raft: Implement the PreVote RPC described in thesis section 9.6

This prevents disruption when a node that has been partitioned
away rejoins the cluster.

Fixes #6522

raft/doc.go

@@ -257,6 +257,12 @@ stale log entries:
 	If candidate receives majority of votes of denials, it reverts back to
 	follower.
 
+  'MsgPreVote' and 'MsgPreVoteResp' are used in an optional two-phase election
+  protocol. When Config.PreVote is true, a pre-election is carried out first
+  (using the same rules as a regular election, and no node increases its term
+  number unless the pre-election indicates that the campaigining node would win.
+  This minimizes disruption when a partitioned node rejoins the cluster.
+
 	'MsgSnap' requests to install a snapshot message. When a node has just
 	become a leader or the leader receives 'MsgProp' message, it calls
 	'bcastAppend' method, which then calls 'sendAppend' method to each

raft/raft.go

@@ -37,6 +37,7 @@ const (
 	StateFollower StateType = iota
 	StateCandidate
 	StateLeader
+	StatePreCandidate
 )
 
 type ReadOnlyOption int
@@ -55,7 +56,11 @@ const (
 
 // Possible values for CampaignType
 const (
-	// campaignElection represents the type of normal election
+	// campaignPreElection represents the first phase of a normal election when
+	// Config.PreVote is true.
+	campaignPreElection CampaignType = "CampaignPreElection"
+	// campaignElection represents a normal (time-based) election (the second phase
+	// of the election when Config.PreVote is true).
 	campaignElection CampaignType = "CampaignElection"
 	// campaignTransfer represents the type of leader transfer
 	campaignTransfer CampaignType = "CampaignTransfer"
@@ -92,6 +97,7 @@ var stmap = [...]string{
 	"StateFollower",
 	"StateCandidate",
 	"StateLeader",
+	"StatePreCandidate",
 }
 
 func (st StateType) String() string {
@@ -149,6 +155,11 @@ type Config struct {
 	// steps down when quorum is not active for an electionTimeout.
 	CheckQuorum bool
 
+	// PreVote enables the Pre-Vote algorithm described in raft thesis section
+	// 9.6. This prevents disruption when a node that has been partitioned away
+	// rejoins the cluster.
+	PreVote bool
+
 	// ReadOnlyOption specifies how the read only request is processed.
 	//
 	// ReadOnlySafe guarantees the linearizability of the read only request by
@@ -236,6 +247,7 @@ type raft struct {
 	heartbeatElapsed int
 
 	checkQuorum bool
+	preVote     bool
 
 	heartbeatTimeout int
 	electionTimeout  int
@@ -280,6 +292,7 @@ func newRaft(c *Config) *raft {
 		heartbeatTimeout: c.HeartbeatTick,
 		logger:           c.Logger,
 		checkQuorum:      c.CheckQuorum,
+		preVote:          c.PreVote,
 		readOnly:         newReadOnly(c.ReadOnlyOption),
 	}
 	for _, p := range peers {
@@ -329,11 +342,22 @@ func (r *raft) nodes() []uint64 {
 // send persists state to stable storage and then sends to its mailbox.
 func (r *raft) send(m pb.Message) {
 	m.From = r.id
-	// do not attach term to MsgProp
-	// proposals are a way to forward to the leader and
-	// should be treated as local message.
-	if m.Type != pb.MsgProp {
-		m.Term = r.Term
+	if m.Type == pb.MsgVote || m.Type == pb.MsgPreVote {
+		if m.Term == 0 {
+			// PreVote RPCs are sent at a term other than our actual term, so the code
+			// that sends these messages is responsible for setting the term.
+			panic(fmt.Sprintf("term should be set when sending %s", m.Type))
+		}
+	} else {
+		if m.Term != 0 {
+			panic(fmt.Sprintf("term should not be set when sending %s (was %d)", m.Type, m.Term))
+		}
+		// do not attach term to MsgProp
+		// proposals are a way to forward to the leader and
+		// should be treated as local message.
+		if m.Type != pb.MsgProp {
+			m.Term = r.Term
+		}
 	}
 	r.msgs = append(r.msgs, m)
 }
@@ -555,6 +579,20 @@ func (r *raft) becomeCandidate() {
 	r.logger.Infof("%x became candidate at term %d", r.id, r.Term)
 }
 
+func (r *raft) becomePreCandidate() {
+	// TODO(xiangli) remove the panic when the raft implementation is stable
+	if r.state == StateLeader {
+		panic("invalid transition [leader -> pre-candidate]")
+	}
+	// Becoming a pre-candidate changes our step functions and state,
+	// but doesn't change anything else. In particular it does not increase
+	// r.Term or change r.Vote.
+	r.step = stepCandidate
+	r.tick = r.tickElection
+	r.state = StatePreCandidate
+	r.logger.Infof("%x became pre-candidate at term %d", r.id, r.Term)
+}
+
 func (r *raft) becomeLeader() {
 	// TODO(xiangli) remove the panic when the raft implementation is stable
 	if r.state == StateFollower {
@@ -583,31 +621,48 @@ func (r *raft) becomeLeader() {
 }
 
 func (r *raft) campaign(t CampaignType) {
-	r.becomeCandidate()
-	if r.quorum() == r.poll(r.id, true) {
-		r.becomeLeader()
+	var term uint64
+	var voteMsg pb.MessageType
+	if t == campaignPreElection {
+		r.becomePreCandidate()
+		voteMsg = pb.MsgPreVote
+		// PreVote RPCs are sent for the next term before we've incremented r.Term.
+		term = r.Term + 1
+	} else {
+		r.becomeCandidate()
+		voteMsg = pb.MsgVote
+		term = r.Term
+	}
+	if r.quorum() == r.poll(r.id, voteRespMsgType(voteMsg), true) {
+		// We won the election after voting for ourselves (which must mean that
+		// this is a single-node cluster). Advance to the next state.
+		if t == campaignPreElection {
+			r.campaign(campaignElection)
+		} else {
+			r.becomeLeader()
+		}
 		return
 	}
 	for id := range r.prs {
 		if id == r.id {
 			continue
 		}
-		r.logger.Infof("%x [logterm: %d, index: %d] sent vote request to %x at term %d",
-			r.id, r.raftLog.lastTerm(), r.raftLog.lastIndex(), id, r.Term)
+		r.logger.Infof("%x [logterm: %d, index: %d] sent %s request to %x at term %d",
+			r.id, r.raftLog.lastTerm(), r.raftLog.lastIndex(), voteMsg, id, r.Term)
 
 		var ctx []byte
 		if t == campaignTransfer {
 			ctx = []byte(t)
 		}
-		r.send(pb.Message{To: id, Type: pb.MsgVote, Index: r.raftLog.lastIndex(), LogTerm: r.raftLog.lastTerm(), Context: ctx})
+		r.send(pb.Message{Term: term, To: id, Type: voteMsg, Index: r.raftLog.lastIndex(), LogTerm: r.raftLog.lastTerm(), Context: ctx})
 	}
 }
 
-func (r *raft) poll(id uint64, v bool) (granted int) {
+func (r *raft) poll(id uint64, t pb.MessageType, v bool) (granted int) {
 	if v {
-		r.logger.Infof("%x received vote from %x at term %d", r.id, id, r.Term)
+		r.logger.Infof("%x received %s from %x at term %d", r.id, t, id, r.Term)
 	} else {
-		r.logger.Infof("%x received vote rejection from %x at term %d", r.id, id, r.Term)
+		r.logger.Infof("%x received %s rejection from %x at term %d", r.id, t, id, r.Term)
 	}
 	if _, ok := r.votes[id]; !ok {
 		r.votes[id] = v
@@ -633,7 +688,11 @@ func (r *raft) Step(m pb.Message) error {
 			}
 
 			r.logger.Infof("%x is starting a new election at term %d", r.id, r.Term)
-			r.campaign(campaignElection)
+			if r.preVote {
+				r.campaign(campaignPreElection)
+			} else {
+				r.campaign(campaignElection)
+			}
 		} else {
 			r.logger.Debugf("%x ignoring MsgHup because already leader", r.id)
 		}
@@ -645,32 +704,47 @@ func (r *raft) Step(m pb.Message) error {
 		// local message
 	case m.Term > r.Term:
 		lead := m.From
-		if m.Type == pb.MsgVote {
+		if m.Type == pb.MsgVote || m.Type == pb.MsgPreVote {
 			force := bytes.Equal(m.Context, []byte(campaignTransfer))
 			inLease := r.checkQuorum && r.lead != None && r.electionElapsed < r.electionTimeout
 			if !force && inLease {
 				// If a server receives a RequestVote request within the minimum election timeout
 				// of hearing from a current leader, it does not update its term or grant its vote
-				r.logger.Infof("%x [logterm: %d, index: %d, vote: %x] ignored vote from %x [logterm: %d, index: %d] at term %d: lease is not expired (remaining ticks: %d)",
-					r.id, r.raftLog.lastTerm(), r.raftLog.lastIndex(), r.Vote, m.From, m.LogTerm, m.Index, r.Term, r.electionTimeout-r.electionElapsed)
+				r.logger.Infof("%x [logterm: %d, index: %d, vote: %x] ignored %s from %x [logterm: %d, index: %d] at term %d: lease is not expired (remaining ticks: %d)",
+					r.id, r.raftLog.lastTerm(), r.raftLog.lastIndex(), r.Vote, m.Type, m.From, m.LogTerm, m.Index, r.Term, r.electionTimeout-r.electionElapsed)
 				return nil
 			}
 			lead = None
 		}
-		r.logger.Infof("%x [term: %d] received a %s message with higher term from %x [term: %d]",
-			r.id, r.Term, m.Type, m.From, m.Term)
-		r.becomeFollower(m.Term, lead)
+		switch {
+		case m.Type == pb.MsgPreVote:
+		// Never change our term in response to a PreVote
+		case m.Type == pb.MsgPreVoteResp && !m.Reject:
+			// We send pre-vote requests with a term in our future. If the
+			// pre-vote is granted, we will increment our term when we get a
+			// quorum. If it is not, the term comes from the node that
+			// rejected our vote so we should become a follower at the new
+			// term.
+		default:
+			r.logger.Infof("%x [term: %d] received a %s message with higher term from %x [term: %d]",
+				r.id, r.Term, m.Type, m.From, m.Term)
+			r.becomeFollower(m.Term, lead)
+		}
 	case m.Term < r.Term:
 		if r.checkQuorum && (m.Type == pb.MsgHeartbeat || m.Type == pb.MsgApp) {
-			// We have received messages from a leader at a lower term. It is possible that these messages were
-			// simply delayed in the network, but this could also mean that this node has advanced its term number
-			// during a network partition, and it is now unable to either win an election or to rejoin the majority
-			// on the old term. If checkQuorum is false, this will be handled by incrementing term numbers in response
-			// to MsgVote with a higher term, but if checkQuorum is true we may not advance the term on MsgVote and
-			// must generate other messages to advance the term. The net result of these two features is to minimize
-			// the disruption caused by nodes that have been removed from the cluster's configuration: a removed node
-			// will send MsgVotes which will be ignored, but it will not receive MsgApp or MsgHeartbeat, so it will not
-			// create disruptive term increases
+			// We have received messages from a leader at a lower term. It is possible
+			// that these messages were simply delayed in the network, but this could
+			// also mean that this node has advanced its term number during a network
+			// partition, and it is now unable to either win an election or to rejoin
+			// the majority on the old term. If checkQuorum is false, this will be
+			// handled by incrementing term numbers in response to MsgVote with a
+			// higher term, but if checkQuorum is true we may not advance the term on
+			// MsgVote and must generate other messages to advance the term. The net
+			// result of these two features is to minimize the disruption caused by
+			// nodes that have been removed from the cluster's configuration: a
+			// removed node will send MsgVotes (or MsgPreVotes) which will be ignored,
+			// but it will not receive MsgApp or MsgHeartbeat, so it will not create
+			// disruptive term increases
 			r.send(pb.Message{To: m.From, Type: pb.MsgAppResp})
 		} else {
 			// ignore other cases
@@ -723,10 +797,10 @@ func stepLeader(r *raft, m pb.Message) {
 		r.appendEntry(m.Entries...)
 		r.bcastAppend()
 		return
-	case pb.MsgVote:
-		r.logger.Infof("%x [logterm: %d, index: %d, vote: %x] rejected vote from %x [logterm: %d, index: %d] at term %d",
-			r.id, r.raftLog.lastTerm(), r.raftLog.lastIndex(), r.Vote, m.From, m.LogTerm, m.Index, r.Term)
-		r.send(pb.Message{To: m.From, Type: pb.MsgVoteResp, Reject: true})
+	case pb.MsgVote, pb.MsgPreVote:
+		r.logger.Infof("%x [logterm: %d, index: %d, vote: %x] rejected %s from %x [logterm: %d, index: %d] at term %d",
+			r.id, r.raftLog.lastTerm(), r.raftLog.lastIndex(), r.Vote, m.Type, m.From, m.LogTerm, m.Index, r.Term)
+		r.send(pb.Message{To: m.From, Type: voteRespMsgType(m.Type), Reject: true})
 		return
 	case pb.MsgReadIndex:
 		if r.quorum() > 1 {
@@ -884,7 +958,18 @@ func stepLeader(r *raft, m pb.Message) {
 	}
 }
 
+// stepCandidate is shared by StateCandidate and StatePreCandidate; the difference is
+// whether they respond to MsgVoteResp or MsgPreVoteResp.
 func stepCandidate(r *raft, m pb.Message) {
+	// Only handle vote responses corresponding to our candidacy (while in
+	// StateCandidate, we may get stale MsgPreVoteResp messages in this term from
+	// our pre-candidate state).
+	var myVoteRespType pb.MessageType
+	if r.state == StatePreCandidate {
+		myVoteRespType = pb.MsgPreVoteResp
+	} else {
+		myVoteRespType = pb.MsgVoteResp
+	}
 	switch m.Type {
 	case pb.MsgProp:
 		r.logger.Infof("%x no leader at term %d; dropping proposal", r.id, r.Term)
@@ -898,22 +983,27 @@ func stepCandidate(r *raft, m pb.Message) {
 	case pb.MsgSnap:
 		r.becomeFollower(m.Term, m.From)
 		r.handleSnapshot(m)
-	case pb.MsgVote:
-		r.logger.Infof("%x [logterm: %d, index: %d, vote: %x] rejected vote from %x [logterm: %d, index: %d] at term %d",
-			r.id, r.raftLog.lastTerm(), r.raftLog.lastIndex(), r.Vote, m.From, m.LogTerm, m.Index, r.Term)
-		r.send(pb.Message{To: m.From, Type: pb.MsgVoteResp, Reject: true})
-	case pb.MsgVoteResp:
-		gr := r.poll(m.From, !m.Reject)
-		r.logger.Infof("%x [quorum:%d] has received %d votes and %d vote rejections", r.id, r.quorum(), gr, len(r.votes)-gr)
+	case pb.MsgVote, pb.MsgPreVote:
+		r.logger.Infof("%x [logterm: %d, index: %d, vote: %x] rejected %s from %x [logterm: %d, index: %d] at term %d",
+			r.id, r.raftLog.lastTerm(), r.raftLog.lastIndex(), r.Vote, m.Type, m.From, m.LogTerm, m.Index, r.Term)
+		r.send(pb.Message{To: m.From, Type: voteRespMsgType(m.Type), Reject: true})
+	case myVoteRespType:
+		gr := r.poll(m.From, m.Type, !m.Reject)
+		r.logger.Infof("%x [quorum:%d] has received %d %s votes and %d vote rejections", r.id, r.quorum(), gr, m.Type, len(r.votes)-gr)
 		switch r.quorum() {
 		case gr:
-			r.becomeLeader()
-			r.bcastAppend()
+			if r.state == StatePreCandidate {
+				r.campaign(campaignElection)
+			} else {
+				r.becomeLeader()
+				r.bcastAppend()
+			}
 		case len(r.votes) - gr:
 			r.becomeFollower(r.Term, None)
 		}
 	case pb.MsgTimeoutNow:
 		r.logger.Debugf("%x [term %d state %v] ignored MsgTimeoutNow from %x", r.id, r.Term, r.state, m.From)
+
 	}
 }
 
@@ -938,17 +1028,17 @@ func stepFollower(r *raft, m pb.Message) {
 		r.electionElapsed = 0
 		r.lead = m.From
 		r.handleSnapshot(m)
-	case pb.MsgVote:
-		if (r.Vote == None || r.Vote == m.From) && r.raftLog.isUpToDate(m.Index, m.LogTerm) {
+	case pb.MsgVote, pb.MsgPreVote:
+		if (r.Vote == None || m.Term > r.Term || r.Vote == m.From) && r.raftLog.isUpToDate(m.Index, m.LogTerm) {
 			r.electionElapsed = 0
-			r.logger.Infof("%x [logterm: %d, index: %d, vote: %x] voted for %x [logterm: %d, index: %d] at term %d",
-				r.id, r.raftLog.lastTerm(), r.raftLog.lastIndex(), r.Vote, m.From, m.LogTerm, m.Index, r.Term)
+			r.logger.Infof("%x [logterm: %d, index: %d, vote: %x] cast %s for %x [logterm: %d, index: %d] at term %d",
+				r.id, r.raftLog.lastTerm(), r.raftLog.lastIndex(), r.Vote, m.Type, m.From, m.LogTerm, m.Index, r.Term)
 			r.Vote = m.From
-			r.send(pb.Message{To: m.From, Type: pb.MsgVoteResp})
+			r.send(pb.Message{To: m.From, Type: voteRespMsgType(m.Type)})
 		} else {
-			r.logger.Infof("%x [logterm: %d, index: %d, vote: %x] rejected vote from %x [logterm: %d, index: %d] at term %d",
-				r.id, r.raftLog.lastTerm(), r.raftLog.lastIndex(), r.Vote, m.From, m.LogTerm, m.Index, r.Term)
-			r.send(pb.Message{To: m.From, Type: pb.MsgVoteResp, Reject: true})
+			r.logger.Infof("%x [logterm: %d, index: %d, vote: %x] rejected %s from %x [logterm: %d, index: %d] at term %d",
+				r.id, r.raftLog.lastTerm(), r.raftLog.lastIndex(), r.Vote, m.Type, m.From, m.LogTerm, m.Index, r.Term)
+			r.send(pb.Message{To: m.From, Type: voteRespMsgType(m.Type), Reject: true})
 		}
 	case pb.MsgTransferLeader:
 		if r.lead == None {
@@ -959,6 +1049,9 @@ func stepFollower(r *raft, m pb.Message) {
 		r.send(m)
 	case pb.MsgTimeoutNow:
 		r.logger.Infof("%x [term %d] received MsgTimeoutNow from %x and starts an election to get leadership.", r.id, r.Term, m.From)
+		// Leadership transfers never use pre-vote even if r.preVote is true; we
+		// know we are not recovering from a partition so there is no need for the
+		// extra round trip.
 		r.campaign(campaignTransfer)
 	case pb.MsgReadIndex:
 		if r.lead == None {

raft/raft_test.go

@@ -288,33 +288,80 @@ func TestProgressPaused(t *testing.T) {
 }
 
 func TestLeaderElection(t *testing.T) {
-	tests := []struct {
-		*network
-		state   StateType
-		expTerm uint64
-	}{
-		{newNetwork(nil, nil, nil), StateLeader, 1},
-		{newNetwork(nil, nil, nopStepper), StateLeader, 1},
-		{newNetwork(nil, nopStepper, nopStepper), StateCandidate, 1},
-		{newNetwork(nil, nopStepper, nopStepper, nil), StateCandidate, 1},
-		{newNetwork(nil, nopStepper, nopStepper, nil, nil), StateLeader, 1},
-
-		// three logs further along than 0, but in the same term so rejections
-		// are returned instead of the votes being ignored.
-		{newNetwork(nil, ents(1), ents(1), ents(1, 1), nil), StateFollower, 1},
-
-		// logs converge
-		{newNetwork(ents(1), nil, ents(2), ents(1), nil), StateLeader, 2},
+	for i, preVote := range []bool{false, true} {
+		var cfg func(*Config)
+		if preVote {
+			cfg = preVoteConfig
+		}
+		tests := []struct {
+			*network
+			state   StateType
+			expTerm uint64
+		}{
+			{newNetworkWithConfig(cfg, nil, nil, nil), StateLeader, 1},
+			{newNetworkWithConfig(cfg, nil, nil, nopStepper), StateLeader, 1},
+			{newNetworkWithConfig(cfg, nil, nopStepper, nopStepper), StateCandidate, 1},
+			{newNetworkWithConfig(cfg, nil, nopStepper, nopStepper, nil), StateCandidate, 1},
+			{newNetworkWithConfig(cfg, nil, nopStepper, nopStepper, nil, nil), StateLeader, 1},
+
+			// three logs further along than 0, but in the same term so rejections
+			// are returned instead of the votes being ignored.
+			{newNetworkWithConfig(cfg, nil, ents(1), ents(1), ents(1, 1), nil), StateFollower, 1},
+
+			// logs converge
+			{newNetworkWithConfig(cfg, ents(1), nil, ents(2), ents(1), nil), StateLeader, 2},
+		}
+
+		for j, tt := range tests {
+			tt.send(pb.Message{From: 1, To: 1, Type: pb.MsgHup})
+			sm := tt.network.peers[1].(*raft)
+			var expState StateType
+			var expTerm uint64
+			if tt.state == StateCandidate && preVote {
+				// In pre-vote mode, an election that fails to complete
+				// leaves the node in pre-candidate state without advancing
+				// the term.
+				expState = StatePreCandidate
+				expTerm = 0
+			} else {
+				expState = tt.state
+				expTerm = tt.expTerm
+			}
+			if sm.state != expState {
+				t.Errorf("#%d.%d: state = %s, want %s", i, j, sm.state, expState)
+			}
+			if g := sm.Term; g != expTerm {
+				t.Errorf("#%d.%d: term = %d, want %d", i, j, g, expTerm)
+			}
+		}
 	}
+}
 
-	for i, tt := range tests {
-		tt.send(pb.Message{From: 1, To: 1, Type: pb.MsgHup})
-		sm := tt.network.peers[1].(*raft)
-		if sm.state != tt.state {
-			t.Errorf("#%d: state = %s, want %s", i, sm.state, tt.state)
+// TestLeaderCycle verifies that each node in a cluster can campaign
+// and be elected in turn. This ensures that elections (including
+// pre-vote) work when not starting from a clean slate (as they do in
+// TestLeaderElection)
+func TestLeaderCycle(t *testing.T) {
+	for _, preVote := range []bool{false, true} {
+		var cfg func(*Config)
+		if preVote {
+			cfg = preVoteConfig
 		}
-		if g := sm.Term; g != tt.expTerm {
-			t.Errorf("#%d: term = %d, want %d", i, g, tt.expTerm)
+		n := newNetworkWithConfig(cfg, nil, nil, nil)
+		for campaignerID := uint64(1); campaignerID <= 3; campaignerID++ {
+			n.send(pb.Message{From: campaignerID, To: campaignerID, Type: pb.MsgHup})
+
+			for _, peer := range n.peers {
+				sm := peer.(*raft)
+				if sm.id == campaignerID && sm.state != StateLeader {
+					t.Errorf("preVote=%v: campaigning node %d state = %v, want StateLeader",
+						preVote, sm.id, sm.state)
+				} else if sm.id != campaignerID && sm.state != StateFollower {
+					t.Errorf("preVote=%v: after campaign of node %d, "+
+						"node %d had state = %v, want StateFollower",
+						preVote, campaignerID, sm.id, sm.state)
+				}
+			}
 		}
 	}
 }
@@ -532,6 +579,76 @@ func TestDuelingCandidates(t *testing.T) {
 	}
 }
 
+func TestDuelingPreCandidates(t *testing.T) {
+	cfgA := newTestConfig(1, []uint64{1, 2, 3}, 10, 1, NewMemoryStorage())
+	cfgB := newTestConfig(2, []uint64{1, 2, 3}, 10, 1, NewMemoryStorage())
+	cfgC := newTestConfig(3, []uint64{1, 2, 3}, 10, 1, NewMemoryStorage())
+	cfgA.PreVote = true
+	cfgB.PreVote = true
+	cfgC.PreVote = true
+	a := newRaft(cfgA)
+	b := newRaft(cfgB)
+	c := newRaft(cfgC)
+
+	nt := newNetwork(a, b, c)
+	nt.cut(1, 3)
+
+	nt.send(pb.Message{From: 1, To: 1, Type: pb.MsgHup})
+	nt.send(pb.Message{From: 3, To: 3, Type: pb.MsgHup})
+
+	// 1 becomes leader since it receives votes from 1 and 2
+	sm := nt.peers[1].(*raft)
+	if sm.state != StateLeader {
+		t.Errorf("state = %s, want %s", sm.state, StateLeader)
+	}
+
+	// 3 campaigns then reverts to follower when its PreVote is rejected
+	sm = nt.peers[3].(*raft)
+	if sm.state != StateFollower {
+		t.Errorf("state = %s, want %s", sm.state, StateFollower)
+	}
+
+	nt.recover()
+
+	// Candidate 3 now increases its term and tries to vote again.
+	// With PreVote, it does not disrupt the leader.
+	nt.send(pb.Message{From: 3, To: 3, Type: pb.MsgHup})
+
+	wlog := &raftLog{
+		storage:   &MemoryStorage{ents: []pb.Entry{{}, {Data: nil, Term: 1, Index: 1}}},
+		committed: 1,
+		unstable:  unstable{offset: 2},
+	}
+	tests := []struct {
+		sm      *raft
+		state   StateType
+		term    uint64
+		raftLog *raftLog
+	}{
+		{a, StateLeader, 1, wlog},
+		{b, StateFollower, 1, wlog},
+		{c, StateFollower, 1, newLog(NewMemoryStorage(), raftLogger)},
+	}
+
+	for i, tt := range tests {
+		if g := tt.sm.state; g != tt.state {
+			t.Errorf("#%d: state = %s, want %s", i, g, tt.state)
+		}
+		if g := tt.sm.Term; g != tt.term {
+			t.Errorf("#%d: term = %d, want %d", i, g, tt.term)
+		}
+		base := ltoa(tt.raftLog)
+		if sm, ok := nt.peers[1+uint64(i)].(*raft); ok {
+			l := ltoa(sm.raftLog)
+			if g := diffu(base, l); g != "" {
+				t.Errorf("#%d: diff:\n%s", i, g)
+			}
+		} else {
+			t.Logf("#%d: empty log", i)
+		}
+	}
+}
+
 func TestCandidateConcede(t *testing.T) {
 	tt := newNetwork(nil, nil, nil)
 	tt.isolate(1)
@@ -586,6 +703,16 @@ func TestSingleNodeCandidate(t *testing.T) {
 	}
 }
 
+func TestSingleNodePreCandidate(t *testing.T) {
+	tt := newNetworkWithConfig(preVoteConfig, nil)
+	tt.send(pb.Message{From: 1, To: 1, Type: pb.MsgHup})
+
+	sm := tt.peers[1].(*raft)
+	if sm.state != StateLeader {
+		t.Errorf("state = %d, want %d", sm.state, StateLeader)
+	}
+}
+
 func TestOldMessages(t *testing.T) {
 	tt := newNetwork(nil, nil, nil)
 	// make 0 leader @ term 3
@@ -1022,65 +1149,71 @@ func TestMsgAppRespWaitReset(t *testing.T) {
 }
 
 func TestRecvMsgVote(t *testing.T) {
-	tests := []struct {
-		state   StateType
-		i, term uint64
-		voteFor uint64
-		wreject bool
-	}{
-		{StateFollower, 0, 0, None, true},
-		{StateFollower, 0, 1, None, true},
-		{StateFollower, 0, 2, None, true},
-		{StateFollower, 0, 3, None, false},
-
-		{StateFollower, 1, 0, None, true},
-		{StateFollower, 1, 1, None, true},
-		{StateFollower, 1, 2, None, true},
-		{StateFollower, 1, 3, None, false},
-
-		{StateFollower, 2, 0, None, true},
-		{StateFollower, 2, 1, None, true},
-		{StateFollower, 2, 2, None, false},
-		{StateFollower, 2, 3, None, false},
-
-		{StateFollower, 3, 0, None, true},
-		{StateFollower, 3, 1, None, true},
-		{StateFollower, 3, 2, None, false},
-		{StateFollower, 3, 3, None, false},
-
-		{StateFollower, 3, 2, 2, false},
-		{StateFollower, 3, 2, 1, true},
-
-		{StateLeader, 3, 3, 1, true},
-		{StateCandidate, 3, 3, 1, true},
-	}
-
-	for i, tt := range tests {
-		sm := newTestRaft(1, []uint64{1}, 10, 1, NewMemoryStorage())
-		sm.state = tt.state
-		switch tt.state {
-		case StateFollower:
-			sm.step = stepFollower
-		case StateCandidate:
-			sm.step = stepCandidate
-		case StateLeader:
-			sm.step = stepLeader
-		}
-		sm.Vote = tt.voteFor
-		sm.raftLog = &raftLog{
-			storage:  &MemoryStorage{ents: []pb.Entry{{}, {Index: 1, Term: 2}, {Index: 2, Term: 2}}},
-			unstable: unstable{offset: 3},
-		}
+	for i, msgType := range []pb.MessageType{pb.MsgVote, pb.MsgPreVote} {
+		tests := []struct {
+			state   StateType
+			i, term uint64
+			voteFor uint64
+			wreject bool
+		}{
+			{StateFollower, 0, 0, None, true},
+			{StateFollower, 0, 1, None, true},
+			{StateFollower, 0, 2, None, true},
+			{StateFollower, 0, 3, None, false},
+
+			{StateFollower, 1, 0, None, true},
+			{StateFollower, 1, 1, None, true},
+			{StateFollower, 1, 2, None, true},
+			{StateFollower, 1, 3, None, false},
+
+			{StateFollower, 2, 0, None, true},
+			{StateFollower, 2, 1, None, true},
+			{StateFollower, 2, 2, None, false},
+			{StateFollower, 2, 3, None, false},
+
+			{StateFollower, 3, 0, None, true},
+			{StateFollower, 3, 1, None, true},
+			{StateFollower, 3, 2, None, false},
+			{StateFollower, 3, 3, None, false},
+
+			{StateFollower, 3, 2, 2, false},
+			{StateFollower, 3, 2, 1, true},
+
+			{StateLeader, 3, 3, 1, true},
+			{StatePreCandidate, 3, 3, 1, true},
+			{StateCandidate, 3, 3, 1, true},
+		}
+
+		for j, tt := range tests {
+			sm := newTestRaft(1, []uint64{1}, 10, 1, NewMemoryStorage())
+			sm.state = tt.state
+			switch tt.state {
+			case StateFollower:
+				sm.step = stepFollower
+			case StateCandidate, StatePreCandidate:
+				sm.step = stepCandidate
+			case StateLeader:
+				sm.step = stepLeader
+			}
+			sm.Vote = tt.voteFor
+			sm.raftLog = &raftLog{
+				storage:  &MemoryStorage{ents: []pb.Entry{{}, {Index: 1, Term: 2}, {Index: 2, Term: 2}}},
+				unstable: unstable{offset: 3},
+			}
 
-		sm.Step(pb.Message{Type: pb.MsgVote, From: 2, Index: tt.i, LogTerm: tt.term})
+			sm.Step(pb.Message{Type: msgType, From: 2, Index: tt.i, LogTerm: tt.term})
 
-		msgs := sm.readMessages()
-		if g := len(msgs); g != 1 {
-			t.Fatalf("#%d: len(msgs) = %d, want 1", i, g)
-			continue
-		}
-		if g := msgs[0].Reject; g != tt.wreject {
-			t.Errorf("#%d, m.Reject = %v, want %v", i, g, tt.wreject)
+			msgs := sm.readMessages()
+			if g := len(msgs); g != 1 {
+				t.Fatalf("#%d.%d: len(msgs) = %d, want 1", i, j, g)
+				continue
+			}
+			if g := msgs[0].Type; g != voteRespMsgType(msgType) {
+				t.Errorf("#%d.%d, m.Type = %v, want %v", i, j, g, voteRespMsgType(msgType))
+			}
+			if g := msgs[0].Reject; g != tt.wreject {
+				t.Errorf("#%d.%d, m.Reject = %v, want %v", i, j, g, tt.wreject)
+			}
 		}
 	}
 }
@@ -1094,14 +1227,22 @@ func TestStateTransition(t *testing.T) {
 		wlead  uint64
 	}{
 		{StateFollower, StateFollower, true, 1, None},
+		{StateFollower, StatePreCandidate, true, 0, None},
 		{StateFollower, StateCandidate, true, 1, None},
 		{StateFollower, StateLeader, false, 0, None},
 
+		{StatePreCandidate, StateFollower, true, 0, None},
+		{StatePreCandidate, StatePreCandidate, true, 0, None},
+		{StatePreCandidate, StateCandidate, true, 1, None},
+		{StatePreCandidate, StateLeader, true, 0, 1},
+
 		{StateCandidate, StateFollower, true, 0, None},
+		{StateCandidate, StatePreCandidate, true, 0, None},
 		{StateCandidate, StateCandidate, true, 1, None},
 		{StateCandidate, StateLeader, true, 0, 1},
 
 		{StateLeader, StateFollower, true, 1, None},
+		{StateLeader, StatePreCandidate, false, 0, None},
 		{StateLeader, StateCandidate, false, 1, None},
 		{StateLeader, StateLeader, true, 0, 1},
 	}
@@ -1122,6 +1263,8 @@ func TestStateTransition(t *testing.T) {
 			switch tt.to {
 			case StateFollower:
 				sm.becomeFollower(tt.wterm, tt.wlead)
+			case StatePreCandidate:
+				sm.becomePreCandidate()
 			case StateCandidate:
 				sm.becomeCandidate()
 			case StateLeader:
@@ -1147,6 +1290,7 @@ func TestAllServerStepdown(t *testing.T) {
 		windex uint64
 	}{
 		{StateFollower, StateFollower, 3, 0},
+		{StatePreCandidate, StateFollower, 3, 0},
 		{StateCandidate, StateFollower, 3, 0},
 		{StateLeader, StateFollower, 3, 1},
 	}
@@ -2176,23 +2320,27 @@ func TestRaftNodes(t *testing.T) {
 }
 
 func TestCampaignWhileLeader(t *testing.T) {
-	r := newTestRaft(1, []uint64{1}, 5, 1, NewMemoryStorage())
-	if r.state != StateFollower {
-		t.Errorf("expected new node to be follower but got %s", r.state)
-	}
-	// We don't call campaign() directly because it comes after the check
-	// for our current state.
-	r.Step(pb.Message{From: 1, To: 1, Type: pb.MsgHup})
-	if r.state != StateLeader {
-		t.Errorf("expected single-node election to become leader but got %s", r.state)
-	}
-	term := r.Term
-	r.Step(pb.Message{From: 1, To: 1, Type: pb.MsgHup})
-	if r.state != StateLeader {
-		t.Errorf("expected to remain leader but got %s", r.state)
-	}
-	if r.Term != term {
-		t.Errorf("expected to remain in term %v but got %v", term, r.Term)
+	for i, preVote := range []bool{false, true} {
+		cfg := newTestConfig(1, []uint64{1}, 5, 1, NewMemoryStorage())
+		cfg.PreVote = preVote
+		r := newRaft(cfg)
+		if r.state != StateFollower {
+			t.Errorf("#%d: expected new node to be follower but got %s", i, r.state)
+		}
+		// We don't call campaign() directly because it comes after the check
+		// for our current state.
+		r.Step(pb.Message{From: 1, To: 1, Type: pb.MsgHup})
+		if r.state != StateLeader {
+			t.Errorf("#%d: expected single-node election to become leader but got %s", i, r.state)
+		}
+		term := r.Term
+		r.Step(pb.Message{From: 1, To: 1, Type: pb.MsgHup})
+		if r.state != StateLeader {
+			t.Errorf("#%d: expected to remain leader but got %s", i, r.state)
+		}
+		if r.Term != term {
+			t.Errorf("#%d: expected to remain in term %v but got %v", i, term, r.Term)
+		}
 	}
 }
 
@@ -2603,6 +2751,12 @@ type network struct {
 // A *stateMachine will get its k, id.
 // When using stateMachine, the address list is always [1, n].
 func newNetwork(peers ...stateMachine) *network {
+	return newNetworkWithConfig(nil, peers...)
+}
+
+// newNetworkWithConfig is like newNetwork but calls the given func to
+// modify the configuration of any state machines it creates.
+func newNetworkWithConfig(configFunc func(*Config), peers ...stateMachine) *network {
 	size := len(peers)
 	peerAddrs := idsBySize(size)
 
@@ -2614,7 +2768,11 @@ func newNetwork(peers ...stateMachine) *network {
 		switch v := p.(type) {
 		case nil:
 			nstorage[id] = NewMemoryStorage()
-			sm := newTestRaft(id, peerAddrs, 10, 1, nstorage[id])
+			cfg := newTestConfig(id, peerAddrs, 10, 1, nstorage[id])
+			if configFunc != nil {
+				configFunc(cfg)
+			}
+			sm := newRaft(cfg)
 			npeers[id] = sm
 		case *raft:
 			v.id = id
@@ -2638,6 +2796,10 @@ func newNetwork(peers ...stateMachine) *network {
 	}
 }
 
+func preVoteConfig(c *Config) {
+	c.PreVote = true
+}
+
 func (nw *network) send(msgs ...pb.Message) {
 	for len(msgs) > 0 {
 		m := msgs[0]

raft/raftpb/raft.pb.go

@@ -92,6 +92,8 @@ const (
 	MsgTimeoutNow     MessageType = 14
 	MsgReadIndex      MessageType = 15
 	MsgReadIndexResp  MessageType = 16
+	MsgPreVote        MessageType = 17
+	MsgPreVoteResp    MessageType = 18
 )
 
 var MessageType_name = map[int32]string{
@@ -112,6 +114,8 @@ var MessageType_name = map[int32]string{
 	14: "MsgTimeoutNow",
 	15: "MsgReadIndex",
 	16: "MsgReadIndexResp",
+	17: "MsgPreVote",
+	18: "MsgPreVoteResp",
 }
 var MessageType_value = map[string]int32{
 	"MsgHup":            0,
@@ -131,6 +135,8 @@ var MessageType_value = map[string]int32{
 	"MsgTimeoutNow":     14,
 	"MsgReadIndex":      15,
 	"MsgReadIndexResp":  16,
+	"MsgPreVote":        17,
+	"MsgPreVoteResp":    18,
 }
 
 func (x MessageType) Enum() *MessageType {
@@ -1836,54 +1842,55 @@ var (
 )
 
 var fileDescriptorRaft = []byte{
-	// 776 bytes of a gzipped FileDescriptorProto
-	0x1f, 0x8b, 0x08, 0x00, 0x00, 0x09, 0x6e, 0x88, 0x02, 0xff, 0x64, 0x54, 0xcd, 0x6e, 0xdb, 0x38,
-	0x10, 0xb6, 0x64, 0xf9, 0x6f, 0xe4, 0x38, 0x0c, 0xe3, 0x5d, 0x10, 0x41, 0xe0, 0xf5, 0x1a, 0x7b,
-	0x30, 0xb2, 0x48, 0x76, 0xd7, 0x87, 0x3d, 0xf4, 0x96, 0xd8, 0x05, 0x12, 0xa0, 0x0e, 0x5a, 0xc7,
-	0xe9, 0xa1, 0x45, 0x51, 0x30, 0x16, 0x2d, 0xbb, 0x8d, 0x44, 0x81, 0xa2, 0xd3, 0xe4, 0x52, 0xf4,
-	0x01, 0xfa, 0x00, 0xbd, 0xf4, 0x19, 0xfa, 0x1a, 0x39, 0xe6, 0x09, 0x8a, 0x26, 0x7d, 0x91, 0x82,
-	0x14, 0x65, 0x4b, 0xf1, 0x8d, 0xfc, 0xbe, 0xe1, 0xcc, 0x37, 0xdf, 0x8c, 0x04, 0x20, 0xe8, 0x54,
-	0x1e, 0x44, 0x82, 0x4b, 0x8e, 0xcb, 0xea, 0x1c, 0x5d, 0xec, 0x34, 0x7d, 0xee, 0x73, 0x0d, 0xfd,
-	0xa3, 0x4e, 0x09, 0xdb, 0xf9, 0x08, 0xa5, 0xa7, 0xa1, 0x14, 0x37, 0xf8, 0x6f, 0x70, 0xc6, 0x37,
-	0x11, 0x23, 0x56, 0xdb, 0xea, 0x36, 0x7a, 0x5b, 0x07, 0xc9, 0xab, 0x03, 0x4d, 0x2a, 0xe2, 0xc8,
-	0xb9, 0xfd, 0xfe, 0x47, 0x61, 0xe4, 0xc8, 0x9b, 0x88, 0x61, 0x02, 0xce, 0x98, 0x89, 0x80, 0xd8,
-	0x6d, 0xab, 0xeb, 0x2c, 0x19, 0x26, 0x02, 0xbc, 0x03, 0xa5, 0x93, 0xd0, 0x63, 0xd7, 0xa4, 0x98,
-	0xa1, 0x4a, 0x73, 0x05, 0x61, 0x0c, 0xce, 0x80, 0x4a, 0x4a, 0x9c, 0xb6, 0xd5, 0xad, 0x8f, 0x1c,
-	0x8f, 0x4a, 0xda, 0xf9, 0x64, 0x01, 0x3a, 0x0b, 0x69, 0x14, 0xcf, 0xb8, 0x1c, 0x32, 0x49, 0x15,
-	0x88, 0xff, 0x07, 0x98, 0xf0, 0x70, 0xfa, 0x36, 0x96, 0x54, 0x26, 0x8a, 0xdc, 0x95, 0xa2, 0x3e,
-	0x0f, 0xa7, 0x67, 0x8a, 0x30, 0xc9, 0x6b, 0x93, 0x14, 0x50, 0xc5, 0x75, 0xa5, 0x9c, 0x2e, 0x53,
-	0x9c, 0x80, 0x16, 0x98, 0xd3, 0xa5, 0x91, 0xce, 0x2b, 0xa8, 0xa6, 0x0a, 0x94, 0x44, 0xa5, 0x40,
-	0xd7, 0x34, 0x12, 0xf1, 0x13, 0xa8, 0x06, 0x46, 0x99, 0x4e, 0xec, 0xf6, 0x48, 0xaa, 0xe5, 0xb1,
-	0x72, 0x93, 0x77, 0x19, 0xdf, 0xf9, 0x5a, 0x84, 0xca, 0x90, 0xc5, 0x31, 0xf5, 0x19, 0xde, 0x07,
-	0x6d, 0x9e, 0x71, 0x78, 0x3b, 0xcd, 0x61, 0xe8, 0x35, 0x8f, 0x9b, 0x60, 0x4b, 0x9e, 0xeb, 0xc4,
-	0x96, 0x5c, 0xb5, 0x31, 0x15, 0xfc, 0x51, 0x1b, 0x0a, 0x59, 0x36, 0xe8, 0xac, 0xcd, 0xa4, 0x05,
-	0x95, 0x4b, 0xee, 0xeb, 0x81, 0x95, 0x32, 0x64, 0x0a, 0xae, 0x6c, 0x2b, 0xaf, 0xdb, 0xb6, 0x0f,
-	0x15, 0x16, 0x4a, 0x31, 0x67, 0x31, 0xa9, 0xb4, 0x8b, 0x5d, 0xb7, 0xb7, 0x91, 0xdb, 0x8c, 0x34,
-	0x95, 0x89, 0xc1, 0xbb, 0x50, 0x9e, 0xf0, 0x20, 0x98, 0x4b, 0x52, 0xcd, 0xe4, 0x32, 0x18, 0xee,
-	0x41, 0x35, 0x36, 0x8e, 0x91, 0x9a, 0x76, 0x12, 0x3d, 0x76, 0x32, 0x75, 0x30, 0x8d, 0x53, 0x19,
-	0x05, 0x7b, 0xc7, 0x26, 0x92, 0x40, 0xdb, 0xea, 0x56, 0xd3, 0x8c, 0x09, 0x86, 0xff, 0x02, 0x48,
-	0x4e, 0xc7, 0xf3, 0x50, 0x12, 0x37, 0x53, 0x33, 0x83, 0x63, 0x02, 0x95, 0x09, 0x0f, 0x25, 0xbb,
-	0x96, 0xa4, 0xae, 0x07, 0x9b, 0x5e, 0x3b, 0x6f, 0xa0, 0x76, 0x4c, 0x85, 0x97, 0xac, 0x4f, 0xea,
-	0xa0, 0xb5, 0xe6, 0x20, 0x01, 0xe7, 0x8a, 0x4b, 0x96, 0xdf, 0x77, 0x85, 0x64, 0x1a, 0x2e, 0xae,
-	0x37, 0xdc, 0xf9, 0x13, 0x6a, 0xcb, 0x75, 0xc5, 0x4d, 0x28, 0x85, 0xdc, 0x63, 0x31, 0xb1, 0xda,
-	0xc5, 0xae, 0x33, 0x4a, 0x2e, 0x9d, 0xcf, 0x16, 0x80, 0x8a, 0xe9, 0xcf, 0x68, 0xe8, 0xeb, 0xa9,
-	0x9f, 0x0c, 0x72, 0x0a, 0xec, 0xf9, 0x00, 0xff, 0x6b, 0x3e, 0x4e, 0x5b, 0xaf, 0xce, 0xef, 0xd9,
-	0x4f, 0x21, 0x79, 0xb7, 0xb6, 0x3d, 0xbb, 0x50, 0x3e, 0xe5, 0x1e, 0x3b, 0x19, 0xe4, 0x75, 0x85,
-	0x1a, 0x53, 0x86, 0xf4, 0x8d, 0x21, 0x4e, 0xce, 0x90, 0xbd, 0xff, 0xa0, 0xb6, 0xfc, 0xe4, 0xf1,
-	0x26, 0xb8, 0xfa, 0x72, 0xca, 0x45, 0x40, 0x2f, 0x51, 0x01, 0x6f, 0xc3, 0xa6, 0x06, 0x56, 0x85,
-	0x91, 0xb5, 0xf7, 0xcd, 0x06, 0x37, 0xb3, 0xc4, 0x18, 0xa0, 0x3c, 0x8c, 0xfd, 0xe3, 0x45, 0x84,
-	0x0a, 0xd8, 0x85, 0xca, 0x30, 0xf6, 0x8f, 0x18, 0x95, 0xc8, 0x32, 0x97, 0xe7, 0x82, 0x47, 0xc8,
-	0x36, 0x51, 0x87, 0x51, 0x84, 0x8a, 0xb8, 0x01, 0x90, 0x9c, 0x47, 0x2c, 0x8e, 0x90, 0x63, 0x02,
-	0x5f, 0x72, 0xc9, 0x50, 0x49, 0x89, 0x30, 0x17, 0xcd, 0x96, 0x0d, 0xab, 0x16, 0x06, 0x55, 0x30,
-	0x82, 0xba, 0x2a, 0xc6, 0xa8, 0x90, 0x17, 0xaa, 0x4a, 0x15, 0x37, 0x01, 0x65, 0x11, 0xfd, 0xa8,
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-	0xdc, 0x70, 0xcb, 0xda, 0x16, 0x06, 0x00, 0x00,
+	// 788 bytes of a gzipped FileDescriptorProto
+	0x1f, 0x8b, 0x08, 0x00, 0x00, 0x09, 0x6e, 0x88, 0x02, 0xff, 0x64, 0x54, 0xcd, 0x6e, 0xdb, 0x46,
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+	0xfd, 0x63, 0xbb, 0xf0, 0xf0, 0xd8, 0x2e, 0xdc, 0x3f, 0xb5, 0xad, 0x87, 0xa7, 0xb6, 0xf5, 0xf5,
+	0xa9, 0x6d, 0x7d, 0xfc, 0xd6, 0x2e, 0x7c, 0x0f, 0x00, 0x00, 0xff, 0xff, 0xaf, 0x8d, 0x65, 0xcb,
+	0x3a, 0x06, 0x00, 0x00,
 }

raft/raftpb/raft.proto

@@ -50,6 +50,8 @@ enum MessageType {
 	MsgTimeoutNow      = 14;
 	MsgReadIndex       = 15;
 	MsgReadIndexResp   = 16;
+	MsgPreVote         = 17;
+	MsgPreVoteResp     = 18;
 }
 
 message Message {

raft/util.go

@@ -52,7 +52,19 @@ func IsLocalMsg(msgt pb.MessageType) bool {
 }
 
 func IsResponseMsg(msgt pb.MessageType) bool {
-	return msgt == pb.MsgAppResp || msgt == pb.MsgVoteResp || msgt == pb.MsgHeartbeatResp || msgt == pb.MsgUnreachable
+	return msgt == pb.MsgAppResp || msgt == pb.MsgVoteResp || msgt == pb.MsgHeartbeatResp || msgt == pb.MsgUnreachable || msgt == pb.MsgPreVoteResp
+}
+
+// voteResponseType maps vote and prevote message types to their corresponding responses.
+func voteRespMsgType(msgt pb.MessageType) pb.MessageType {
+	switch msgt {
+	case pb.MsgVote:
+		return pb.MsgVoteResp
+	case pb.MsgPreVote:
+		return pb.MsgPreVoteResp
+	default:
+		panic(fmt.Sprintf("not a vote message: %s", msgt))
+	}
 }
 
 // EntryFormatter can be implemented by the application to provide human-readable formatting

raft/util_test.go

@@ -93,6 +93,8 @@ func TestIsLocalMsg(t *testing.T) {
 		{pb.MsgTimeoutNow, false},
 		{pb.MsgReadIndex, false},
 		{pb.MsgReadIndexResp, false},
+		{pb.MsgPreVote, false},
+		{pb.MsgPreVoteResp, false},
 	}
 
 	for i, tt := range tests {