MIT6.5840-2023-Lab2C: Raft-Persistence

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前置知识

见上一篇 Lab2A。

实验内容

实现 RAFT，分为四个 part：leader election、log、persistence、log compaction。

实验环境

OS：WSL-Ubuntu-18.04
golang：go1.17.6 linux/amd64

Part 2C: persistence

大部分的bug都与这张图有关。如果前两次lab通过了千次以上测试，这边应该问题不大。注意rpc前后的状态判断。
在这里插入图片描述

实现持久化，重启后能快速恢复。真正的实现将在每次更改时在磁盘写下 raft 的持久状态，并在重新启动后从磁盘中读取状态。lab 实现时在 Persister 中存储和恢复。currentTerm、votedFor、log[]
persist 和 readPersist：通过 labgob实现。

// save Raft's persistent state to stable storage,
// where it can later be retrieved after a crash and restart.
// see paper's Figure 2 for a description of what should be persistent.
// before you've implemented snapshots, you should pass nil as the
// second argument to persister.Save().
// after you've implemented snapshots, pass the current snapshot
// (or nil if there's not yet a snapshot).
func (rf *Raft) persist() {// Your code here (2C).// Example:w := new(bytes.Buffer)e := labgob.NewEncoder(w)e.Encode(rf.currentTerm)e.Encode(rf.votedFor)e.Encode(rf.log)raftstate := w.Bytes()rf.persister.Save(raftstate, nil)
}// restore previously persisted state.
func (rf *Raft) readPersist(data []byte) {if data == nil || len(data) < 1 { // bootstrap without any state?return}// Your code here (2C).// Example:r := bytes.NewBuffer(data)d := labgob.NewDecoder(r)var currentTerm, votedFor intvar logs []Entryif d.Decode(&currentTerm) != nil ||d.Decode(&votedFor) != nil ||d.Decode(&logs) != nil {log.Println("decode fail")} else {rf.currentTerm = currentTermrf.votedFor = votedForrf.log = logslog.Println("restore success")}
}

优化：避免 leader 每次只往前移动一位；若日志很长的话在一段时间内无法达到冲突位置。若 leader 发送心跳时接收到的回复是 false，leader 会减小发送的 AppendEntriesArgs 中的 rf.nextIndex，但是这种每次仅减小 1 的方案无法在有限的时间内确定跟其他服务器发生冲突的下标位置。

func (rf *Raft) AppendEntries(args *AppendEntriesArgs, reply *AppendEntriesReply) {// Your code here (2A, 2B).rf.mu.Lock()defer rf.mu.Unlock()if args.Term > rf.currentTerm { // all serverrf.state = Followerrf.currentTerm = args.Termrf.votedFor = -1rf.persist()}if args.Term < rf.currentTerm {reply.Success = falsereply.Term = rf.currentTerm} else if len(rf.log)-1 < args.PrevLogIndex ||rf.log[args.PrevLogIndex].Term != args.PrevLogTerm {log.Println(rf.me, "mismatch", "len(rf.log)", len(rf.log), "args.PrevLogIndex", args.PrevLogIndex)reply.Success = falsereply.Term = rf.currentTermtimeout := time.Duration(250+rand.Intn(300)) * time.Millisecondrf.expiryTime = time.Now().Add(timeout)if args.PrevLogIndex > len(rf.log)-1 {reply.ConflictIndex = len(rf.log)reply.ConflictTerm = -1} else {reply.ConflictTerm = rf.log[args.PrevLogIndex].Termreply.ConflictIndex = 1for i := args.PrevLogIndex - 1; i >= 0; i-- {if rf.log[i].Term != reply.ConflictTerm {reply.ConflictIndex += ibreak}}}} else {reply.Success = truereply.Term = rf.currentTermtimeout := time.Duration(250+rand.Intn(300)) * time.Millisecondrf.expiryTime = time.Now().Add(timeout)// // delete// rf.log = rf.log[:args.PrevLogIndex+1]// // append// rf.log = append(rf.log, args.Entries...)insertIndex := args.PrevLogIndex + 1argsLogIndex := 0for {if insertIndex >= len(rf.log) ||argsLogIndex >= len(args.Entries) ||rf.log[insertIndex].Term != args.Entries[argsLogIndex].Term {break}insertIndex++argsLogIndex++}// for _, e := range rf.log {// 	log.Print(e)// }// for _, e := range args.Entries {// 	log.Print(e)// }if argsLogIndex < len(args.Entries) {rf.log = append(rf.log[:insertIndex], args.Entries[argsLogIndex:]...)rf.persist()}if args.LeaderCommit > rf.commitIndex {rf.commitIndex = int(math.Min(float64(args.LeaderCommit), float64(len(rf.log)-1)))log.Println(rf.me, "follower update commitIndex", "args.LeaderCommit", args.LeaderCommit, "len(rf.log)", len(rf.log), "rf.commitIndex", rf.commitIndex)}}}

// 修改前
// if args.PrevLogIndex > 0 {
// 	rf.nextIndex[i] = args.PrevLogIndex
// }
// 修改后if reply.ConflictTerm >= 0 {lastIndex := -1for i := len(rf.log) - 1; i >= 0; i-- {if rf.log[i].Term == reply.ConflictTerm {lastIndex = ibreak}}if lastIndex >= 0 {rf.nextIndex[i] = lastIndex + 1} else {rf.nextIndex[i] = reply.ConflictIndex}} else {rf.nextIndex[i] = reply.ConflictIndex}