#golang#raft#leader#network

raft.go

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(),
		// 返回自己节点的任期
		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)
	// 获取本节点认为的leader
	// 不是处于leadershipTransfer
	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")
		// 自己成为follow
		r.setState(Follower)
		// 成为current term
		r.setCurrentTerm(req.Term)
		resp.Term = req.Term
	}

	// Check if we have voted yet
	// 是否有投票过
	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
	}
	// 获取上次投票的结果
	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
	// 有投票过，
	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)
			// 已经投票了
			resp.Granted = true
		}
		// 别的节点，那么直接false
		return
	}

	// Reject if their term is older
	lastIdx, lastTerm := r.getLastEntry()
	// 本地的term大于上次的log term，那么直接vote false
	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
	}

	// term相同，并且相同的term,本地logindex 的大于请求lastLogIndex
	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
	// 持久化投票结果
	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
}