Comment 2 for bug 1718259

~stub,

That's odd. At least from what I can see in the Juju code, there are no checks for otherEnd's scope and there are watchers for different relation objects.

If this end is global - should send
If this end is not global - check if the other end is a principal for this watcher - should send
If this end is not global and the other end is not a principal - should send (subordinate <-> subordinate)

https://github.com/juju/juju/blob/juju-2.2.4/apiserver/uniter/subordinaterelationwatcher.go#L118-L139
func (w *subRelationsWatcher) shouldSendCheck(key string) (bool, error) {
...
 if thisEnd.Scope == charm.ScopeGlobal {
  return true, nil
 }

 // Only allow container relations if the other end is our
 // principal or the other end is a subordinate.
 otherEnds, err := rel.RelatedEndpoints(w.app.Name())
 if err != nil {
  return false, errors.Trace(err)
 }
 for _, otherEnd := range otherEnds {
  if otherEnd.ApplicationName == w.principalName {
   return true, nil
  }
  otherApp, err := w.backend.Application(otherEnd.ApplicationName)
  if err != nil {
   return false, errors.Trace(err)
  }
  if !otherApp.IsPrincipal() {
   return true, nil
  }
 }

No checks for scope here either:
https://github.com/juju/juju/blob/juju-2.2.4/apiserver/uniter/uniter.go#L1691-L1705

This behavior of setting relation scope to 'container' if one of two sides has ScopeContainer goes way back (to 5 years ago):
https://github.com/juju/juju/blame/juju-2.2.4/state/state.go#L1661-L1668
func (st *State) AddRelation(eps ...Endpoint) (r *Relation, err error) {
...
 // If either endpoint has container scope, so must the other; and the
 // applications's series must also match, because they'll be deployed to
 // the same machines.
 matchSeries := true
 if eps[0].Scope == charm.ScopeContainer {
  eps[1].Scope = charm.ScopeContainer
 } else if eps[1].Scope == charm.ScopeContainer {
  eps[0].Scope = charm.ScopeContainer

eps ~ Endpoint ~ Relation
https://github.com/juju/juju/blob/juju-2.2.4/api/uniter/endpoint.go#L13-L15
Relation
https://github.com/juju/charm/blob/v6-unstable/meta.go#L125-L134

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Which Juju version was used when you noticed the undefined behavior with one side Global and another side Container-scoped?