这篇文章介绍 etcd-raft 的 PreVote 机制，避免由于网络分区导致 candidate 的 term 不断增大。
Election timeout 之后，发送 type 为 pb.MsgHup 的请求，进入选举阶段：
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// tickElection is run by followers and candidates after r.electionTimeout.
func (r *raft) tickElection() {
r.electionElapsed++
if r.promotable() && r.pastElectionTimeout() {
r.electionElapsed = 0
r.Step(pb.Message{From: r.id, Type: pb.MsgHup})
}
}
如果设置了 preVote 为 true，则先进入 prevote 阶段。调用 r.campaign 传入 type campaignPreElection：
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func (r *raft) Step(m pb.Message) error {
switch m.Type {
case pb.MsgHup:
if r.state != StateLeader {
r.logger.Infof("%x is starting a new election at term %d", r.id, r.Term)
if r.preVote {
// 如果 preVote 设置为 true，先发起 campaignPreElection
r.campaign(campaignPreElection)
} else {
r.campaign(campaignElection)
}
} else {
r.logger.Debugf("%x ignoring MsgHup because already leader", r.id)
}
}
return nil
}
campaign 方法处理选举逻辑。如果是 campaignPreElection，设置节点状态为 StatePreCandidate，此时不会递增节点的 Term（避免 term 增长过快）。然后向其他 peers 发送 type 为 pb.MsgPreVote 的请求：
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func (r *raft) campaign(t CampaignType) {
var term uint64
var voteMsg pb.MessageType
if t == campaignPreElection {
r.becomePreCandidate() // state 设置为 StatePreCandidate
voteMsg = pb.MsgPreVote // msg type 设置为 preVote
// PreVote RPCs are sent for the next term before we've incremented r.Term.
term = r.Term + 1 // preVote 不会递增 r.Term
} else {
...
}
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) // prevote 成功，可以发起 campaignElection 了
} else {
...
}
return
}
// 广播 pb.MsgPreVote
for id := range r.prs {
if id == r.id {
continue
}
var ctx []byte
r.send(pb.Message{Term: term, To: id, Type: voteMsg, Index: r.raftLog.lastIndex(), LogTerm: r.raftLog.lastTerm(), Context: ctx})
}
}
看看 stepCandidate 如何处理 pb.MsgPreVote 请求的回包。检查选票是否达到 quorum 数量，如果已经达到，prevote 成功，可以发起真正的选举了，调用 r.campaign 传入 type campaignElection：
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// stepCandidate is shared by StateCandidate and StatePreCandidate; the difference is
// whether they respond to MsgVoteResp or MsgPreVoteResp.
func stepCandidate(r *raft, m pb.Message) error {
// 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 myVoteRespType:
gr := r.poll(m.From, m.Type, !m.Reject)
switch r.quorum() {
case gr:
if r.state == StatePreCandidate {
r.campaign(campaignElection) // prevote 成功，可以发起 campaignElection
} else {
...
}
case len(r.votes) - gr: // prevote 失败（m.Reject 为 true，此时 m.Term > r.Term），转为 follower 角色
// pb.MsgPreVoteResp contains future term of pre-candidate
// m.Term > r.Term; reuse r.Term
r.becomeFollower(r.Term, None)
}
}
return nil
}
如果 campaign 类型为 campaignElection，则调用 r.becomeCandidate，此时设置节点状态为 StateCandidate，递增节点的 Term，并向其他 peers 发送 pb.MsgVote 请求：
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func (r *raft) campaign(t CampaignType) {
var term uint64
var voteMsg pb.MessageType
if t == campaignPreElection {
...
} else {
r.becomeCandidate() // become cancdidate，term 递增
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 {
...
} else {
r.becomeLeader() // 得到 quorum 的选票，选举 leader 成功，become leader
}
return
}
// 广播 pb.MsgVote，进行选举
for id := range r.prs {
if id == r.id {
continue
}
var ctx []byte
r.send(pb.Message{Term: term, To: id, Type: voteMsg, Index: r.raftLog.lastIndex(), LogTerm: r.raftLog.lastTerm(), Context: ctx})
}
}
收到 pb.MsgVote 的回包后，同样检查是否选票数量是否达到 quorum，成功则该节点当选 leader：
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func stepCandidate(r *raft, m pb.Message) error {
// 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 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:
if r.state == StatePreCandidate {
...
} else {
r.becomeLeader() // 收到 quorum 选票，选举成功
r.bcastAppend() // 广播 append 消息
}
case len(r.votes) - gr:
// pb.MsgPreVoteResp contains future term of pre-candidate
// m.Term > r.Term; reuse r.Term
r.becomeFollower(r.Term, None)
}
}
return nil
}
