ETCD-raft笔记0. 引言
该篇博客基于etcd v3.5.7版本,首先会简单介绍etcd/raft对Raft选举部分的算法优化,然后通过源码分析etcd/raft的选举实现。
1. etcd对于raft选举算法优化措施
该优化措施均在raft博士论文中有讲解
etcd/raft实现的与选举有关的优化有Pre-Vote、Check Quorum、和Leader Lease。在这三种优化中,只有Pre-Vote和Leader Lease最初是对选举过程的优化,Check Quorum是为了更高效地实现线性一致性读(Linearizable Read)而做出的优化,但是由于Leader Lease需要依赖Check Quorum,因此也放在这讲。
1.1 Pre-Vote
如下图所示,当Raft集群的网络发生分区时,会出现节点数达不到quorum(达成共识至少需要的节点数)的分区,如图中的Partition 1。
在节点数能够达到quorum的分区中,选举流程会正常进行,该分区中的所有节点的term最终会稳定为新选举出的leader节点的term。不幸的是,在节点数无法达到quorum的分区中,如果该分区中没有leader节点,因为节点总是无法收到数量达到quorum的投票而不会选举出新的leader,所以该分区中的节点在election timeout超时后,会增大term并发起下一轮选举,这导致该分区中的节点的term会不断增大。
如果网络一直没有恢复,这是没有问题的。但是,如果网络分区恢复,此时,达不到quorum的分区中的节点的term值会远大于能够达到quorum的分区中的节点的term,这会导致能够达到quorum的分区的leader退位(step down)并增大自己的term到更大的term,使集群产生一轮不必要的选举。
Pre-Vote机制就是为了解决这一问题而设计的,其解决的思路在于不允许达不到quorum的分区正常进入投票流程,也就避免了其term号的增大。为此,Pre-Vote引入了“预投票”,也就是说,当节点election timeout超时时,它们不会立即增大自身的term并请求投票,而是先发起一轮预投票。收到预投票请求的节点不会退位。只有当节点收到了达到quorum的预投票响应时,节点才能增大自身term号并发起投票请求。这样,达不到quorum的分区中的节点永远无法增大term,也就不会在分区恢复后引起不必要的一轮投票。
1.2 Check Quorum
在Raft算法中,保证线性一致性读取的最简单的方式,就是讲读请求同样当做一条Raft提议,通过与其它日志相同的方式执行,因此这种方式也叫作Log Read。显然,Log Read的性能很差。而在很多系统中,读多写少的负载是很常见的场景。因此,为了提高读取的性能,就要试图绕过日志机制。
但是,直接绕过日志机制从leader读取,可能会读到陈旧的数据,也就是说存在stale read的问题。在下图的场景中,假设网络分区前,Node 5是整个集群的leader。在网络发生分区后,Partition 0分区中选举出了新leader,也就是图中的Node 1。
但是,由于网络分区,Node 5无法收到Partition 0中节点的消息,Node 5不会意识到集群中出现了新的leader。此时,虽然它不能成功地完成日志提交,但是如果读取时绕过了日志,它还是能够提供读取服务的。这会导致连接到Node 5的client读取到陈旧的数据。
Check Quorum可以减轻这一问题带来的影响,其机制也非常简单:让leader每隔一段时间主动地检查follower是否活跃。如果活跃的follower数量达不到quorum,那么说明该leader可能是分区前的旧leader,所以此时该leader会主动退位转为follower。
需要注意的是,Check Quorum并不能完全避免stale read的发生,只能减小其发生时间,降低影响。如果需要严格的线性一致性,需要通过其它机制实现。
1.3 Leader Lease
分布式系统中的网络环境十分复杂,有时可能出现网络不完全分区的情况,即整个整个网络拓补图是一个连通图,但是可能并非任意的两个节点都能互相访问。
这种现象不止会出现在网络故障中,还会出现在成员变更中。在通过ConfChange
移除节点时,不同节点应用该ConfChange
的时间可能不同,这也可能导致这一现象发生——TODO (举个例子)。
在上图的场景下,Node 1与Node 2之间无法通信。如果它们之间的通信中断前,Node 1是集群的leader,在通信中断后,Node 2无法再收到来自Node 1的心跳。因此,Node 2会开始选举。如果在Node 2发起选举前,Node 1和Node 3中都没有新的日志,那么Node 2仍可以收到能达到quorum的投票(来自Node 2本身的投票和来自Node 3的投票),并成为leader。
Leader Lease机制对投票引入了一条新的约束以解决这一问题:当节点在election timeout超时前,如果收到了leader的消息,那么它不会为其它发起投票或预投票请求的节点投票。也就是说,Leader Lease机制会阻止了正常工作的集群中的节点给其它节点投票。
Leader Lease需要依赖Check Quorum机制才能正常工作。接下来笔者通过一个例子说明其原因。
假如在一个5个节点组成的Raft集群中,出现了下图中的分区情况:Node 1与Node 2互通,Node 3、Node 4、Node 5之间两两互通、Node 5与任一节点不通。在网络分区前,Node 1是集群的leader。
在既没有Leader Lease也没有Check Quorum的情况下,Node 3、Node 4会因收不到leader的心跳而发起投票,因为Node 2、Node 3、Node 4互通,该分区节点数能达到quorum,因此它们可以选举出新的leader。
而在使用了Leader Lease而不使用Check Quorum的情况下,由于Node 2仍能够收到原leader Node 1的心跳,受Leader Lease机制的约束,它不会为其它节点投票。这会导致即使整个集群中存在可用节点数达到quorum的分区,但是集群仍无法正常工作。
而如果同时使用了Leader Lease和Check Quorum,那么在上图的情况下,Node 1会在election timeout超时后因检测不到数量达到quorum的活跃节点而退位为follower。这样,Node 2、Node 3、Node 4之间的选举可以正常进行。
1.4 引入的新问题与解决方案
引入Pre-Vote和Check Quorum(etcd/raft的实现中,开启Check Quorum会自动开启Leader Lease)会为Raft算法引入一些新的问题。
当一个节点收到了term比自己低的消息时,原本的逻辑是直接忽略该消息,因为term比自己低的消息仅可能是因网络延迟的迟到的旧消息。然而,开启了这些机制后,在如下的场景中会出现问题:
场景1: 如上图所示,在开启了Check Quorum / Leader Lease后(假设没有开启Pre-Vote,Pre-Vote的问题在下一场景中讨论),数量达不到quorum的分区中的leader会退位,且该分区中的节点永远都无法选举出leader,因此该分区的节点的term会不断增大。当该分区与整个集群的网络恢复后,由于开启了Check Quorum / Leader Lease,即使该分区中的节点有更大的term,由于原分区的节点工作正常,它们的选举请求会被丢弃。同时,由于该节点的term比原分区的leader节点的term大,因此它会丢弃原分区的leader的请求。这样,该节点永远都无法重新加入集群,也无法当选新leader。(详见issue #5451、issue #5468)。
场景2: Pre-Vote机制也有类似的问题。如上图所示,假如发起预投票的节点,在预投票通过后正要发起正式投票的请求时出现网络分区。此时,该节点的term会高于原集群的term。而原集群因没有收到真正的投票请求,不会更新term,继续正常运行。在网络分区恢复后,原集群的term低于分区节点的term,但是日志比分区节点更新。此时,该节点发起的预投票请求因没有日志落后会被丢弃,而原集群leader发给该节点的请求会因term比该节点小而被丢弃。同样,该节点永远都无法重新加入集群,也无法当选新leader。(详见issue #8501、issue #8525)。
场景3: 在更复杂的情况中,比如,在变更配置时,开启了原本没有开启的Pre-Vote机制。此时可能会出现与上一条类似的情况,即可能因term更高但是log更旧的节点的存在导致整个集群的死锁,所有节点都无法预投票成功。这种情况比上一种情况更危险,上一种情况只有之前分区的节点无法加入集群,在这种情况下,整个集群都会不可用。(详见issue #8501、issue #8525)。
为了解决以上问题,节点在收到term比自己低的请求时,需要做特殊的处理。处理逻辑也很简单:
- 如果收到了term比当前节点term低的leader的消息,且集群开启了Check Quorum / Leader Lease或Pre-Vote,那么发送一条term为当前term的消息,令term低的节点成为follower。(针对场景1、场景2)
- 对于term比当前节点term低的预投票请求,无论是否开启了Check Quorum / Leader Lease或Pre-Vote,都要通过一条term为当前term的消息,迫使其转为follower并更新term。(针对场景3)
2. etcd中Raft选举的实现2.1 发起vote或pre-vote流程2.1.1 Election timeout
在集群刚启动时,所有节点的状态都为 follower
,等待超时触发 leader election
。超时时间由 Config
设置。etcd/raft
没有用真实时间而是使用逻辑时钟,当调用 tick
的次数超过指定次数时触发超时事件。 对于 follower
和 candidate
而言,tick
中会判断是否超时,若超时则会本地生成一个 MsgHup
类型的消息触发 leader election
:
// tickElection is run by followers and candidates after r.electionTimeout.func (r *raft) tickElection() {r.electionElapsed++if r.promotable() && r.pastElectionTimeout() {r.electionElapsed = 0r.Step(pb.Message{From: r.id, Type: pb.MsgHup})}}
2.1.2 MsgHup消息处理与hup方法
etcd/raft通过raft
结构体的Step
方法实现Raft状态机的状态转移。Step
方法是消息处理的入口,不同 state
处理的消息不同且处理方式不同,所以有多个 step
方法:
raft.Step()
: 消息处理的入口,做一些共性的检查,如term
,或处理所有状态都需要处理的消息。若需要更进一步处理,会根据状态 调用下面的方法:raft.stepLeader()
:leader
状态的消息处理方法;raft.stepFollower()
:follower
状态的消息处理方法;raft.stepCandidate()
:candidate
状态的消息处理方法。
func (r *raft) Step(m pb.Message) error {// ... ...switch m.Type {case pb.MsgHup:if r.preVote {r.hup(campaignPreElection)} else {r.hup(campaignElection)}// ... ...}// ... ...}
Step
方法在处理MsgHup
消息时,会根据当前配置中是否开启了Pre-Vote
机制,以不同的CampaignType
调用hup
方法。CampaignType
是一种枚举类型(go语言的枚举实现方式),其可能值如下表所示。
值 | 描述 |
---|---|
campaignPreElection | 表示Pre-Vote的预选举阶段。 |
campaignElection | 表示正常的选举阶段(仅超时选举,不包括Leader Transfer)。 |
campaignTransfer | 表示Leader Transfer阶段。 |
接下来对hup
的实现进行分析。
func (r *raft) hup(t CampaignType) {if r.state == StateLeader {r.logger.Debugf("%x ignoring MsgHup because already leader", r.id)return}if !r.promotable() {r.logger.Warningf("%x is unpromotable and can not campaign", r.id)return}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}r.logger.Infof("%x is starting a new election at term %d", r.id, r.Term)r.campaign(t)}
// promotable indicates whether state machine can be promoted to leader,// which is true when its own id is in progress list.func (r *raft) promotable() bool {pr := r.prs.Progress[r.id]return pr != nil && !pr.IsLearner && !r.raftLog.hasPendingSnapshot()}
总结当节点出现以下情况时不能发起选举:
- 节点被移出集群
- 节点是learner
- 节点还有未保存到稳定存储的snapshot
- 节点有还未被应用的集群配置变更
ConfChange
消息
2.1.3 campaign
官方注释很详细了,因此不多废笔墨解释
// campaign transitions the raft instance to candidate state. This must only be// called after verifying that this is a legitimate transition.func (r *raft) campaign(t CampaignType) { // 因为调用campaign的方法不止有hup,campaign方法首先还是会检查promotable()是否为真。if !r.promotable() {// This path should not be hit (callers are supposed to check), but// better safe than sorry.r.logger.Warningf("%x is unpromotable; campaign() should have been called", r.id)}var term uint64var voteMsg pb.MessageTypeif 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.MsgVoteterm = r.Term}if _, _, res := r.poll(r.id, voteRespMsgType(voteMsg), true); res == quorum.VoteWon {// 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}var ids []uint64{//won't send requestVote to learners, beacause learners[] are not in incoming[] and outgoing[]idMap := r.prs.Voters.IDs()ids = make([]uint64, 0, len(idMap))for id := range idMap {ids = append(ids, id)}sort.Slice(ids, func(i, j int) bool { return ids[i] < ids[j] })}for _, id := range ids {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 []byteif 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})}}
至此,该节点已向其他节点发送MsgVote或MsgPreVote消息
2.2 节点收到vote或pre-vote消息处理流程
处理vote或pre-vote消息都在Step
方法内,不会进入各自的step方法,有效的MsgPreVote
必须满足其中一个条件(m.Term > r.Term)
官方注释很详细,简单易理解,因此不多废笔墨解释
func (r *raft) Step(m pb.Message) error {// Handle the message term, which may result in our stepping down to a follower.switch {case m.Term == 0:// local messagecase m.Term > r.Term: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.electionTimeoutif !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 votereturn nil}}switch {case m.Type == pb.MsgPreVote:// Never change our term in response to a PreVotecase 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:if m.Type == pb.MsgApp || m.Type == pb.MsgHeartbeat || m.Type == pb.MsgSnap {r.becomeFollower(m.Term, m.From)} else {r.becomeFollower(m.Term, None)}}case m.Term r.Term)// ...and we believe the candidate is up to date.if canVote && r.raftLog.isUpToDate(m.Index, m.LogTerm) {// Note: it turns out that that learners must be allowed to cast votes.// This seems counter- intuitive but is necessary in the situation in which// a learner has been promoted (i.e. is now a voter) but has not learned// about this yet.// For example, consider a group in which id=1 is a learner and id=2 and// id=3 are voters. A configuration change promoting 1 can be committed on// the quorum `{2,3}` without the config change being appended to the// learner's log. If the leader (say 2) fails, there are de facto two// voters remaining. Only 3 can win an election (due to its log containing// all committed entries), but to do so it will need 1 to vote. But 1// considers itself a learner and will continue to do so until 3 has// stepped up as leader, replicates the conf change to 1, and 1 applies it.// Ultimately, by receiving a request to vote, the learner realizes that// the candidate believes it to be a voter, and that it should act// accordingly. The candidate's config may be stale, too; but in that case// it won't win the election, at least in the absence of the bug discussed// in:// https://github.com/etcd-io/etcd/issues/7625#issuecomment-488798263.// When responding to Msg{Pre,}Vote messages we include the term// from the message, not the local term. To see why, consider the// case where a single node was previously partitioned away and// it's local term is now out of date. If we include the local term// (recall that for pre-votes we don't update the local term), the// (pre-)campaigning node on the other end will proceed to ignore// the message (it ignores all out of date messages).// The term in the original message and current local term are the// same in the case of regular votes, but different for pre-votes.r.send(pb.Message{To: m.From, Term: m.Term, Type: voteRespMsgType(m.Type)})if m.Type == pb.MsgVote {// Only record real votes.r.electionElapsed = 0r.Vote = m.From}} else {r.send(pb.Message{To: m.From, Term: r.Term, Type: voteRespMsgType(m.Type), Reject: true})}default: // ...........}return nil}
注意:节点同意投票消息带的是m.Term
,拒绝投票消息是r.Term
,如果拒接MsgPreVote
消息,那么发送pre-vote消息的节点就变为
在r.Term
的follower
,在2.3.1节内体现
2.3 节点收到处理MsgPreVoteResp或MsgVoteResp消息流程2.3.1 Step内处理
根据2.2节可以看到Step
内有这样一段代码:在2.2节最后有解释,官方也给了详细注释
switch {case m.Type == pb.MsgPreVote:// Never change our term in response to a PreVotecase 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:if m.Type == pb.MsgApp || m.Type == pb.MsgHeartbeat || m.Type == pb.MsgSnap {r.becomeFollower(m.Term, m.From)} else {r.becomeFollower(m.Term, None)}}
2.3.2 stepCandidate内处理
case myVoteRespType:gr, rj, res := r.poll(m.From, m.Type, !m.Reject)r.logger.Infof("%x has received %d %s votes and %d vote rejections", r.id, gr, m.Type, rj)switch res {case quorum.VoteWon:if r.state == StatePreCandidate {r.campaign(campaignElection)} else {r.becomeLeader()r.bcastAppend()}case quorum.VoteLost:// pb.MsgPreVoteResp contains future term of pre-candidate// m.Term > r.Term; reuse r.Termr.becomeFollower(r.Term, None)}
如果预投票成功,则发起新一轮正式投票。如果正式投票成功,则转为leader,接着后续操作
2.4 转变领导者身份2.4.1 becomeLeader()
func (r *raft) becomeLeader() {// TODO(xiangli) remove the panic when the raft implementation is stableif r.state == StateFollower {panic("invalid transition [follower -> leader]")}r.step = stepLeaderr.reset(r.Term)r.tick = r.tickHeartbeatr.lead = r.idr.state = StateLeader// Followers enter replicate mode when they've been successfully probed// (perhaps after having received a snapshot as a result). The leader is// trivially in this state. Note that r.reset() has initialized this// progress with the last index already.r.prs.Progress[r.id].BecomeReplicate()// Conservatively set the pendingConfIndex to the last index in the// log. There may or may not be a pending config change, but it's// safe to delay any future proposals until we commit all our// pending log entries, and scanning the entire tail of the log// could be expensive.r.pendingConfIndex = r.raftLog.lastIndex()emptyEnt := pb.Entry{Data: nil}if !r.appendEntry(emptyEnt) {// This won't happen because we just called reset() above.r.logger.Panic("empty entry was dropped")}// As a special case, don't count the initial empty entry towards the// uncommitted log quota. This is because we want to preserve the// behavior of allowing one entry larger than quota if the current// usage is zero.r.reduceUncommittedSize([]pb.Entry{emptyEnt})r.logger.Infof("%x became leader at term %d", r.id, r.Term)}
从candidate
转变为leader
,需要在自己的log中append一条当前term的日志,并广播给其他节点