我们知道选举超时会发起选举，我们看下，具体的流程是怎样的？
发起人
func (r *raft) becomeFollower(term uint64, lead uint64) {
r.step = stepFollower
r.reset(term)
r.tick = r.tickElection
r.lead = lead
r.state = StateFollower
r.logger.Infof("%x became follower at term %d", r.id, r.Term)
}
func (r *raft) becomeCandidate() {
// TODO(xiangli) remove the panic when the raft implementation is stable
if r.state == StateLeader {
panic("invalid transition [leader -> candidate]")
}
r.step = stepCandidate
r.reset(r.Term + 1)
r.tick = r.tickElection
r.Vote = r.id
r.state = StateCandidate
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.votes = make(map[uint64]bool)
r.tick = r.tickElection
r.lead = None
r.state = StatePreCandidate
r.logger.Infof("%x became pre-candidate at term %d", r.id, r.Term)
}
我们看到Follwer，Candidate，PreCandidate都会进行选举计时(tickElection)。
tickElection
func (r *raft) tickElection() {
r.electionElapsed++
if r.promotable() && r.pastElectionTimeout() {
r.electionElapsed = 0
r.Step(pb.Message{From: r.id, Type: pb.MsgHup})
}
}
func (r *raft) pastElectionTimeout() bool {
return r.electionElapsed >= r.randomizedElectionTimeout
}
func (r *raft) resetRandomizedElectionTimeout() {
r.randomizedElectionTimeout = r.electionTimeout + globalRand.Intn(r.electionTimeout)
}
基本如前所述，就是选举超时发生后，会重新发起选举
有个重点是随机的超时时间，为了避免大家同时超时，又同时发起投票，导致票数过不了半的问题，随机超时是解决这个性价比最高的办法。
Step
case pb.MsgHup:
if r.state != StateLeader {
ents, err := r.raftLog.slice(r.raftLog.applied+1, r.raftLog.committed+1, noLimit)
if err != nil {
r.logger.Panicf("unexpected error getting unapplied entries (%v)", err)
}
if n := numOfPendingConf(ents); n != 0 && r.raftLog.committed > r.raftLog.applied {
r.logger.Warningf("%x cannot campaign at term %d since there are still %d pending configuration changes to apply", r.id, r.Term, n)
return nil
}
r.logger.Infof("%x is starting a new election at term %d", r.id, r.Term)
if r.preVote {
r.campaign(campaignPreElection)
} else {
r.campaign(campaignElection)
}
} else {
r.logger.Debugf("%x ignoring MsgHup because already leader", r.id)
}
如果该节点还有配置变更的没有写入状态机的话，那么他没有资格发起选举。
如果配置了PreVote，发起选举r.campaign(campaignPreElection)
如果没有配置PreVote的话，发起选举r.campaign(campaignElection)
campaign
func (r *raft) campaign(t CampaignType) {
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 %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{Term: term, To: id, Type: voteMsg, Index: r.raftLog.lastIndex(), LogTerm: r.raftLog.lastTerm(), Context: ctx})
}
}
首先成为Candidate，主要是重置各个节点的进度，然后先给自己投一票，自己的任期当然要+1，不然你要玩假的么？
这里注意的是发出的消息类型是MsgVote，而且任期是term
r.becomeCandidate()
voteMsg = pb.MsgVote
term = r.Term
func (r *raft) becomeCandidate() {
// TODO(xiangli) remove the panic when the raft implementation is stable
if r.state == StateLeader {
panic("invalid transition [leader -> candidate]")
}
r.step = stepCandidate
r.reset(r.Term + 1)
r.tick = r.tickElection
r.Vote = r.id
r.state = StateCandidate
r.logger.Infof("%x became candidate at term %d", r.id, r.Term)
}
如果之前是准选举，那么清空投票机，lead清零，不做太多改变，
这里注意的是发出的消息类型是MsgPreVote，而且任期是term+1
r.becomePreCandidate()
voteMsg = pb.MsgPreVote
// PreVote RPCs are sent for the next term before we've incremented r.Term.
term = r.Term + 1
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.votes = make(map[uint64]bool)
r.tick = r.tickElection
r.lead = None
r.state = StatePreCandidate
r.logger.Infof("%x became pre-candidate at term %d", r.id, r.Term)
}
其次，要给成员报告自己最后一位的(任期+index)，给大家校验。让对方判断要不要投票给自己。
如果是campaignTransfer，附在ctx里面一起发出去。
总结
MsgVote和MsgPreVote的部分见
作者：Pillar_Zhong
链接：https://www.jianshu.com/p/8d581424264a
来源：简书
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