// maybeCommit attempts to advance the commit index. Returns true if the commit
// index changed (in which case the caller should call r.bcastAppend). This can
// only be called in StateLeader.
func (r *raft) maybeCommit() bool {
	defer traceCommit(r)

	return r.raftLog.maybeCommit(entryID{term: r.Term, index: r.trk.Committed()})
}
…
func (r *raft) becomeCandidate() {
	// TODO(xiangli) remove the panic when the raft implementation is stable
	if r.state == StateLeader {
		panic("invalid transition [leader -> candidate]")
	}
	r.step = stepCandidate
	r.reset(r.Term + 1)
	r.tick = r.tickElection
	r.Vote = r.id
	r.state = StateCandidate
	r.logger.Infof("%x became candidate at term %d", r.id, r.Term)

	traceBecomeCandidate(r)
}
…
	case pb.MsgVote, pb.MsgPreVote:
		// We can vote if this is a repeat of a vote we've already cast...
		canVote := r.Vote == m.GetFrom() ||
			// ...we haven't voted and we don't think there's a leader yet in this term...
			(r.Vote == None && r.lead == None) ||
			// ...or this is a PreVote for a future term...
			(m.GetType() == pb.MsgPreVote && m.GetTerm() > r.Term)
		// ...and we believe the candidate is up to date.
		lastID := r.raftLog.lastEntryID()
		candLastID := entryID{term: m.GetLogTerm(), index: m.GetIndex()}
		if canVote && r.raftLog.isUpToDate(candLastID) {
…
			r.send(&pb.Message{To: m.From, Term: m.Term, Type: voteRespMsgType(m.GetType()).Enum()})
			if m.GetType() == pb.MsgVote {
				// Only record real votes.
				r.electionElapsed = 0
				r.Vote = m.GetFrom()
			}
		} else {
			r.logger.Infof("%x [logterm: %d, index: %d, vote: %x] rejected %s from %x [logterm: %d, index: %d] at term %d",
				r.id, lastID.term, lastID.index, r.Vote, m.GetType(), m.GetFrom(), candLastID.term, candLastID.index, r.Term)
			r.send(&pb.Message{To: m.From, Term: new(r.Term), Type: voteRespMsgType(m.GetType()).Enum(), Reject: new(true)})
		}
Warm-up
