raft 学习笔记
raft
选举过程
跟随者一个心跳周期没有收到心跳，触发选举，但是为了选举安全性，和领导者失联的时间必须超过一个心跳周期开始
状态变化
跟随者 ---> 候选者 ---> 领导者
处理交互请求，例如 追加日志，选举投票等
// processRPC is called to handle an incoming RPC request. This must only be
// called from the main thread.
func (r *Raft) processRPC(rpc RPC) {
if err := r.checkRPCHeader(rpc); err != nil {
rpc.Respond(nil, err)
return
}
switch cmd := rpc.Command.(type) {
case *AppendEntriesRequest:
r.appendEntries(rpc, cmd)
case *RequestVoteRequest:
r.requestVote(rpc, cmd)
case *InstallSnapshotRequest:
r.installSnapshot(rpc, cmd)
case *TimeoutNowRequest:
r.timeoutNow(rpc, cmd)
default:
r.logger.Error("got unexpected command",
"command", hclog.Fmt("%#v", rpc.Command))
rpc.Respond(nil, fmt.Errorf("unexpected command"))
}
}
启动
// run is a long running goroutine that runs the Raft FSM.
func (r *Raft) run() {
for {
// Check if we are doing a shutdown
select {
case <-r.shutdownCh:
// Clear the leader to prevent forwarding
r.setLeader("")
return
default:
}
// Enter into a sub-FSM
switch r.getState() {
case Follower:
r.runFollower() // 默认 follower
case Candidate:
r.runCandidate()
case Leader:
r.runLeader()
}
}
}
执行跟随者，转化为候选者
// runFollower runs the FSM for a follower.
func (r *Raft) runFollower() {
didWarn := false
r.logger.Info("entering follower state", "follower", r, "leader", r.Leader())
metrics.IncrCounter([]string{"raft", "state", "follower"}, 1)
heartbeatTimer := randomTimeout(r.config().HeartbeatTimeout)
for r.getState() == Follower {
select {
//省略部分代码
case <-heartbeatTimer:
// 心跳超时，开始选举
// Restart the heartbeat timer
hbTimeout := r.config().HeartbeatTimeout
heartbeatTimer = randomTimeout(hbTimeout)
// Check if we have had a successful contact
lastContact := r.LastContact()
if time.Now().Sub(lastContact) < hbTimeout {
continue
}
// Heartbeat failed! Transition to the candidate state
lastLeader := r.Leader()
r.setLeader("")
if r.configurations.latestIndex == 0 {
if !didWarn {
r.logger.Warn("no known peers, aborting election")
didWarn = true
}
} else if r.configurations.latestIndex == r.configurations.committedIndex &&
!hasVote(r.configurations.latest, r.localID) {
if !didWarn {
r.logger.Warn("not part of stable configuration, aborting election")
didWarn = true
}
} else {
r.logger.Warn("heartbeat timeout reached, starting election", "last-leader", lastLeader)
metrics.IncrCounter([]string{"raft", "transition", "heartbeat_timeout"}, 1)
// 转化为候选者
r.setState(Candidate)
return
}
case <-r.shutdownCh:
return
}
}
}
执行候选者，先为自己投票，得到票数大于规定票数，则
变成leader
// runCandidate runs the FSM for a candidate.
func (r *Raft) runCandidate() {
r.logger.Info("entering candidate state", "node", r, "term", r.getCurrentTerm()+1)
metrics.IncrCounter([]string{"raft", "state", "candidate"}, 1)
// Start vote for us, and set a timeout
voteCh := r.electSelf()
// Make sure the leadership transfer flag is reset after each run. Having this
// flag will set the field LeadershipTransfer in a RequestVoteRequst to true,
// which will make other servers vote even though they have a leader already.
// It is important to reset that flag, because this priviledge could be abused
// otherwise.
defer func() { r.candidateFromLeadershipTransfer = false }()
electionTimer := randomTimeout(r.config().ElectionTimeout)
// Tally the votes, need a simple majority
grantedVotes := 0
votesNeeded := r.quorumSize()
r.logger.Debug("votes", "needed", votesNeeded)
for r.getState() == Candidate {
select {
case rpc := <-r.rpcCh:
r.processRPC(rpc)
case vote := <-voteCh:
// Check if the term is greater than ours, bail
if vote.Term > r.getCurrentTerm() {
r.logger.Debug("newer term discovered, fallback to follower")
r.setState(Follower)
r.setCurrentTerm(vote.Term)
return
}
// Check if the vote is granted
if vote.Granted {
grantedVotes++
r.logger.Debug("vote granted", "from", vote.voterID, "term", vote.Term, "tally", grantedVotes)
}
// Check if we've become the leader
if grantedVotes >= votesNeeded {
r.logger.Info("election won", "tally", grantedVotes)
r.setState(Leader)
r.setLeader(r.localAddr)
return
}
}
}
}
发起投票
// Start vote for us, and set a timeout
voteCh := r.electSelf()
// electSelf is used to send a RequestVote RPC to all peers, and vote for
// ourself. This has the side affecting of incrementing the current term. The
// response channel returned is used to wait for all the responses (including a
// vote for ourself). This must only be called from the main thread.
func (r *Raft) electSelf() <-chan *voteResult {
// Create a response channel
respCh := make(chan *voteResult, len(r.configurations.latest.Servers))
// Increment the term
r.setCurrentTerm(r.getCurrentTerm() + 1)
// Construct the request
lastIdx, lastTerm := r.getLastEntry()
req := &RequestVoteRequest{
RPCHeader: r.getRPCHeader(),
Term: r.getCurrentTerm(),
Candidate: r.trans.EncodePeer(r.localID, r.localAddr),
LastLogIndex: lastIdx,
LastLogTerm: lastTerm,
LeadershipTransfer: r.candidateFromLeadershipTransfer,
}
// Construct a function to ask for a vote
askPeer := func(peer Server) {
r.goFunc(func() {
defer metrics.MeasureSince([]string{"raft", "candidate", "electSelf"}, time.Now())
resp := &voteResult{voterID: peer.ID}
err := r.trans.RequestVote(peer.ID, peer.Address, req, &resp.RequestVoteResponse)
if err != nil {
r.logger.Error("failed to make requestVote RPC",
"target", peer,
"error", err)
resp.Term = req.Term
resp.Granted = false
}
respCh <- resp
})
}
// For each peer, request a vote
for _, server := range r.configurations.latest.Servers {
if server.Suffrage == Voter {
if server.ID == r.localID {
// Persist a vote for ourselves
if err := r.persistVote(req.Term, req.Candidate); err != nil {
r.logger.Error("failed to persist vote", "error", err)
return nil
}
// Include our own vote
respCh <- &voteResult{
RequestVoteResponse: RequestVoteResponse{
RPCHeader: r.getRPCHeader(),
Term: req.Term,
Granted: true,
},
voterID: r.localID,
}
} else {
askPeer(server)
}
}
}
return respCh
}
给候选者投票， 只有任期比我大且日志比我长的人才投票，相同任期只投一票
// requestVote is invoked when we get an request vote RPC call.
func (r *Raft) requestVote(rpc RPC, req *RequestVoteRequest) {
defer metrics.MeasureSince([]string{"raft", "rpc", "requestVote"}, time.Now())
r.observe(*req)
// Setup a response
resp := &RequestVoteResponse{
RPCHeader: r.getRPCHeader(),
Term: r.getCurrentTerm(),
Granted: false,
}
var rpcErr error
defer func() {
rpc.Respond(resp, rpcErr)
}()
// Version 0 servers will panic unless the peers is present. It's only
// used on them to produce a warning message.
if r.protocolVersion < 2 {
resp.Peers = encodePeers(r.configurations.latest, r.trans)
}
// Check if we have an existing leader [who's not the candidate] and also
// check the LeadershipTransfer flag is set. Usually votes are rejected if
// there is a known leader. But if the leader initiated a leadership transfer,
// vote!
candidate := r.trans.DecodePeer(req.Candidate)
if leader := r.Leader(); leader != "" && leader != candidate && !req.LeadershipTransfer {
r.logger.Warn("rejecting vote request since we have a leader",
"from", candidate,
"leader", leader)
return
}
// Ignore an older term
if req.Term < r.getCurrentTerm() {
return
}
// Increase the term if we see a newer one
if req.Term > r.getCurrentTerm() {
// Ensure transition to follower
r.logger.Debug("lost leadership because received a requestVote with a newer term")
r.setState(Follower)
r.setCurrentTerm(req.Term)
resp.Term = req.Term
}
// Check if we have voted yet
lastVoteTerm, err := r.stable.GetUint64(keyLastVoteTerm)
if err != nil && err.Error() != "not found" {
r.logger.Error("failed to get last vote term", "error", err)
return
}
lastVoteCandBytes, err := r.stable.Get(keyLastVoteCand)
if err != nil && err.Error() != "not found" {
r.logger.Error("failed to get last vote candidate", "error", err)
return
}
// Check if we've voted in this election before
if lastVoteTerm == req.Term && lastVoteCandBytes != nil {
r.logger.Info("duplicate requestVote for same term", "term", req.Term)
if bytes.Compare(lastVoteCandBytes, req.Candidate) == 0 {
r.logger.Warn("duplicate requestVote from", "candidate", candidate)
resp.Granted = true
}
return
}
// Reject if their term is older
lastIdx, lastTerm := r.getLastEntry()
if lastTerm > req.LastLogTerm {
r.logger.Warn("rejecting vote request since our last term is greater",
"candidate", candidate,
"last-term", lastTerm,
"last-candidate-term", req.LastLogTerm)
return
}
if lastTerm == req.LastLogTerm && lastIdx > req.LastLogIndex {
r.logger.Warn("rejecting vote request since our last index is greater",
"candidate", candidate,
"last-index", lastIdx,
"last-candidate-index", req.LastLogIndex)
return
}
// Persist a vote for safety
if err := r.persistVote(req.Term, req.Candidate); err != nil {
r.logger.Error("failed to persist vote", "error", err)
return
}
resp.Granted = true
r.setLastContact()
return
}
当选leader 后，立即给其他成员发送命令，使其成为 跟随者
复制过程
跟随者 接受领导者的日志
// runFollower runs the FSM for a follower.
func (r *Raft) runFollower() {
didWarn := false
r.logger.Info("entering follower state", "follower", r, "leader", r.Leader())
metrics.IncrCounter([]string{"raft", "state", "follower"}, 1)
heartbeatTimer := randomTimeout(r.config().HeartbeatTimeout)
for r.getState() == Follower {
select {
case rpc := <-r.rpcCh:
r.processRPC(rpc)
// 忽略部分代码
}
}
}
处理日志
// appendEntries is invoked when we get an append entries RPC call. This must
// only be called from the main thread.
func (r *Raft) appendEntries(rpc RPC, a *AppendEntriesRequest) {
defer metrics.MeasureSince([]string{"raft", "rpc", "appendEntries"}, time.Now())
// Setup a response
resp := &AppendEntriesResponse{
RPCHeader: r.getRPCHeader(),
Term: r.getCurrentTerm(),
LastLog: r.getLastIndex(),
Success: false,
NoRetryBackoff: false,
}
var rpcErr error
defer func() {
rpc.Respond(resp, rpcErr)
}()
// Ignore an older term
if a.Term < r.getCurrentTerm() {
return
}
// Increase the term if we see a newer one, also transition to follower
// if we ever get an appendEntries call
if a.Term > r.getCurrentTerm() || r.getState() != Follower {
// Ensure transition to follower
r.setState(Follower)
r.setCurrentTerm(a.Term)
resp.Term = a.Term
}
// Save the current leader
r.setLeader(r.trans.DecodePeer(a.Leader))
// Verify the last log entry
if a.PrevLogEntry > 0 {
lastIdx, lastTerm := r.getLastEntry()
var prevLogTerm uint64
if a.PrevLogEntry == lastIdx {
prevLogTerm = lastTerm
} else {
var prevLog Log
if err := r.logs.GetLog(a.PrevLogEntry, &prevLog); err != nil {
r.logger.Warn("failed to get previous log",
"previous-index", a.PrevLogEntry,
"last-index", lastIdx,
"error", err)
resp.NoRetryBackoff = true
return
}
prevLogTerm = prevLog.Term
}
if a.PrevLogTerm != prevLogTerm {
r.logger.Warn("previous log term mis-match",
"ours", prevLogTerm,
"remote", a.PrevLogTerm)
resp.NoRetryBackoff = true
return
}
}
// Process any new entries
if len(a.Entries) > 0 {
start := time.Now()
// Delete any conflicting entries, skip any duplicates
lastLogIdx, _ := r.getLastLog()
var newEntries []*Log
for i, entry := range a.Entries {
if entry.Index > lastLogIdx {
newEntries = a.Entries[i:]
break
}
var storeEntry Log
if err := r.logs.GetLog(entry.Index, &storeEntry); err != nil {
r.logger.Warn("failed to get log entry",
"index", entry.Index,
"error", err)
return
}
if entry.Term != storeEntry.Term {
r.logger.Warn("clearing log suffix",
"from", entry.Index,
"to", lastLogIdx)
if err := r.logs.DeleteRange(entry.Index, lastLogIdx); err != nil {
r.logger.Error("failed to clear log suffix", "error", err)
return
}
if entry.Index <= r.configurations.latestIndex {
r.setLatestConfiguration(r.configurations.committed, r.configurations.committedIndex)
}
newEntries = a.Entries[i:]
break
}
}
if n := len(newEntries); n > 0 {
// Append the new entries
if err := r.logs.StoreLogs(newEntries); err != nil {
r.logger.Error("failed to append to logs", "error", err)
// TODO: leaving r.getLastLog() in the wrong
// state if there was a truncation above
return
}
// Handle any new configuration changes
for _, newEntry := range newEntries {
r.processConfigurationLogEntry(newEntry)
}
// Update the lastLog
last := newEntries[n-1]
r.setLastLog(last.Index, last.Term)
}
metrics.MeasureSince([]string{"raft", "rpc", "appendEntries", "storeLogs"}, start)
}
// Update the commit index
if a.LeaderCommitIndex > 0 && a.LeaderCommitIndex > r.getCommitIndex() {
start := time.Now()
idx := min(a.LeaderCommitIndex, r.getLastIndex())
r.setCommitIndex(idx)
if r.configurations.latestIndex <= idx {
r.setCommittedConfiguration(r.configurations.latest, r.configurations.latestIndex)
}
r.processLogs(idx, nil)
metrics.MeasureSince([]string{"raft", "rpc", "appendEntries", "processLogs"}, start)
}
// Everything went well, set success
resp.Success = true
r.setLastContact()
return
}
