etcdserver/server.go
func (s *EtcdServer) applyEntryNormal(e *raftpb.Entry) {
	shouldApplyV3 := false
	if e.Index > s.consistIndex.ConsistentIndex() {
		// set the consistent index of current executing entry
		s.consistIndex.setConsistentIndex(e.Index)
		shouldApplyV3 = true
	}

	// raft state machine may generate noop entry when leader confirmation.
	// skip it in advance to avoid some potential bug in the future
	if len(e.Data) == 0 {
		select {
		case s.forceVersionC <- struct{}{}:
		default:
		}
		// promote lessor when the local member is leader and finished
		// applying all entries from the last term.
		if s.isLeader() {
			s.lessor.Promote(s.Cfg.electionTimeout())
		}
		return
	}

	var raftReq pb.InternalRaftRequest
	if !pbutil.MaybeUnmarshal(&raftReq, e.Data) { // backward compatible
		var r pb.Request
		rp := &r
		pbutil.MustUnmarshal(rp, e.Data)
		s.w.Trigger(r.ID, s.applyV2Request((*RequestV2)(rp)))
		return
	}
	if raftReq.V2 != nil {
		req := (*RequestV2)(raftReq.V2)
		s.w.Trigger(req.ID, s.applyV2Request(req))
		return
	}

	// do not re-apply applied entries.
	if !shouldApplyV3 {
		return
	}

	id := raftReq.ID
	if id == 0 {
		id = raftReq.Header.ID
	}

	var ar *applyResult
	needResult := s.w.IsRegistered(id)
	if needResult || !noSideEffect(&raftReq) {
		if !needResult && raftReq.Txn != nil {
			removeNeedlessRangeReqs(raftReq.Txn)
		}
		ar = s.applyV3.Apply(&raftReq)
	}

	if ar == nil {
		return
	}

	if ar.err != ErrNoSpace || len(s.alarmStore.Get(pb.AlarmType_NOSPACE)) > 0 {
		s.w.Trigger(id, ar)
		return
	}

	if lg := s.getLogger(); lg != nil {
		lg.Warn(
			"message exceeded backend quota; raising alarm",
			zap.Int64("quota-size-bytes", s.Cfg.QuotaBackendBytes),
			zap.String("quota-size", humanize.Bytes(uint64(s.Cfg.QuotaBackendBytes))),
			zap.Error(ar.err),
		)
	} else {
		plog.Errorf("applying raft message exceeded backend quota")
	}

	s.goAttach(func() {
		a := &pb.AlarmRequest{
			MemberID: uint64(s.ID()),
			Action:   pb.AlarmRequest_ACTIVATE,
			Alarm:    pb.AlarmType_NOSPACE,
		}
		s.raftRequest(s.ctx, pb.InternalRaftRequest{Alarm: a})
		s.w.Trigger(id, ar)
	})
}
func (s *EtcdServer) applyEntryNormal(e *raftpb.Entry) {
	shouldApplyV3 := false
	if e.Index > s.consistIndex.ConsistentIndex() {
		// set the consistent index of current executing entry
		s.consistIndex.setConsistentIndex(e.Index)
		shouldApplyV3 = true
	}

	// raft state machine may generate noop entry when leader confirmation.
	// skip it in advance to avoid some potential bug in the future
	if len(e.Data) == 0 {
		select {
		case s.forceVersionC <- struct{}{}:
		default:
		}
		// promote lessor when the local member is leader and finished
		// applying all entries from the last term.
		if s.isLeader() {
			s.lessor.Promote(s.Cfg.electionTimeout())
		}
		return
	}

	var raftReq pb.InternalRaftRequest
	if !pbutil.MaybeUnmarshal(&raftReq, e.Data) { // backward compatible
		var r pb.Request
		rp := &r
		pbutil.MustUnmarshal(rp, e.Data)
		s.w.Trigger(r.ID, s.applyV2Request((*RequestV2)(rp)))
		return
	}
	if raftReq.V2 != nil {
		req := (*RequestV2)(raftReq.V2)
		s.w.Trigger(req.ID, s.applyV2Request(req))
		return
	}

	// do not re-apply applied entries.
	if !shouldApplyV3 {
		return
	}

	id := raftReq.ID
	if id == 0 {
		id = raftReq.Header.ID
	}

	var ar *applyResult
	needResult := s.w.IsRegistered(id)
	if needResult || !noSideEffect(&raftReq) {
		if !needResult && raftReq.Txn != nil {
			removeNeedlessRangeReqs(raftReq.Txn)
		}
		ar = s.applyV3.Apply(&raftReq)
	}

	if ar == nil {
		return
	}

	if ar.err != ErrNoSpace || len(s.alarmStore.Get(pb.AlarmType_NOSPACE)) > 0 {
		s.w.Trigger(id, ar)
		return
	}

	if lg := s.getLogger(); lg != nil {
		lg.Warn(
			"message exceeded backend quota; raising alarm",
			zap.Int64("quota-size-bytes", s.Cfg.QuotaBackendBytes),
			zap.String("quota-size", humanize.Bytes(uint64(s.Cfg.QuotaBackendBytes))),
			zap.Error(ar.err),
		)
	} else {
		plog.Errorf("applying raft message exceeded backend quota")
	}

	s.goAttach(func() {
		a := &pb.AlarmRequest{
			MemberID: uint64(s.ID()),
			Action:   pb.AlarmRequest_ACTIVATE,
			Alarm:    pb.AlarmType_NOSPACE,
		}
		s.raftRequest(s.ctx, pb.InternalRaftRequest{Alarm: a})
		s.w.Trigger(id, ar)
	})
}
// applyEntryNormal apples an EntryNormal type raftpb request to the EtcdServer