func (a *applierV2store) Put(r *pb.Request) Response {
	ttlOptions := toTTLOptions(r)
	exists, existsSet := pbutil.GetBool(r.PrevExist)
	switch {
	case existsSet:
		if exists {
			if r.PrevIndex == 0 && r.PrevValue == "" {
				return toResponse(a.s.store.Update(r.Path, r.Val, ttlOptions))
			} else {
				return toResponse(a.s.store.CompareAndSwap(r.Path, r.PrevValue, r.PrevIndex, r.Val, ttlOptions))
			}
		}
		return toResponse(a.s.store.Create(r.Path, r.Dir, r.Val, false, ttlOptions))
	case r.PrevIndex > 0 || r.PrevValue != "":
		return toResponse(a.s.store.CompareAndSwap(r.Path, r.PrevValue, r.PrevIndex, r.Val, ttlOptions))
	default:
		if storeMemberAttributeRegexp.MatchString(r.Path) {
			id := membership.MustParseMemberIDFromKey(path.Dir(r.Path))
			var attr membership.Attributes
			if err := json.Unmarshal([]byte(r.Val), &attr); err != nil {
				plog.Panicf("unmarshal %s should never fail: %v", r.Val, err)
			}
			a.s.cluster.UpdateAttributes(id, attr)
			// return an empty response since there is no consumer.
			return Response{}
		}
		if r.Path == membership.StoreClusterVersionKey() {
			a.s.cluster.SetVersion(semver.Must(semver.NewVersion(r.Val)))
			// return an empty response since there is no consumer.
			return Response{}
		}
		return toResponse(a.s.store.Set(r.Path, r.Dir, r.Val, ttlOptions))
	}
}

func (s *EtcdServer) updateClusterVersion(ver string) {
	if s.cluster.Version() == nil {
		plog.Infof("setting up the initial cluster version to %s", version.Cluster(ver))
	} else {
		plog.Infof("updating the cluster version from %s to %s", version.Cluster(s.cluster.Version().String()), version.Cluster(ver))
	}
	req := pb.Request{
		Method: "PUT",
		Path:   membership.StoreClusterVersionKey(),
		Val:    ver,
	}
	ctx, cancel := context.WithTimeout(context.Background(), s.Cfg.ReqTimeout())
	_, err := s.Do(ctx, req)
	cancel()
	switch err {
	case nil:
		return
	case ErrStopped:
		plog.Infof("aborting update cluster version because server is stopped")
		return
	default:
		plog.Errorf("error updating cluster version (%v)", err)
	}
}

// applyRequest interprets r as a call to store.X and returns a Response interpreted
// from store.Event
func (s *EtcdServer) applyRequest(r pb.Request) Response {
	f := func(ev *store.Event, err error) Response {
		return Response{Event: ev, err: err}
	}

	refresh, _ := pbutil.GetBool(r.Refresh)
	ttlOptions := store.TTLOptionSet{Refresh: refresh}
	if r.Expiration != 0 {
		ttlOptions.ExpireTime = time.Unix(0, r.Expiration)
	}

	switch r.Method {
	case "POST":
		return f(s.store.Create(r.Path, r.Dir, r.Val, true, ttlOptions))
	case "PUT":
		exists, existsSet := pbutil.GetBool(r.PrevExist)
		switch {
		case existsSet:
			if exists {
				if r.PrevIndex == 0 && r.PrevValue == "" {
					return f(s.store.Update(r.Path, r.Val, ttlOptions))
				} else {
					return f(s.store.CompareAndSwap(r.Path, r.PrevValue, r.PrevIndex, r.Val, ttlOptions))
				}
			}
			return f(s.store.Create(r.Path, r.Dir, r.Val, false, ttlOptions))
		case r.PrevIndex > 0 || r.PrevValue != "":
			return f(s.store.CompareAndSwap(r.Path, r.PrevValue, r.PrevIndex, r.Val, ttlOptions))
		default:
			if storeMemberAttributeRegexp.MatchString(r.Path) {
				id := membership.MustParseMemberIDFromKey(path.Dir(r.Path))
				var attr membership.Attributes
				if err := json.Unmarshal([]byte(r.Val), &attr); err != nil {
					plog.Panicf("unmarshal %s should never fail: %v", r.Val, err)
				}
				s.cluster.UpdateAttributes(id, attr)
				// return an empty response since there is no consumer.
				return Response{}
			}
			if r.Path == membership.StoreClusterVersionKey() {
				s.cluster.SetVersion(semver.Must(semver.NewVersion(r.Val)))
				// return an empty response since there is no consumer.
				return Response{}
			}
			return f(s.store.Set(r.Path, r.Dir, r.Val, ttlOptions))
		}
	case "DELETE":
		switch {
		case r.PrevIndex > 0 || r.PrevValue != "":
			return f(s.store.CompareAndDelete(r.Path, r.PrevValue, r.PrevIndex))
		default:
			return f(s.store.Delete(r.Path, r.Dir, r.Recursive))
		}
	case "QGET":
		return f(s.store.Get(r.Path, r.Recursive, r.Sorted))
	case "SYNC":
		s.store.DeleteExpiredKeys(time.Unix(0, r.Time))
		return Response{}
	default:
		// This should never be reached, but just in case:
		return Response{err: ErrUnknownMethod}
	}
}