MIT 6.824: Distributed Systems是麻省理工大学的研究生公开课，主讲分布式系统。

该课程一共包含Lab1-4共4个大作业，Lab1是实现Mapreduce原型，Lab2-4是实现Raft以及基于Raft实现分布式KV存储。

我正在实现Lab2，也就是Raft核心算法，它被划分成了Lab2A、Lab2B、Lab2C三个子任务：

Lab2A：实现leader election、heartbeat。
Lab2B：实现Log replication。
Lab2C：实现state persistent。

这个拆解符合Raft算法的描述，也就是选主、日志同步、状态存储，是可以分开实现的，最终凑出一个完整的Raft实现。

学习资料

以MIT课程教案与视频为主：

教案：https://pdos.csail.mit.edu/6.824/labs/lab-raft.html
中文视频（因盈利原因，缺少Lab2B~Lab2C）：https://www.bilibili.com/video/BV1x7411M7Sf
英文视频（带字幕，可英可中）：https://www.youtube.com/watch?v=cQP8WApzIQQ&list=PLrw6a1wE39_tb2fErI4-WkMbsvGQk9_UB

中英文对照学习Raft论文：

中文：https://yuerblog.cc/wp-content/uploads/raft-zh_cn.pdf
英文：https://pdos.csail.mit.edu/6.824/papers/raft-extended.pdf

实现Raft的时候基本就盯着Figure2的图片即可：

Lab2A实现

因为课程分为Lab2A、Lab2B、Lab2C，为了能留住每个子任务的代码状态，我决定把每个子Lab的代码实现贴到博客上，方便拆分回看，我不会讲解代码细节。

下面是我的Lab2A实现，已经通过了课程的单元测试：

结构体定义

// 日志项
type LogEntry struct {
	Command interface{}
	Term    int
}

// 当前角色
const ROLE_LEADER = "Leader"
const ROLE_FOLLOWER = "Follower"
const ROLE_CANDIDATES = "Candidates"

//
// A Go object implementing a single Raft peer.
//
type Raft struct {
	mu        sync.Mutex          // Lock to protect shared access to this peer's state
	peers     []*labrpc.ClientEnd // RPC end points of all peers
	persister *Persister          // Object to hold this peer's persisted state
	me        int                 // this peer's index into peers[]
	dead      int32               // set by Kill()

	// Your data here (2A, 2B, 2C).
	// Look at the paper's Figure 2 for a description of what
	// state a Raft server must maintain.

	// 所有服务器，持久化状态（lab-2A不要求持久化）
	currentTerm int         // 见过的最大任期
	votedFor    int         // 记录在currentTerm任期投票给谁了
	log         []*LogEntry // 操作日志

	// 所有服务器，易失状态
	commitIndex int // 已知的最大已提交索引
	lastApplied int // 当前应用到状态机的索引

	// 仅Leader，易失状态（成为leader时重置）
	nextIndex  []int //	每个follower的log同步起点索引（初始为leader log的最后一项）
	matchIndex []int // 每个follower的log同步进度（初始为0），和nextIndex强关联

	// 所有服务器，选举相关状态
	role              string    // 身份
	leaderId          int       // leader的id
	lastActiveTime    time.Time // 上次活跃时间（刷新时机：收到leader心跳、给其他candidates投票、请求其他节点投票）
	lastBroadcastTime time.Time // 作为leader，上次的广播时间
}

//
// example RequestVote RPC arguments structure.
// field names must start with capital letters!
//
type RequestVoteArgs struct {
	// Your data here (2A, 2B).
	Term         int
	CandidateId  int
	LastLogIndex int
	LastLogTerm  int
}

//
// example RequestVote RPC reply structure.
// field names must start with capital letters!
//
type RequestVoteReply struct {
	// Your data here (2A).
	Term        int
	VoteGranted bool
}

type AppendEntriesArgs struct {
	Term         int
	LeaderId     int
	PrevLogIndex int
	PrevLogTerm  int
	Entries      []*LogEntry
	LeaderCommit int
}

type AppendEntriesReply struct {
	Term    int
	Success bool
}

// 日志项

type LogEntry struct {

Command interface{}

Term int

}

// 当前角色

const ROLE_LEADER = "Leader"

const ROLE_FOLLOWER = "Follower"

const ROLE_CANDIDATES = "Candidates"

// A Go object implementing a single Raft peer.

type Raft struct {

mu sync.Mutex // Lock to protect shared access to this peer's state

peers []*labrpc.ClientEnd // RPC end points of all peers

persister *Persister // Object to hold this peer's persisted state

me int // this peer's index into peers[]

dead int32 // set by Kill()

// Your data here (2A, 2B, 2C).

// Look at the paper's Figure 2 for a description of what

// state a Raft server must maintain.

// 所有服务器，持久化状态（lab-2A不要求持久化）

currentTerm int // 见过的最大任期

votedFor int // 记录在currentTerm任期投票给谁了

log []*LogEntry // 操作日志

// 所有服务器，易失状态

commitIndex int // 已知的最大已提交索引

lastApplied int // 当前应用到状态机的索引

// 仅Leader，易失状态（成为leader时重置）

nextIndex []int // 每个follower的log同步起点索引（初始为leader log的最后一项）

matchIndex []int // 每个follower的log同步进度（初始为0），和nextIndex强关联

// 所有服务器，选举相关状态

role string // 身份

leaderId int // leader的id

lastActiveTime time.Time // 上次活跃时间（刷新时机：收到leader心跳、给其他candidates投票、请求其他节点投票）

lastBroadcastTime time.Time // 作为leader，上次的广播时间

}

// example RequestVote RPC arguments structure.

// field names must start with capital letters!

type RequestVoteArgs struct {

// Your data here (2A, 2B).

Term int

CandidateId int

LastLogIndex int

LastLogTerm int

}

// example RequestVote RPC reply structure.

// field names must start with capital letters!

type RequestVoteReply struct {

// Your data here (2A).

Term int

VoteGranted bool

}

type AppendEntriesArgs struct {

Term int

LeaderId int

PrevLogIndex int

PrevLogTerm int

Entries []*LogEntry

LeaderCommit int

}

type AppendEntriesReply struct {

Term int

Success bool

}

Make 程序入口

//
func Make(peers []*labrpc.ClientEnd, me int,
	persister *Persister, applyCh chan ApplyMsg) *Raft {
	rf := &Raft{}
	rf.peers = peers
	rf.persister = persister
	rf.me = me

	// Your initialization code here (2A, 2B, 2C).
	rf.role = ROLE_FOLLOWER
	rf.leaderId = -1
	rf.votedFor = -1
	rf.lastActiveTime = time.Now()

	// initialize from state persisted before a crash
	rf.readPersist(persister.ReadRaftState())

	// election逻辑
	go rf.electionLoop()
	// leader逻辑
	go rf.appendEntriesLoop()

	DPrintf("Raftnode[%d]启动", me)

	return rf
}

func Make(peers []*labrpc.ClientEnd, me int,

persister *Persister, applyCh chan ApplyMsg) *Raft {

rf := &Raft{}

rf.peers = peers

rf.persister = persister

rf.me = me

// Your initialization code here (2A, 2B, 2C).

rf.role = ROLE_FOLLOWER

rf.leaderId = -1

rf.votedFor = -1

rf.lastActiveTime = time.Now()

// initialize from state persisted before a crash

rf.readPersist(persister.ReadRaftState())

// election逻辑

go rf.electionLoop()

// leader逻辑

go rf.appendEntriesLoop()

DPrintf("Raftnode[%d]启动", me)

return rf

}

GetState（单元测试会调用）

// return currentTerm and whether this server
// believes it is the leader.
func (rf *Raft) GetState() (int, bool) {
	rf.mu.Lock()
	defer rf.mu.Unlock()

	var term int
	var isleader bool
	// Your code here (2A).
	term = rf.currentTerm
	isleader = rf.role == ROLE_LEADER
	return term, isleader
}

// return currentTerm and whether this server

// believes it is the leader.

func (rf *Raft) GetState() (int, bool) {

rf.mu.Lock()

defer rf.mu.Unlock()

var term int

var isleader bool

// Your code here (2A).

term = rf.currentTerm

isleader = rf.role == ROLE_LEADER

return term, isleader

}

RequestVote相关

//
// example RequestVote RPC handler.
//
func (rf *Raft) RequestVote(args *RequestVoteArgs, reply *RequestVoteReply) {
	// Your code here (2A, 2B).
	rf.mu.Lock()
	defer rf.mu.Unlock()

	reply.Term = rf.currentTerm
	reply.VoteGranted = false

	DPrintf("RaftNode[%d] Handle RequestVote, CandidatesId[%d] Term[%d] CurrentTerm[%d] LastLogIndex[%d] LastLogTerm[%d] votedFor[%d]",
		rf.me, args.CandidateId, args.Term, rf.currentTerm, args.LastLogIndex, args.LastLogTerm, rf.votedFor)
	defer func() {
		DPrintf("RaftNode[%d] Return RequestVote, CandidatesId[%d] VoteGranted[%v] ", rf.me, args.CandidateId, reply.VoteGranted)
	}()

	// 任期不如我大，拒绝投票
	if args.Term < rf.currentTerm {
		return
	}

	// 发现更大的任期，则转为该任期的follower
	if args.Term > rf.currentTerm {
		rf.currentTerm = args.Term
		rf.role = ROLE_FOLLOWER
		rf.votedFor = -1
		rf.leaderId = -1
		// 继续向下走，进行投票
	}

	// 每个任期，只能投票给1人
	if rf.votedFor == -1 || rf.votedFor == args.CandidateId {
		// candidate的日志必须比我的新
		// 1, 最后一条log，任期大的更新
		// 2，更长的log则更新
		lastLogTerm := 0
		if len(rf.log) != 0 {
			lastLogTerm = rf.log[len(rf.log)-1].Term
		}
		if args.LastLogTerm < lastLogTerm || args.LastLogIndex < len(rf.log) {
			return
		}
		rf.votedFor = args.CandidateId
		reply.VoteGranted = true
		rf.lastActiveTime = time.Now() // 为其他人投票，那么重置自己的下次投票时间
	}
	rf.persist()
}

func (rf *Raft) sendRequestVote(server int, args *RequestVoteArgs, reply *RequestVoteReply) bool {
	ok := rf.peers[server].Call("Raft.RequestVote", args, reply)
	return ok
}

func (rf *Raft) electionLoop() {
	for !rf.killed() {
		time.Sleep(1 * time.Millisecond)

		func() {
			rf.mu.Lock()
			defer rf.mu.Unlock()

			now := time.Now()
			timeout := time.Duration(200+rand.Int31n(150)) * time.Millisecond // 超时随机化
			elapses := now.Sub(rf.lastActiveTime)
			// follower -> candidates
			if rf.role == ROLE_FOLLOWER {
				if elapses >= timeout {
					rf.role = ROLE_CANDIDATES
					DPrintf("RaftNode[%d] Follower -> Candidate", rf.me)
				}
			}
			// 请求vote
			if rf.role == ROLE_CANDIDATES && elapses >= timeout {
				rf.lastActiveTime = now // 重置下次选举时间

				rf.currentTerm += 1 // 发起新任期
				rf.votedFor = rf.me // 该任期投了自己
				rf.persist()

				// 请求投票req
				args := RequestVoteArgs{
					Term:         rf.currentTerm,
					CandidateId:  rf.me,
					LastLogIndex: len(rf.log),
				}
				if len(rf.log) != 0 {
					args.LastLogTerm = rf.log[len(rf.log)-1].Term
				}

				rf.mu.Unlock()

				DPrintf("RaftNode[%d] RequestVote starts, Term[%d] LastLogIndex[%d] LastLogTerm[%d]", rf.me, args.Term,
					args.LastLogIndex, args.LastLogTerm)

				// 并发RPC请求vote
				type VoteResult struct {
					peerId int
					resp   *RequestVoteReply
				}
				voteCount := 1   // 收到投票个数（先给自己投1票）
				finishCount := 1 // 收到应答个数
				voteResultChan := make(chan *VoteResult, len(rf.peers))
				for peerId := 0; peerId < len(rf.peers); peerId++ {
					go func(id int) {
						if id == rf.me {
							return
						}
						resp := RequestVoteReply{}
						if ok := rf.sendRequestVote(id, &args, &resp); ok {
							voteResultChan <- &VoteResult{peerId: id, resp: &resp}
						} else {
							voteResultChan <- &VoteResult{peerId: id, resp: nil}
						}
					}(peerId)
				}

				maxTerm := 0
				for {
					select {
					case voteResult := <-voteResultChan:
						finishCount += 1
						if voteResult.resp != nil {
							if voteResult.resp.VoteGranted {
								voteCount += 1
							}
							if voteResult.resp.Term > maxTerm {
								maxTerm = voteResult.resp.Term
							}
						}
						// 得到大多数vote后，立即离开
						if finishCount == len(rf.peers) || voteCount > len(rf.peers)/2 {
							goto VOTE_END
						}
					}
				}
			VOTE_END:
				rf.mu.Lock()
				defer func() {
					DPrintf("RaftNode[%d] RequestVote ends, finishCount[%d] voteCount[%d] Role[%s] maxTerm[%d] currentTerm[%d]", rf.me, finishCount, voteCount,
						rf.role, maxTerm, rf.currentTerm)
				}()
				// 如果角色改变了，则忽略本轮投票结果
				if rf.role != ROLE_CANDIDATES {
					return
				}
				// 发现了更高的任期，切回follower
				if maxTerm > rf.currentTerm {
					rf.role = ROLE_FOLLOWER
					rf.leaderId = -1
					rf.currentTerm = maxTerm
					rf.votedFor = -1
					rf.persist()
					return
				}
				// 赢得大多数选票，则成为leader
				if voteCount > len(rf.peers)/2 {
					rf.role = ROLE_LEADER
					rf.leaderId = rf.me
					rf.lastBroadcastTime = time.Unix(0, 0) // 令appendEntries广播立即执行
					return
				}
			}
		}()
	}
}

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// example RequestVote RPC handler.

func (rf *Raft) RequestVote(args *RequestVoteArgs, reply *RequestVoteReply) {

// Your code here (2A, 2B).

rf.mu.Lock()

defer rf.mu.Unlock()

reply.Term = rf.currentTerm

reply.VoteGranted = false

DPrintf("RaftNode[%d] Handle RequestVote, CandidatesId[%d] Term[%d] CurrentTerm[%d] LastLogIndex[%d] LastLogTerm[%d] votedFor[%d]",

rf.me, args.CandidateId, args.Term, rf.currentTerm, args.LastLogIndex, args.LastLogTerm, rf.votedFor)

defer func() {

DPrintf("RaftNode[%d] Return RequestVote, CandidatesId[%d] VoteGranted[%v] ", rf.me, args.CandidateId, reply.VoteGranted)

}()

// 任期不如我大，拒绝投票

if args.Term < rf.currentTerm {

return

}

// 发现更大的任期，则转为该任期的follower

if args.Term > rf.currentTerm {

rf.currentTerm = args.Term

rf.role = ROLE_FOLLOWER

rf.votedFor = -1

rf.leaderId = -1

// 继续向下走，进行投票

}

// 每个任期，只能投票给1人

if rf.votedFor == -1 || rf.votedFor == args.CandidateId {

// candidate的日志必须比我的新

// 1, 最后一条log，任期大的更新

// 2，更长的log则更新

lastLogTerm := 0

if len(rf.log) != 0 {

lastLogTerm = rf.log[len(rf.log)-1].Term

}

if args.LastLogTerm < lastLogTerm || args.LastLogIndex < len(rf.log) {

return

}

rf.votedFor = args.CandidateId

reply.VoteGranted = true

rf.lastActiveTime = time.Now() // 为其他人投票，那么重置自己的下次投票时间

}

rf.persist()

}

func (rf *Raft) sendRequestVote(server int, args *RequestVoteArgs, reply *RequestVoteReply) bool {

ok := rf.peers[server].Call("Raft.RequestVote", args, reply)

return ok

}

func (rf *Raft) electionLoop() {

for !rf.killed() {

time.Sleep(1 * time.Millisecond)

func() {

rf.mu.Lock()

defer rf.mu.Unlock()

now := time.Now()

timeout := time.Duration(200+rand.Int31n(150)) * time.Millisecond // 超时随机化

elapses := now.Sub(rf.lastActiveTime)

// follower -> candidates

if rf.role == ROLE_FOLLOWER {

if elapses >= timeout {

rf.role = ROLE_CANDIDATES

DPrintf("RaftNode[%d] Follower -> Candidate", rf.me)

}

// 请求vote

if rf.role == ROLE_CANDIDATES && elapses >= timeout {

rf.lastActiveTime = now // 重置下次选举时间

rf.currentTerm += 1 // 发起新任期

rf.votedFor = rf.me // 该任期投了自己

rf.persist()

// 请求投票req

args := RequestVoteArgs{

Term: rf.currentTerm,

CandidateId: rf.me,

LastLogIndex: len(rf.log),

}

if len(rf.log) != 0 {

args.LastLogTerm = rf.log[len(rf.log)-1].Term

}

rf.mu.Unlock()

DPrintf("RaftNode[%d] RequestVote starts, Term[%d] LastLogIndex[%d] LastLogTerm[%d]", rf.me, args.Term,

args.LastLogIndex, args.LastLogTerm)

// 并发RPC请求vote

type VoteResult struct {

peerId int

resp *RequestVoteReply

}

voteCount := 1 // 收到投票个数（先给自己投1票）

finishCount := 1 // 收到应答个数

voteResultChan := make(chan *VoteResult, len(rf.peers))

for peerId := 0; peerId < len(rf.peers); peerId++ {

go func(id int) {

if id == rf.me {

return

}

resp := RequestVoteReply{}

if ok := rf.sendRequestVote(id, &args, &resp); ok {

voteResultChan <- &VoteResult{peerId: id, resp: &resp}

} else {

voteResultChan <- &VoteResult{peerId: id, resp: nil}

}

}(peerId)

}

maxTerm := 0

for {

select {

case voteResult := <-voteResultChan:

finishCount += 1

if voteResult.resp != nil {

if voteResult.resp.VoteGranted {

voteCount += 1

}

if voteResult.resp.Term > maxTerm {

maxTerm = voteResult.resp.Term

}

// 得到大多数vote后，立即离开

if finishCount == len(rf.peers) || voteCount > len(rf.peers)/2 {

goto VOTE_END

}

VOTE_END:

rf.mu.Lock()

defer func() {

DPrintf("RaftNode[%d] RequestVote ends, finishCount[%d] voteCount[%d] Role[%s] maxTerm[%d] currentTerm[%d]", rf.me, finishCount, voteCount,

rf.role, maxTerm, rf.currentTerm)

}()

// 如果角色改变了，则忽略本轮投票结果

if rf.role != ROLE_CANDIDATES {

return

}

// 发现了更高的任期，切回follower

if maxTerm > rf.currentTerm {

rf.role = ROLE_FOLLOWER

rf.leaderId = -1

rf.currentTerm = maxTerm

rf.votedFor = -1

rf.persist()

return

}

// 赢得大多数选票，则成为leader

if voteCount > len(rf.peers)/2 {

rf.role = ROLE_LEADER

rf.leaderId = rf.me

rf.lastBroadcastTime = time.Unix(0, 0) // 令appendEntries广播立即执行

return

}

}()

}

AppendEnties（仅心跳）

func (rf *Raft) AppendEntries(args *AppendEntriesArgs, reply *AppendEntriesReply) {
	rf.mu.Lock()
	defer rf.mu.Unlock()

	DPrintf("RaftNode[%d] Handle AppendEntries, LeaderId[%d] Term[%d] CurrentTerm[%d] role=[%s]",
		rf.me, args.LeaderId, args.Term, rf.currentTerm, rf.role)
	defer func() {
		DPrintf("RaftNode[%d] Return AppendEntries, LeaderId[%d] Term[%d] CurrentTerm[%d] role=[%s]",
			rf.me, args.LeaderId, args.Term, rf.currentTerm, rf.role)
	}()

	reply.Term = rf.currentTerm
	reply.Success = false

	if args.Term < rf.currentTerm {
		return
	}

	// 发现更大的任期，则转为该任期的follower
	if args.Term > rf.currentTerm {
		rf.currentTerm = args.Term
		rf.role = ROLE_FOLLOWER
		rf.votedFor = -1
		rf.leaderId = -1
		// 继续向下走
	}

	// 认识新的leader
	rf.leaderId = args.LeaderId
	// 刷新活跃时间
	rf.lastActiveTime = time.Now()

	// 日志操作lab-2A不实现
	rf.persist()
}

func (rf *Raft) sendAppendEntries(server int, args *AppendEntriesArgs, reply *AppendEntriesReply) bool {
	ok := rf.peers[server].Call("Raft.AppendEntries", args, reply)
	return ok
}

// lab-2A只做心跳，不考虑log同步
func (rf *Raft) appendEntriesLoop() {
	for !rf.killed() {
		time.Sleep(1 * time.Millisecond)

		func() {
			rf.mu.Lock()
			defer rf.mu.Unlock()

			// 只有leader才向外广播心跳
			if rf.role != ROLE_LEADER {
				return
			}

			// 100ms广播1次
			now := time.Now()
			if now.Sub(rf.lastBroadcastTime) < 100*time.Millisecond {
				return
			}
			rf.lastBroadcastTime = time.Now()

			// 并发RPC心跳
			type AppendResult struct {
				peerId int
				resp   *AppendEntriesReply
			}

			for peerId := 0; peerId < len(rf.peers); peerId++ {
				if peerId == rf.me {
					continue
				}

				args := AppendEntriesArgs{}
				args.Term = rf.currentTerm
				args.LeaderId = rf.me
				// log相关字段在lab-2A不处理
				go func(id int, args1 *AppendEntriesArgs) {
					DPrintf("RaftNode[%d] appendEntries starts, myTerm[%d] peerId[%d]", rf.me, args1.Term, id)
					reply := AppendEntriesReply{}
					if ok := rf.sendAppendEntries(id, args1, &reply); ok {
						rf.mu.Lock()
						defer rf.mu.Unlock()
						if reply.Term > rf.currentTerm { // 变成follower
							rf.role = ROLE_FOLLOWER
							rf.leaderId = -1
							rf.currentTerm = reply.Term
							rf.votedFor = -1
							rf.persist()
						}
						DPrintf("RaftNode[%d] appendEntries ends, peerTerm[%d] myCurrentTerm[%d] myRole[%s]", rf.me, reply.Term, rf.currentTerm, rf.role)
					}
				}(peerId, &args)
			}
		}()
	}
}

func (rf *Raft) AppendEntries(args *AppendEntriesArgs, reply *AppendEntriesReply) {

rf.mu.Lock()

defer rf.mu.Unlock()

DPrintf("RaftNode[%d] Handle AppendEntries, LeaderId[%d] Term[%d] CurrentTerm[%d] role=[%s]",

rf.me, args.LeaderId, args.Term, rf.currentTerm, rf.role)

defer func() {

DPrintf("RaftNode[%d] Return AppendEntries, LeaderId[%d] Term[%d] CurrentTerm[%d] role=[%s]",

rf.me, args.LeaderId, args.Term, rf.currentTerm, rf.role)

}()

reply.Term = rf.currentTerm

reply.Success = false

if args.Term < rf.currentTerm {

return

}

// 发现更大的任期，则转为该任期的follower

if args.Term > rf.currentTerm {

rf.currentTerm = args.Term

rf.role = ROLE_FOLLOWER

rf.votedFor = -1

rf.leaderId = -1

// 继续向下走

}

// 认识新的leader

rf.leaderId = args.LeaderId

// 刷新活跃时间

rf.lastActiveTime = time.Now()

// 日志操作lab-2A不实现

rf.persist()

}

func (rf *Raft) sendAppendEntries(server int, args *AppendEntriesArgs, reply *AppendEntriesReply) bool {

ok := rf.peers[server].Call("Raft.AppendEntries", args, reply)

return ok

}

// lab-2A只做心跳，不考虑log同步

func (rf *Raft) appendEntriesLoop() {

for !rf.killed() {

time.Sleep(1 * time.Millisecond)

func() {

rf.mu.Lock()

defer rf.mu.Unlock()

// 只有leader才向外广播心跳

if rf.role != ROLE_LEADER {

return

}

// 100ms广播1次

now := time.Now()

if now.Sub(rf.lastBroadcastTime) < 100*time.Millisecond {

return

}

rf.lastBroadcastTime = time.Now()

// 并发RPC心跳

type AppendResult struct {

peerId int

resp *AppendEntriesReply

}

for peerId := 0; peerId < len(rf.peers); peerId++ {

if peerId == rf.me {

continue

}

args := AppendEntriesArgs{}

args.Term = rf.currentTerm

args.LeaderId = rf.me

// log相关字段在lab-2A不处理

go func(id int, args1 *AppendEntriesArgs) {

DPrintf("RaftNode[%d] appendEntries starts, myTerm[%d] peerId[%d]", rf.me, args1.Term, id)

reply := AppendEntriesReply{}

if ok := rf.sendAppendEntries(id, args1, &reply); ok {

rf.mu.Lock()

defer rf.mu.Unlock()

if reply.Term > rf.currentTerm { // 变成follower

rf.role = ROLE_FOLLOWER

rf.leaderId = -1

rf.currentTerm = reply.Term

rf.votedFor = -1

rf.persist()

}

DPrintf("RaftNode[%d] appendEntries ends, peerTerm[%d] myCurrentTerm[%d] myRole[%s]", rf.me, reply.Term, rf.currentTerm, rf.role)

}

}(peerId, &args)

}

}()

}

总结Lab2A

一把大锁保护好状态，RPC期间释放锁，RPC结束后注意状态二次判定
request/response都要先判断term > currentTerm，转换follower
一个currentTerm只能voteFor其他节点1次
注意candidates请求vote的时间随机性
注意requestVote得到大多数投票后立即结束等待剩余RPC
注意成为leader后尽快appendEntries心跳，否则其他节点又会成为candidates
注意几个刷新选举超时时间的逻辑点

其他了解：

虽然Lab2A不要求关注persist状态的落地（currentTerm,voteFor,[]log），但是我稍微扫了一眼代码，发现它的persist其实还是写在内存里，供单元测试去check结果，因此猜测这门课可能不会真的实现raft如何持久化log这个存储部分（从目前来看的确是这样的）。

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MIT 6.824: Distributed Systems- 实现Raft Lab2A

学习资料

Lab2A实现

结构体定义

Make 程序入口

GetState（单元测试会调用）

RequestVote相关

AppendEnties（仅心跳）

总结Lab2A

《MIT 6.824: Distributed Systems- 实现Raft Lab2A》有1个想法

发表评论取消回复

学习资料

Lab2A实现

结构体定义

Make 程序入口

GetState（单元测试会调用）

RequestVote相关

AppendEnties（仅心跳）

总结Lab2A

《MIT 6.824: Distributed Systems- 实现Raft Lab2A》有1个想法

发表评论 取消回复

发表评论取消回复