func (r *EtcdRegistry) getJobFromModel(jm jobModel) *job.Job {
	var err error
	var unit *unit.Unit

	// New-style Jobs should have a populated UnitHash, and the contents of the Unit are stored separately in the Registry
	if !jm.UnitHash.Empty() {
		unit = r.getUnitByHash(jm.UnitHash)
		if unit == nil {
			log.Warningf("No Unit found in Registry for Job(%s)", jm.Name)
			return nil
		}
		if unit.Hash() != jm.UnitHash {
			log.Errorf("Unit Hash %s does not match expected %s for Job(%s)!", unit.Hash(), jm.UnitHash, jm.Name)
			return nil
		}
	} else {
		// Old-style Jobs had "Payloads" instead of Units, also stored separately in the Registry
		unit, err = r.getUnitFromLegacyPayload(jm.Name)
		if err != nil {
			log.Errorf("Error retrieving legacy payload for Job(%s)", jm.Name)
			return nil
		} else if unit == nil {
			log.Warningf("No Payload found in Registry for Job(%s)", jm.Name)
			return nil
		}

		log.Infof("Migrating legacy Payload(%s)", jm.Name)
		if err := r.storeOrGetUnit(*unit); err != nil {
			log.Warningf("Unable to migrate legacy Payload: %v", err)
		}
	}

	return job.NewJob(jm.Name, *unit)
}

// Load writes the given Unit to disk, subscribing to relevant dbus
// events, and, if necessary, instructing the systemd daemon to reload.
func (m *SystemdUnitManager) Load(name string, u unit.Unit) error {
	err := m.writeUnit(name, u.String())
	if err != nil {
		return err
	}

	m.subscriptions.Add(name)
	return m.daemonReload()
}

// Load writes the given Unit to disk, subscribing to relevant dbus
// events, caching the Unit's Hash, and, if necessary, instructing the systemd
// daemon to reload.
func (m *systemdUnitManager) Load(name string, u unit.Unit) error {
	m.mutex.Lock()
	defer m.mutex.Unlock()
	err := m.writeUnit(name, u.String())
	if err != nil {
		return err
	}
	m.hashes[name] = u.Hash()
	return m.daemonReload()
}

func (ur *unitsResource) destroy(rw http.ResponseWriter, req *http.Request, item string) {
	if validateContentType(req) != nil {
		sendError(rw, http.StatusNotAcceptable, errors.New("application/json is only supported Content-Type"))
		return
	}

	var du schema.DeletableUnit
	dec := json.NewDecoder(req.Body)
	err := dec.Decode(&du)
	if err != nil {
		sendError(rw, http.StatusBadRequest, fmt.Errorf("unable to decode body: %v", err))
		return
	}

	var u *unit.Unit
	if len(du.FileContents) > 0 {
		u, err = decodeUnitContents(du.FileContents)
		if err != nil {
			sendError(rw, http.StatusBadRequest, fmt.Errorf("invalid fileContents: %v", err))
			return
		}
	}

	j, err := ur.reg.Job(item)
	if err != nil {
		log.Errorf("Failed fetching Job(%s): %v", item, err)
		sendError(rw, http.StatusInternalServerError, nil)
		return
	}

	if j == nil {
		sendError(rw, http.StatusNotFound, errors.New("unit does not exist"))
		return
	}

	if u != nil && u.Hash() != j.Unit.Hash() {
		sendError(rw, http.StatusConflict, errors.New("hash of provided fileContents does not match that of existing unit"))
		return
	}

	err = ur.reg.DestroyJob(item)
	if err != nil {
		log.Errorf("Failed destroying Job(%s): %v", item, err)
		sendError(rw, http.StatusInternalServerError, nil)
		return
	}

	rw.WriteHeader(http.StatusNoContent)
}

func (ur *unitsResource) update(rw http.ResponseWriter, j *job.Job, ds job.JobState, cmp *unit.Unit) {
	// Assert that the Job's Unit matches the Unit in the request, if provided
	if cmp != nil && cmp.Hash() != j.Unit.Hash() {
		sendError(rw, http.StatusConflict, errors.New("hash of provided fileContents does not match that of existing unit"))
		return
	}

	err := ur.reg.SetJobTargetState(j.Name, ds)
	if err != nil {
		log.Errorf("Failed setting target state of Job(%s): %v", j.Name, err)
		sendError(rw, http.StatusInternalServerError, nil)
		return
	}

	rw.WriteHeader(http.StatusNoContent)
}

func (r *EtcdRegistry) getJobFromObjectNode(node *etcd.Node) (*job.Job, error) {
	var err error
	var jm jobModel
	if err = unmarshal(node.Value, &jm); err != nil {
		return nil, err
	}

	var unit *unit.Unit

	// New-style Jobs should have a populated UnitHash, and the contents of the Unit are stored separately in the Registry
	if !jm.UnitHash.Empty() {
		unit = r.getUnitByHash(jm.UnitHash)
		if unit == nil {
			log.Warningf("No Unit found in Registry for Job(%s)", jm.Name)
			return nil, nil
		}
		if unit.Hash() != jm.UnitHash {
			log.Errorf("Unit Hash %s does not match expected %s for Job(%s)!", unit.Hash(), jm.UnitHash, jm.Name)
			return nil, nil
		}
	} else {
		// Old-style Jobs had "Payloads" instead of Units, also stored separately in the Registry
		unit, err = r.getUnitFromLegacyPayload(jm.Name)
		if err != nil {
			log.Errorf("Error retrieving legacy payload for Job(%s)", jm.Name)
			return nil, nil
		} else if unit == nil {
			log.Warningf("No Payload found in Registry for Job(%s)", jm.Name)
			return nil, nil
		}

		log.Infof("Migrating legacy Payload(%s)", jm.Name)
		if err := r.storeOrGetUnit(*unit); err != nil {
			log.Warningf("Unable to migrate legacy Payload: %v", err)
		}

		jm.UnitHash = unit.Hash()
		log.Infof("Updating Job(%s) with legacy payload Hash(%s)", jm.Name, jm.UnitHash)
		if err := r.updateJobObjectNode(&jm, node.ModifiedIndex); err != nil {
			log.Warningf("Unable to update Job(%s) with legacy payload Hash(%s): %v", jm.Name, jm.UnitHash, err)
		}
	}

	return job.NewJob(jm.Name, *unit), nil
}

// NewJob creates a new Job based on the given name and Unit.
// The returned Job has a populated UnitHash and empty JobState and
// UnitState. nil is returned on failure.
func NewJob(name string, unit unit.Unit) *Job {
	return &Job{
		Name:      name,
		State:     nil,
		Unit:      unit,
		UnitHash:  unit.Hash(),
		UnitState: nil,
	}
}

func (r *EtcdRegistry) storeOrGetUnit(u unit.Unit) (err error) {
	json, err := marshal(u)
	if err != nil {
		return err
	}

	req := etcd.Create{
		Key:   r.hashedUnitPath(u.Hash()),
		Value: json,
	}
	_, err = r.etcd.Do(&req)
	// unit is already stored
	if err != nil && isNodeExist(err) {
		// TODO(jonboulle): verify more here?
		err = nil
	}
	return
}

func (r *EtcdRegistry) storeOrGetUnit(u unit.Unit) (err error) {
	key := r.hashedUnitPath(u.Hash())
	json, err := marshal(u)
	if err != nil {
		return err
	}

	log.V(3).Infof("Storing Unit(%s) in Registry: %s", u.Hash(), json)
	_, err = r.etcd.Create(key, json, 0)
	// unit is already stored
	if err != nil && err.(*goetcd.EtcdError).ErrorCode == etcd.EcodeNodeExist {
		log.V(2).Infof("Unit(%s) already exists in Registry", u.Hash())
		// TODO(jonboulle): verify more here?
		err = nil
	}
	return
}