fmt.Errorf("cannot access member directory: %v", terr)
	}

	sstats := &stats.ServerStats{
		Name: cfg.Name,
		ID:   id.String(),
	}
	sstats.Initialize()
	lstats := stats.NewLeaderStats(id.String())

	heartbeat := time.Duration(cfg.TickMs) * time.Millisecond
	srv = &EtcdServer{
		readych:   make(chan struct{}),
		Cfg:       cfg,
		snapCount: cfg.SnapCount,
		errorc:    make(chan error, 1),
		store:     st,
		r: raftNode{
			isIDRemoved: func(id uint64) bool { return cl.IsIDRemoved(types.ID(id)) },
			Node:        n,
			ticker:      time.Tick(heartbeat),
			// set up contention detectors for raft heartbeat message.
			// expect to send a heartbeat within 2 heartbeat intervals.
			td:          contention.NewTimeoutDetector(2 * heartbeat),
			heartbeat:   heartbeat,
			raftStorage: s,
			storage:     NewStorage(w, ss),
			msgSnapC:    make(chan raftpb.Message, maxInFlightMsgSnap),
			readStateC:  make(chan raft.ReadState, 1),
		},
		id:            id,
		attributes:    membership.Attributes{Name: cfg.Name, ClientURLs: cfg.ClientURLs.StringSlice()},
		cluster:       cl,
		stats:         sstats,
		lstats:        lstats,
		SyncTicker:    time.Tick(500 * time.Millisecond),
		peerRt:        prt,
		reqIDGen:      idutil.NewGenerator(uint16(id), time.Now()),
		forceVersionC: make(chan struct{}),
	}

	srv.applyV2 = &applierV2store{store: srv.store, cluster: srv.cluster}

	srv.be = be
	minTTL := time.Duration((3*cfg.ElectionTicks)/2) * heartbeat

	// always recover lessor before kv. When we recover the mvcc.KV it will reattach keys to its leases.
	// If we recover mvcc.KV first, it will attach the keys to the wrong lessor before it recovers.
	srv.lessor = lease.NewLessor(srv.be, int64(math.Ceil(minTTL.Seconds())))
	srv.kv = mvcc.New(srv.be, srv.lessor, &srv.consistIndex)
	if beExist {
		kvindex := srv.kv.ConsistentIndex()
		// TODO: remove kvindex != 0 checking when we do not expect users to upgrade
		// etcd from pre-3.0 release.
		if snapshot != nil && kvindex < snapshot.Metadata.Index {
			if kvindex != 0 {
				return nil, fmt.Errorf("database file (%v index %d) does not match with snapshot (index %d).", bepath, kvindex, snapshot.Metadata.Index)
			}
			plog.Warningf("consistent index never saved (snapshot index=%d)", snapshot.Metadata.Index)
		}
	}
	srv.consistIndex.setConsistentIndex(srv.kv.ConsistentIndex())

	srv.authStore = auth.NewAuthStore(srv.be,
		func(index uint64) <-chan struct{} {
			return srv.applyWait.Wait(index)
		})
	if h := cfg.AutoCompactionRetention; h != 0 {
		srv.compactor = compactor.NewPeriodic(h, srv.kv, srv)
		srv.compactor.Run()
	}

	srv.applyV3Base = &applierV3backend{srv}
	if err = srv.restoreAlarms(); err != nil {
		return nil, err
	}

	// TODO: move transport initialization near the definition of remote
	tr := &rafthttp.Transport{
		TLSInfo:     cfg.PeerTLSInfo,
		DialTimeout: cfg.peerDialTimeout(),
		ID:          id,
		URLs:        cfg.PeerURLs,
		ClusterID:   cl.ID(),
		Raft:        srv,
		Snapshotter: ss,
		ServerStats: sstats,
		LeaderStats: lstats,
		ErrorC:      srv.errorc,
	}
	if err = tr.Start(); err != nil {
		return nil, err
	}
	// add all remotes into transport
	for _, m := range remotes {
		if m.ID != id {
			tr.AddRemote(m.ID, m.PeerURLs)
		}
	}
	for _, m := range cl.Members() {
		if m.ID != id {
			tr.AddPeer(m.ID, m.PeerURLs)
		}
	}
	srv.r.transport = tr

	return srv, nil
}

func (s *EtcdServer) adjustTicks() {
	clusterN := len(s.cluster.Members())

	// single-node fresh start, or single-node recovers from snapshot
	if clusterN == 1 {
		ticks := s.Cfg.ElectionTicks - 1
		plog.Infof("%s as single-node; fast-forwarding %d ticks (election ticks %d)", s.ID(), ticks, s.Cfg.ElectionTicks)
		s.r.advanceTicks(ticks)
		return
	}

	// retry up to "rafthttp.ConnReadTimeout", which is 5-sec
	// until peer connection reports; otherwise:
	// 1. all connections failed, or
	// 2. no active peers, or
	// 3. restarted single-node with no snapshot
	// then, do nothing, because advancing ticks would have no effect
	waitTime := rafthttp.ConnReadTimeout
	itv := 50 * time.Millisecond
	for i := int64(0); i < int64(waitTime/itv); i++ {
		select {
		case <-time.After(itv):
		case <-s.stopping:
			return
		}

		peerN := s.r.transport.ActivePeers()
		if peerN > 1 {
			// multi-node received peer connection reports
			// adjust ticks, in case slow leader message receive
			ticks := s.Cfg.ElectionTicks - 2
			plog.Infof("%s initialzed peer connection; fast-forwarding %d ticks (election ticks %d) with %d active peer(s)", s.ID(), ticks, s.Cfg.ElectionTicks, peerN)
			s.r.advanceTicks(ticks)
			return
		}
	}
}

// Start prepares and starts server in a new goroutine. It is no longer safe to
// modify a server's fields after it has been sent to Start.
// It also starts a goroutine to publish its server information.
func (s *EtcdServer) Start() {
	s.start()
	s.goAttach(func() { s.adjustTicks() })
	s.goAttach(func() { s.publish(s.Cfg.ReqTimeout()) })
	s.goAttach(s.purgeFile)
	s.goAttach(func() { monitorFileDescriptor(s.stopping) })
	s.goAttach(s.monitorVersions)
	s.goAttach(s.linearizableReadLoop)
}

// start prepares and starts server in a new goroutine. It is no longer safe to
// modify a server's fields after it has been sent to Start.
// This function is just used for testing.
func (s *EtcdServer) start() {
	if s.snapCount == 0 {
		plog.Infof("set snapshot count to default %d", DefaultSnapCount)
		s.snapCount = DefaultSnapCount
	}
	s.w = wait.New()
	s.applyWait = wait.NewTimeList()
	s.done = make(chan struct{})
	s.stop = make(chan struct{})
	s.stopping = make(chan struct{})
	s.readwaitc = make(chan struct{}, 1)
	s.readNotifier = newNotifier()
	if s.ClusterVersion() != nil {
		plog.Infof("starting server... [version: %v, cluster version: %v]", version.Version, version.Cluster(s.ClusterVersion().String()))
	} else {
		plog.Infof("starting server... [version: %v, cluster version: to_be_decided]", version.Version)
	}
	// TODO: if this is an empty log, writes all peer infos
	// into the first entry
	go s.run()
}

func (s *EtcdServer) purgeFile() {
	var serrc, werrc <-chan error
	if s.Cfg.MaxSnapFiles > 0 {
		serrc = fileutil.PurgeFile(s.Cfg.SnapDir(), "snap", s.Cfg.MaxSnapFiles, purgeFileInterval, s.done)
	}
	if s.Cfg.MaxWALFiles > 0 {
		werrc = fileutil.PurgeFile(s.Cfg.WALDir(), "wal", s.Cfg.MaxWALFiles, purgeFileInterval, s.done)
	}
	select {
	case e := <-werrc:
		plog.Fatalf("failed to purge wal file %v", e)
	case e := <-serrc:
		plog.Fatalf("failed to purge snap file %v", e)
	case <-s.stopping:
		return
	}
}

func (s *EtcdServer) ID() types.ID { return s.id }

func (s *EtcdServer) Cluster() *membership.RaftCluster { return s.cluster }

func (s *EtcdServer) RaftHandler() http.Handler { return s.r.transport.Handler() }

func (s *EtcdServer) Lessor() lease.Lessor { return s.lessor }

func (s *EtcdServer) ApplyWait() <-chan struct{} { return s.applyWait.Wait(s.getCommittedIndex()) }

func (s *EtcdServer) Process(ctx context.Context, m raftpb.Message) error {
	if s.cluster.IsIDRemoved(types.ID(m.From)) {
		plog.Warningf("reject message from removed member %s", types.ID(m.From).String())
		return httptypes.NewHTTPError(http.StatusForbidden, "cannot process message from removed member")
	}
	if m.Type == raftpb.MsgApp {
		s.stats.RecvAppendReq(types.ID(m.From).String(), m.Size())
	}
	return s.r.Step(ctx, m)
}

func (s *EtcdServer) IsIDRemoved(id uint64) bool { return s.cluster.IsIDRemoved(types.ID(id)) }

func (s *EtcdServer) ReportUnreachable(id uint64) { s.r.ReportUnreachable(id) }

// ReportSnapshot reports snapshot sent status to the raft state machine,
// and clears the used snapshot from the snapshot store.
func (s *EtcdServer) ReportSnapshot(id uint64, status raft.SnapshotStatus) {
	s.r.ReportSnapshot(id, status)
}

type etcdProgress struct {
	confState raftpb.ConfState
	snapi     uint64
	appliedt  uint64
	appliedi  uint64
}

// raftReadyHandler contains a set of EtcdServer operations to be called by raftNode,
// and helps decouple state machine logic from Raft algorithms.
// TODO: add a state machine interface to apply the commit entries and do snapshot/recover
type raftReadyHandler struct {
	updateLeadership     func()
	updateCommittedIndex func(uint64)
}

func (s *EtcdServer) run() {
	snapshot, err := s.r.raftStorage.Snapshot()
	if err != nil {
		plog.Panicf("get snapshot from raft storage error: %v", err)
	}

	var (
		smu   sync.RWMutex
		syncC <-chan time.Time
	)
	setSyncC := func(ch <-chan time.Time) {
		smu.Lock()
		syncC = ch
		smu.Unlock()
	}
	getSyncC := func() (ch <-chan time.Time) {
		smu.RLock()
		ch = syncC
		smu.RUnlock()
		return
	}
	rh := &raftReadyHandler{
		updateLeadership: func() {
			if !s.isLeader() {
				if s.lessor != nil {
					s.lessor.Demote()
				}
				if s.compactor != nil {
					s.compactor.Pause()
				}
				setSyncC(nil)
			} else {
				setSyncC(s.SyncTicker)
				if s.compactor != nil {
					s.compactor.Resume()
				}
			}

			// TODO: remove the nil checking
			// current test utility does not provide the stats
			if s.stats != nil {
				s.stats.BecomeLeader()
			}
			if s.r.td != nil {
				s.r.td.Reset()
			}
		},
		updateCommittedIndex: func(ci uint64) {
			cci := s.getCommittedIndex()
			if ci > cci {
				s.setCommittedIndex(ci)
			}
		},
	}
	s.r.start(rh)

	// asynchronously accept apply packets, dispatch progress in-order
	sched := schedule.NewFIFOScheduler()
	ep := etcdProgress{
		confState: snapshot.Metadata.ConfState,
		snapi:     snapshot.Metadata.Index,
		appliedt:  snapshot.Metadata.Term,
		appliedi:  snapshot.Metadata.Index,
	}

	defer func() {
		s.wgMu.Lock() // block concurrent waitgroup adds in goAttach while stopping
		close(s.stopping)
		s.wgMu.Unlock()

		sched.Stop()

		// wait for gouroutines before closing raft so wal stays open
		s.wg.Wait()

		// must stop raft after scheduler-- etcdserver can leak rafthttp pipelines
		// by adding a peer after raft stops the transport
		s.r.stop()

		// kv, lessor and backend can be nil if running without v3 enabled
		// or running unit tests.
		if s.lessor != nil {
			s.lessor.Stop()
		}
		if s.kv != nil {
			s.kv.Close()
		}
		if s.authStore != nil {
			s.authStore.Close()
		}
		if s.be != nil {
			s.be.Close()
		}
		if s.compactor != nil {
			s.compactor.Stop()
		}
		close(s.done)
	}()

	var expiredLeaseC <-chan []*lease.Lease
	if s.lessor != nil {
		expiredLeaseC = s.lessor.ExpiredLeasesC()
	}

	for {
		select {
		case ap := <-s.r.apply():
			f := func(context.Context) { s.applyAll(&ep, &ap) }
			sched.Schedule(f)
		case leases := <-expiredLeaseC:
			s.goAttach(func() {
				// Increases throughput of expired leases deletion process through parallelization
				c := make(chan struct{}, maxPendingRevokes)
				for _, lease := range leases {
					select {
					case c <- struct{}{}:
					case <-s.stopping:
						return
					}
					lid := lease.ID
					s.goAttach(func() {
						s.LeaseRevoke(context.TODO(), &pb.LeaseRevokeRequest{ID: int64(lid)})
						<-c
					})
				}
			})
		case err := <-s.errorc:
			plog.Errorf("%s", err)
			plog.Infof("the data-dir used by this member must be removed.")
			return
		case <-getSyncC():
			if s.store.HasTTLKeys() {
				s.sync(s.Cfg.ReqTimeout())
			}
		case <-s.stop:
			return
		}
	}
}

func (s *EtcdServer) applyAll(ep *etcdProgress, apply *apply) {
	s.applySnapshot(ep, apply)
	st := time.Now()
	s.applyEntries(ep, apply)
	d := time.Since(st)
	entriesNum := len(apply.entries)
	if entriesNum != 0 && d > time.Duration(entriesNum)*warnApplyDuration {
		plog.Warningf("apply entries took too long [%v for %d entries]", d, len(apply.entries))
		plog.Warningf("avoid queries with large range/delete range!")
	}
	proposalsApplied.Set(float64(ep.appliedi))
	s.applyWait.Trigger(ep.appliedi)
	// wait for the raft routine to finish the disk writes before triggering a
	// snapshot. or applied index might be greater than the last index in raft
	// storage, since the raft routine might be slower than apply routine.
	<-apply.raftDone

	s.triggerSnapshot(ep)
	select {
	// snapshot requested via send()
	case m := <-s.r.msgSnapC:
		merged := s.createMergedSnapshotMessage(m, ep.appliedt, ep.appliedi, ep.confState)
		s.sendMergedSnap(merged)
	default:
	}
}

func (s *EtcdServer) applySnapshot(ep *etcdProgress, apply *apply) {
	if raft.IsEmptySnap(apply.snapshot) {
		return
	}

	plog.Infof("applying snapshot at index %d...", ep.snapi)
	defer plog.Infof("finished applying incoming snapshot at index %d", ep.snapi)

	if apply.snapshot.Metadata.Index <= ep.appliedi {
		plog.Panicf("snapshot index [%d] should > appliedi[%d] + 1",
			apply.snapshot.Metadata.Index, ep.appliedi)
	}

	snapfn, err := s.r.storage.DBFilePath(apply.snapshot.Metadata.Index)
	if err != nil {
		plog.Panicf("get database snapshot file path error: %v", err)
	}

	fn := filepath.Join(s.Cfg.SnapDir(), databaseFilename)
	if err := os.Rename(snapfn, fn); err != nil {
		plog.Panicf("rename snapshot file error: %v", err)
	}

	newbe := backend.NewDefaultBackend(fn)

	// always recover lessor before kv. When we recover the mvcc.KV it will reattach keys to its leases.
	// If we recover mvcc.KV first, it will attach the keys to the wrong lessor before it recovers.
	if s.lessor != nil {
		plog.Info("recovering lessor...")
		s.lessor.Recover(newbe, s.kv)
		plog.Info("finished recovering lessor")
	}

	plog.Info("restoring mvcc store...")

	if err := s.kv.Restore(newbe); err != nil {
		plog.Panicf("restore KV error: %v", err)
	}
	s.consistIndex.setConsistentIndex(s.kv.ConsistentIndex())

	plog.Info("finished restoring mvcc store")

	// Closing old backend might block until all the txns
	// on the backend are finished.
	// We do not want to wait on closing the old backend.
	s.bemu.Lock()
	oldbe := s.be
	go func() {
		plog.Info("closing old backend...")
		defer plog.Info("finished closing old backend")

		if err := oldbe.Close(); err != nil {
			plog.Panicf("close backend error: %v", err)
		}
	}()

	s.be = newbe
	s.bemu.Unlock()

	plog.Info("recovering alarms...")
	if err := s.restoreAlarms(); err != nil {
		plog.Panicf("restore alarms error: %v", err)
	}
	plog.Info("finished recovering alarms")

	if s.authStore != nil {
		plog.Info("recovering auth store...")
		s.authStore.Recover(newbe)
		plog.Info("finished recovering auth store")
	}

	plog.Info("recovering store v2...")
	if err := s.store.Recovery(apply.snapshot.Data); err != nil {
		plog.Panicf("recovery store error: %v", err)
	}
	plog.Info("finished recovering store v2")

	s.cluster.SetBackend(s.be)
	plog.Info("recovering cluster configuration...")
	s.cluster.Recover(api.UpdateCapability)
	plog.Info("finished recovering cluster configuration")

	plog.Info("removing old peers from network...")
	// recover raft transport
	s.r.transport.RemoveAllPeers()
	plog.Info("finished removing old peers from network")

	plog.Info("adding peers from new cluster configuration into network...")
	for _, m := range s.cluster.Members() {
		if m.ID == s.ID() {
			continue
		}
		s.r.transport.AddPeer(m.ID, m.PeerURLs)
	}
	plog.Info("finished adding peers from new cluster configuration into network...")

	ep.appliedt = apply.snapshot.Metadata.Term
	ep.appliedi = apply.snapshot.Metadata.Index
	ep.snapi = ep.appliedi
	ep.confState = apply.snapshot.Metadata.ConfState
}

func (s *EtcdServer) applyEntries(ep *etcdProgress, apply *apply) {
	if len(apply.entries) == 0 {
		return
	}
	firsti := apply.entries[0].Index
	if firsti > ep.appliedi+1 {
		plog.Panicf("first index of committed entry[%d] should <= appliedi[%d] + 1", firsti, ep.appliedi)
	}
	var ents []raftpb.Entry
	if ep.appliedi+1-firsti < uint64(len(apply.entries)) {
		ents = apply.entries[ep.appliedi+1-firsti:]
	}
	if len(ents) == 0 {
		return
	}
	var shouldstop bool
	if ep.appliedt, ep.appliedi, shouldstop = s.apply(ents, &ep.confState); shouldstop {
		go s.stopWithDelay(10*100*time.Millisecond, fmt.Errorf("the member has been permanently removed from the cluster"))
	}
}

func (s *EtcdServer) triggerSnapshot(ep *etcdProgress) {
	if ep.appliedi-ep.snapi <= s.snapCount {
		return
	}

	plog.Infof("start to snapshot (applied: %d, lastsnap: %d)", ep.appliedi, ep.snapi)
	s.snapshot(ep.appliedi, ep.confState)
	ep.snapi = ep.appliedi
}

func (s *EtcdServer) isMultiNode() bool {
	return s.cluster != nil && len(s.cluster.MemberIDs()) > 1
}

func (s *EtcdServer) isLeader() bool {
	return uint64(s.ID()) == s.Lead()
}

// transferLeadership transfers the leader to the given transferee.
// TODO: maybe expose to client?
func (s *EtcdServer) transferLeadership(ctx context.Context, lead, transferee uint64) error {
	now := time.Now()
	interval := time.Duration(s.Cfg.TickMs) * time.Millisecond

	plog.Infof("%s starts leadership transfer from %s to %s", s.ID(), types.ID(lead), types.ID(transferee))
	s.r.TransferLeadership(ctx, lead, transferee)
	for s.Lead() != transferee {
		select {
		case <-ctx.Done(): // time out
			return ErrTimeoutLeaderTransfer
		case <-time.After(interval):
		}
	}

	// TODO: drain all requests, or drop all messages to the old leader

	plog.Infof("%s finished leadership transfer from %s to %s (took %v)", s.ID(), types.ID(lead), types.ID(transferee), time.Since(now))
	return nil
}

// TransferLeadership transfers the leader to the chosen transferee.
func (s *EtcdServer) TransferLeadership() error {
	if !s.isLeader() {
		plog.Printf("skipped leadership transfer for stopping non-leader member")
		return nil
	}

	if !s.isMultiNode() {
		plog.Printf("skipped leadership transfer for single member cluster")
		return nil
	}

	transferee, ok := longestConnected(s.r.transport, s.cluster.MemberIDs())
	if !ok {
		return ErrUnhealthy
	}

	tm := s.Cfg.ReqTimeout()
	ctx, cancel := context.WithTimeout(context.TODO(), tm)
	err := s.transferLeadership(ctx, s.Lead(), uint64(transferee))
	cancel()
	return err
}

// HardStop stops the server without coordination with other members in the cluster.
func (s *EtcdServer) HardStop() {
	select {
	case s.stop <- struct{}{}:
	case <-s.done:
		return
	}
	<-s.done
}

// Stop stops the server gracefully, and shuts down the running goroutine.
// Stop should be called after a Start(s), otherwise it will block forever.
// When stopping leader, Stop transfers its leadership to one of its peers
// before stopping the server.
func (s *EtcdServer) Stop() {
	if err := s.TransferLeadership(); err != nil {
		plog.Warningf("%s failed to transfer leadership (%v)", s.ID(), err)
	}
	s.HardStop()
}

// ReadyNotify returns a channel that will be closed when the server
// is ready to serve client requests
func (s *EtcdServer) ReadyNotify() <-chan struct{} { return s.readych }

func (s *EtcdServer) stopWithDelay(d time.Duration, err error) {
	select {
	case <-time.After(d):
	case <-s.done:
	}
	select {
	case s.errorc <- err:
	default:
	}
}

// StopNotify returns a channel that receives a empty struct
// when the server is stopped.
func (s *EtcdServer) StopNotify() <-chan struct{} { return s.done }

func (s *EtcdServer) SelfStats() []byte { return s.stats.JSON() }

func (s *EtcdServer) LeaderStats() []byte {
	lead := atomic.LoadUint64(&s.r.lead)
	if lead != uint64(s.id) {
		return nil
	}
	return s.lstats.JSON()
}

func (s *EtcdServer) StoreStats() []byte { return s.store.JsonStats() }

func (s *EtcdServer) checkMembershipOperationPermission(ctx context.Context) error {
	if s.authStore == nil {
		// In the context of ordinal etcd process, s.authStore will never be nil.
		// This branch is for handling cases in server_test.go
		return nil
	}

	// Note that this permission check is done in the API layer,
	// so TOCTOU problem can be caused potentially in a schedule like this:
	// update membership with user A -> revoke root role of A -> apply membership change
	// in the state machine layer
	// However, both of membership change and role management requires the root privilege.
	// So careful operation by admins can prevent the problem.
	authInfo, err := s.AuthStore().AuthInfoFromCtx(ctx)
	if err != nil {
		return err
	}

	return s.AuthStore().IsAdminPermitted(authInfo)
}

func (s *EtcdServer) AddMember(ctx context.Context, memb membership.Member) error {
	if err := s.checkMembershipOperationPermission(ctx); err != nil {
		return err
	}

	if s.Cfg.StrictReconfigCheck {
		// by default StrictReconfigCheck is enabled; reject new members if unhealthy
		if !s.cluster.IsReadyToAddNewMember() {
			plog.Warningf("not enough started members, rejecting member add %+v", memb)
			return ErrNotEnoughStartedMembers
		}
		if !isConnectedFullySince(s.r.transport, time.Now().Add(-HealthInterval), s.ID(), s.cluster.Members()) {
			plog.Warningf("not healthy for reconfigure, rejecting member add %+v", memb)
			return ErrUnhealthy
		}
	}

	// TODO: move Member to protobuf type
	b, err := json.Marshal(memb)
	if err != nil {
		return err
	}
	cc := raftpb.ConfChange{
		Type:    raftpb.ConfChangeAddNode,
		NodeID:  uint64(memb.ID),
		Context: b,
	}
	return s.configure(ctx, cc)
}

func (s *EtcdServer) RemoveMember(ctx context.Context, id uint64) error {
	if err := s.checkMembershipOperationPermission(ctx); err != nil {
		return err
	}

	// by default StrictReconfigCheck is enabled; reject removal if leads to quorum loss
	if err := s.mayRemoveMember(types.ID(id)); err != nil {
		return err
	}

	cc := raftpb.ConfChange{
		Type:   raftpb.ConfChangeRemoveNode,
		NodeID: id,
	}
	return s.configure(ctx, cc)
}

func (s *EtcdServer) mayRemoveMember(id types.ID) error {
	if !s.Cfg.StrictReconfigCheck {
		return nil
	}

	if !s.cluster.IsReadyToRemoveMember(uint64(id)) {
		plog.Warningf("not enough started members, rejecting remove member %s", id)
		return ErrNotEnoughStartedMembers
	}

	// downed member is safe to remove since it's not part of the active quorum
	if t := s.r.transport.ActiveSince(id); id != s.ID() && t.IsZero() {
		return nil
	}

	// protect quorum if some members are down
	m := s.cluster.Members()
	active := numConnectedSince(s.r.transport, time.Now().Add(-HealthInterval), s.ID(), m)
	if (active - 1) < 1+((len(m)-1)/2) {
		plog.Warningf("reconfigure breaks active quorum, rejecting remove member %s", id)
		return ErrUnhealthy
	}

	return nil
}

func (s *EtcdServer) UpdateMember(ctx context.Context, memb membership.Member) error {
	b, merr := json.Marshal(memb)
	if merr != nil {
		return merr
	}

	if err := s.checkMembershipOperationPermission(ctx); err != nil {
		return err
	}
	cc := raftpb.ConfChange{
		Type:    raftpb.ConfChangeUpdateNode,
		NodeID:  uint64(memb.ID),
		Context: b,
	}
	return s.configure(ctx, cc)
}

// Implement the RaftTimer interface

func (s *EtcdServer) Index() uint64 { return atomic.LoadUint64(&s.r.index) }

func (s *EtcdServer) Term() uint64 { return atomic.LoadUint64(&s.r.term) }

// Lead is only for testing purposes.
// TODO: add Raft server interface to expose raft related info:
// Index, Term, Lead, Committed, Applied, LastIndex, etc.
func (s *EtcdServer) Lead() uint64 { return atomic.LoadUint64(&s.r.lead) }

func (s *EtcdServer) Leader() types.ID { return types.ID(s.Lead()) }

// configure sends a configuration change through consensus and
// then waits for it to be applied to the server. It
// will block until the change is performed or there is an error.
func (s *EtcdServer) configure(ctx context.Context, cc raftpb.ConfChange) error {
	cc.ID = s.reqIDGen.Next()
	ch := s.w.Register(cc.ID)
	start := time.Now()
	if err := s.r.ProposeConfChange(ctx, cc); err != nil {
		s.w.Trigger(cc.ID, nil)
		return err
	}
	select {
	case x := <-ch:
		if err, ok := x.(error); ok {
			return err
		}
		if x != nil {
			plog.Panicf("return type should always be error")
		}
		return nil
	case <-ctx.Done():
		s.w.Trigger(cc.ID, nil) // GC wait
		return s.parseProposeCtxErr(ctx.Err(), start)
	case <-s.stopping:
		return ErrStopped
	}
}

// sync proposes a SYNC request and is non-blocking.
// This makes no guarantee that the request will be proposed or performed.
// The request will be canceled after the given timeout.
func (s *EtcdServer) sync(timeout time.Duration) {
	ctx, cancel := context.WithTimeout(context.Background(), timeout)
	req := pb.Request{
		Method: "SYNC",
		ID:     s.reqIDGen.Next(),
		Time:   time.Now().UnixNano(),
	}
	data := pbutil.MustMarshal(&req)
	// There is no promise that node has leader when do SYNC request,
	// so it uses goroutine to propose.
	s.goAttach(func() {
		s.r.Propose(ctx, data)
		cancel()
	})
}

// publish registers server information into the cluster. The information
// is the JSON representation of this server's member struct, updated with the
// static clientURLs of the server.
// The function keeps attempting to register until it succeeds,
// or its server is stopped.
func (s *EtcdServer) publish(timeout time.Duration) {
	b, err := json.Marshal(s.attributes)
	if err != nil {
		plog.Panicf("json marshal error: %v", err)
		return
	}
	req := pb.Request{
		Method: "PUT",
		Path:   membership.MemberAttributesStorePath(s.id),
		Val:    string(b),
	}

	for {
		ctx, cancel := context.WithTimeout(context.Background(), timeout)
		_, err := s.Do(ctx, req)
		cancel()
		switch err {
		case nil:
			close(s.readych)
			plog.Infof("published %+v to cluster %s", s.attributes, s.cluster.ID())
			return
		case ErrStopped:
			plog.Infof("aborting publish because server is stopped")
			return
		default:
			plog.Errorf("publish error: %v", err)
		}
	}
}

func (s *EtcdServer) sendMergedSnap(merged snap.Message) {
	atomic.AddInt64(&s.inflightSnapshots, 1)

	s.r.transport.SendSnapshot(merged)
	s.goAttach(func() {
		select {
		case ok := <-merged.CloseNotify():
			// delay releasing inflight snapshot for another 30 seconds to
			// block log compaction.
			// If the follower still fails to catch up, it is probably just too slow
			// to catch up. We cannot avoid the snapshot cycle anyway.
			if ok {
				select {
				case <-time.After(releaseDelayAfterSnapshot):
				case <-s.stopping:
				}
			}
			atomic.AddInt64(&s.inflightSnapshots, -1)
		case <-s.stopping:
			return
		}
	})
}

// apply takes entries received from Raft (after it has been committed) and
// applies them to the current state of the EtcdServer.
// The given entries should not be empty.
func (s *EtcdServer) apply(es []raftpb.Entry, confState *raftpb.ConfState) (appliedt uint64, appliedi uint64, shouldStop bool) {
	for i := range es {
		e := es[i]
		switch e.Type {
		case raftpb.EntryNormal:
			s.applyEntryNormal(&e)
		case raftpb.EntryConfChange:
			// set the consistent index of current executing entry
			if e.Index > s.consistIndex.ConsistentIndex() {
				s.consistIndex.setConsistentIndex(e.Index)
			}
			var cc raftpb.ConfChange
			pbutil.MustUnmarshal(&cc, e.Data)
			removedSelf, err := s.applyConfChange(cc, confState)
			s.setAppliedIndex(e.Index)
			shouldStop = shouldStop || removedSelf
			s.w.Trigger(cc.ID, err)
		default:
			plog.Panicf("entry type should be either EntryNormal or EntryConfChange")
		}
		atomic.StoreUint64(&s.r.index, e.Index)
		atomic.StoreUint64(&s.r.term, e.Term)
		appliedt = e.Term
		appliedi = e.Index
	}
	return appliedt, appliedi, shouldStop
}

// applyEntryNormal apples an EntryNormal type raftpb request to the EtcdServer
func (s *EtcdServer) applyEntryNormal(e *raftpb.Entry) {
	shouldApplyV3 := false
	if e.Index > s.consistIndex.ConsistentIndex() {
		// set the consistent index of current executing entry
		s.consistIndex.setConsistentIndex(e.Index)
		shouldApplyV3 = true
	}
	defer s.setAppliedIndex(e.Index)

	// raft state machine may generate noop entry when leader confirmation.
	// skip it in advance to avoid some potential bug in the future
	if len(e.Data) == 0 {
		select {
		case s.forceVersionC <- struct{}{}:
		default:
		}
		// promote lessor when the local member is leader and finished
		// applying all entries from the last term.
		if s.isLeader() {
			s.lessor.Promote(s.Cfg.electionTimeout())
		}
		return
	}

	var raftReq pb.InternalRaftRequest
	if !pbutil.MaybeUnmarshal(&raftReq, e.Data) { // backward compatible
		var r pb.Request
		pbutil.MustUnmarshal(&r, e.Data)
		s.w.Trigger(r.ID, s.applyV2Request(&r))
		return
	}
	if raftReq.V2 != nil {
		req := raftReq.V2
		s.w.Trigger(req.ID, s.applyV2Request(req))
		return
	}

	// do not re-apply applied entries.
	if !shouldApplyV3 {
		return
	}

	id := raftReq.ID
	if id == 0 {
		id = raftReq.Header.ID
	}

	var ar *applyResult
	needResult := s.w.IsRegistered(id)
	if needResult || !noSideEffect(&raftReq) {
		if !needResult && raftReq.Txn != nil {
			removeNeedlessRangeReqs(raftReq.Txn)
		}
		ar = s.applyV3.Apply(&raftReq)
	}

	if ar == nil {
		return
	}

	if ar.err != ErrNoSpace || len(s.alarmStore.Get(pb.AlarmType_NOSPACE)) > 0 {
		s.w.Trigger(id, ar)
		return
	}

	plog.Errorf("applying raft message exceeded backend quota")
	s.goAttach(func() {
		a := &pb.AlarmRequest{
			MemberID: uint64(s.ID()),
			Action:   pb.AlarmRequest_ACTIVATE,
			Alarm:    pb.AlarmType_NOSPACE,
		}
		r := pb.InternalRaftRequest{Alarm: a}
		s.processInternalRaftRequest(context.TODO(), r)
		s.w.Trigger(id, ar)
	})
}

// applyConfChange applies a ConfChange to the server. It is only
// invoked with a ConfChange that has already passed through Raft
func (s *EtcdServer) applyConfChange(cc raftpb.ConfChange, confState *raftpb.ConfState) (bool, error) {
	if err := s.cluster.ValidateConfigurationChange(cc); err != nil {
		cc.NodeID = raft.None
		s.r.ApplyConfChange(cc)
		return false, err
	}
	*confState = *s.r.ApplyConfChange(cc)
	switch cc.Type {
	case raftpb.ConfChangeAddNode:
		m := new(membership.Member)
		if err := json.Unmarshal(cc.Context, m); err != nil {
			plog.Panicf("unmarshal member should never fail: %v", err)
		}
		if cc.NodeID != uint64(m.ID) {
			plog.Panicf("nodeID should always be equal to member ID")
		}
		s.cluster.AddMember(m)
		if m.ID != s.id {
			s.r.transport.AddPeer(m.ID, m.PeerURLs)
		}
	case raftpb.ConfChangeRemoveNode:
		id := types.ID(cc.NodeID)
		s.cluster.RemoveMember(id)
		if id == s.id {
			return true, nil
		}
		s.r.transport.RemovePeer(id)
	case raftpb.ConfChangeUpdateNode:
		m := new(membership.Member)
		if err := json.Unmarshal(cc.Context, m); err != nil {
			plog.Panicf("unmarshal member should never fail: %v", err)
		}
		if cc.NodeID != uint64(m.ID) {
			plog.Panicf("nodeID should always be equal to member ID")
		}
		s.cluster.UpdateRaftAttributes(m.ID, m.RaftAttributes)
		if m.ID != s.id {
			s.r.transport.UpdatePeer(m.ID, m.PeerURLs)
		}
	}
	return false, nil
}

// TODO: non-blocking snapshot
func (s *EtcdServer) snapshot(snapi uint64, confState raftpb.ConfState) {
	clone := s.store.Clone()
	// commit kv to write metadata (for example: consistent index) to disk.
	// KV().commit() updates the consistent index in backend.
	// All operations that update consistent index must be called sequentially
	// from applyAll function.
	// So KV().Commit() cannot run in parallel with apply. It has to be called outside
	// the go routine created below.
	s.KV().Commit()

	s.goAttach(func() {
		d, err := clone.SaveNoCopy()
		// TODO: current store will never fail to do a snapshot
		// what should we do if the store might fail?
		if err != nil {
			plog.Panicf("store save should never fail: %v", err)
		}
		snap, err := s.r.raftStorage.CreateSnapshot(snapi, &confState, d)
		if err != nil {
			// the snapshot was done asynchronously with the progress of raft.
			// raft might have already got a newer snapshot.
			if err == raft.ErrSnapOutOfDate {
				return
			}
			plog.Panicf("unexpected create snapshot error %v", err)
		}
		// SaveSnap saves the snapshot and releases the locked wal files
		// to the snapshot index.
		if err = s.r.storage.SaveSnap(snap); err != nil {
			plog.Fatalf("save snapshot error: %v", err)
		}
		plog.Infof("saved snapshot at index %d", snap.Metadata.Index)

		// When sending a snapshot, etcd will pause compaction.
		// After receives a snapshot, the slow follower needs to get all the entries right after
		// the snapshot sent to catch up. If we do not pause compaction, the log entries right after
		// the snapshot sent might already be compacted. It happens when the snapshot takes long time
		// to send and save. Pausing compaction avoids triggering a snapshot sending cycle.
		if atomic.LoadInt64(&s.inflightSnapshots) != 0 {
			plog.Infof("skip compaction since there is an inflight snapshot")
			return
		}

		// keep some in memory log entries for slow followers.
		compacti := uint64(1)
		if snapi > numberOfCatchUpEntries {
			compacti = snapi - numberOfCatchUpEntries
		}
		err = s.r.raftStorage.Compact(compacti)
		if err != nil {
			// the compaction was done asynchronously with the progress of raft.
			// raft log might already been compact.
			if err == raft.ErrCompacted {
				return
			}
			plog.Panicf("unexpected compaction error %v", err)
		}
		plog.Infof("compacted raft log at %d", compacti)
	})
}

// CutPeer drops messages to the specified peer.
func (s *EtcdServer) CutPeer(id types.ID) {
	tr, ok := s.r.transport.(*rafthttp.Transport)
	if ok {
		tr.CutPeer(id)
	}
}

// MendPeer recovers the message dropping behavior of the given peer.
func (s *EtcdServer) MendPeer(id types.ID) {
	tr, ok := s.r.transport.(*rafthttp.Transport)
	if ok {
		tr.MendPeer(id)
	}
}

func (s *EtcdServer) PauseSending() { s.r.pauseSending() }

func (s *EtcdServer) ResumeSending() { s.r.resumeSending() }

func (s *EtcdServer) ClusterVersion() *semver.Version {
	if s.cluster == nil {
		return nil
	}
	return s.cluster.Version()
}

// monitorVersions checks the member's version every monitorVersionInterval.
// It updates the cluster version if all members agrees on a higher one.
// It prints out log if there is a member with a higher version than the
// local version.
func (s *EtcdServer) monitorVersions() {
	for {
		select {
		case <-s.forceVersionC:
		case <-time.After(monitorVersionInterval):
		case <-s.stopping:
			return
		}

		if s.Leader() != s.ID() {
			continue
		}

		v := decideClusterVersion(getVersions(s.cluster, s.id, s.peerRt))
		if v != nil {
			// only keep major.minor version for comparison
			v = &semver.Version{
				Major: v.Major,
				Minor: v.Minor,
			}
		}

		// if the current version is nil:
		// 1. use the decided version if possible
		// 2. or use the min cluster version
		if s.cluster.Version() == nil {
			verStr := version.MinClusterVersion
			if v != nil {
				verStr = v.String()
			}
			s.goAttach(func() { s.updateClusterVersion(verStr) })
			continue
		}

		// update cluster version only if the decided version is greater than
		// the current cluster version
		if v != nil && s.cluster.Version().LessThan(*v) {
			s.goAttach(func() { s.updateClusterVersion(v.String()) })
		}
	}
}

func (s *EtcdServer) updateClusterVersion(ver string) {
	if s.cluster.Version() == nil {
		plog.Infof("setting up the initial cluster version to %s", version.Cluster(ver))
	} else {
		plog.Infof("updating the cluster version from %s to %s", version.Cluster(s.cluster.Version().String()), version.Cluster(ver))
	}
	req := pb.Request{
		Method: "PUT",
		Path:   membership.StoreClusterVersionKey(),
		Val:    ver,
	}
	ctx, cancel := context.WithTimeout(context.Background(), s.Cfg.ReqTimeout())
	_, err := s.Do(ctx, req)
	cancel()
	switch err {
	case nil:
		return
	case ErrStopped:
		plog.Infof("aborting update cluster version because server is stopped")
		return
	default:
		plog.Errorf("error updating cluster version (%v)", err)
	}
}

func (s *EtcdServer) parseProposeCtxErr(err error, start time.Time) error {
	switch err {
	case context.Canceled:
		return ErrCanceled
	case context.DeadlineExceeded:
		curLeadElected := s.r.leadElectedTime()
		prevLeadLost := curLeadElected.Add(-2 * time.Duration(s.Cfg.ElectionTicks) * time.Duration(s.Cfg.TickMs) * time.Millisecond)
		if start.After(prevLeadLost) && start.Before(curLeadElected) {
			return ErrTimeoutDueToLeaderFail
		}

		lead := types.ID(atomic.LoadUint64(&s.r.lead))
		switch lead {
		case types.ID(raft.None):
			// TODO: return error to specify it happens because the cluster does not have leader now
		case s.ID():
			if !isConnectedToQuorumSince(s.r.transport, start, s.ID(), s.cluster.Members()) {
				return ErrTimeoutDueToConnectionLost
			}
		default:
			if !isConnectedSince(s.r.transport, start, lead) {
				return ErrTimeoutDueToConnectionLost
			}
		}

		return ErrTimeout
	default:
		return err
	}
}

func (s *EtcdServer) KV() mvcc.ConsistentWatchableKV { return s.kv }
func (s *EtcdServer) Backend() backend.Backend {
	s.bemu.Lock()
	defer s.bemu.Unlock()
	return s.be
}

func (s *EtcdServer) AuthStore() auth.AuthStore { return s.authStore }

func (s *EtcdServer) restoreAlarms() error {
	s.applyV3 = s.newApplierV3()
	as, err := alarm.NewAlarmStore(s)
	if err != nil {
		return err
	}
	s.alarmStore = as
	if len(as.Get(pb.AlarmType_NOSPACE)) > 0 {
		s.applyV3 = newApplierV3Capped(s.applyV3)
	}
	return nil
}

func (s *EtcdServer) getAppliedIndex() uint64 {
	return atomic.LoadUint64(&s.appliedIndex)
}

func (s *EtcdServer) setAppliedIndex(v uint64) {
	atomic.StoreUint64(&s.appliedIndex, v)
}

func (s *EtcdServer) getCommittedIndex() uint64 {
	return atomic.LoadUint64(&s.committedIndex)
}

func (s *EtcdServer) setCommittedIndex(v uint64) {
	atomic.StoreUint64(&s.committedIndex, v)
}

// goAttach creates a goroutine on a given function and tracks it using
// the etcdserver waitgroup.
func (s *EtcdServer) goAttach(f func()) {
	s.wgMu.RLock() // this blocks with ongoing close(s.stopping)
	defer s.wgMu.RUnlock()
	select {
	case <-s.stopping:
		plog.Warning("server has stopped (skipping goAttach)")
		return
	default:
	}

	// now safe to add since waitgroup wait has not started yet
	s.wg.Add(1)
	go func() {
		defer s.wg.Done()
		f()
	}()
}