relation_set in relation-departed behavior is unclear

[dshcherb]

https://jujucharms.com/docs/2.3/authors-relations-in-depth#departing-relations

• For every known related unit (those which have joined and not yet departed), run the relation-departed hook.
• Run the relation-broken hook.
• depart from its scope in the relation.

So what's the difference between relation-departed and relation-broken? Think of relation-departed as the "saying goodbye" event. While in the latter state, relation settings can still be read (with relation-get) and a relation can even still be set (with relation-set). Once relation-broken fires, however, the relation no longer exists.

I think there is a logical mistake + unclear behavior description.

"the latter" should be "the former" because relation-departed runs first. In other words:

departed == can still read and write to relation data

But if I can still do relation_set in -relation-departed how will the other side react to that to read this new relation data? Will it receive another -relation-changed event or not? It is not clear at all.

What are the ordering guarantees for -departed in this case?

To make use of relation_set in -departed more information is needed. If there are two applications A and B in would be useful to know what happens after juju remove-relation A B:

1. Do all units in A get a chance to run relation-departed before all units in B? Or there is no order to that?
2. Can relation-set be used on all A and B units? How will they receive relation data updates before -relation-broken?
3. Is there a guarantee that no -broken event will be triggered while there are still -departed events to be processed on individual units of A or B?

It would make sense to me if something like that was described:

1. Any A or B unit can do relation_set in -relation-departed
   OR relation_set cannot be used in -relation-departed;
2. Any A or B unit can do relation_get in -relation-broken to retrieve settings set in -relation-departed
   OR relation_get and relation_set cannot be used in -relation-broken
   OR there is no way to do a relation_set after -relation-departed

I think it is a valid use-case to send some data over a relation after an operator runs remove-relation because there may be some information needed on A or B units from the other side for proper cleanup besides the information already present on a given relation when -relation-departed fires.

[dshcherb]

Similar Juju bug https://bugs.launchpad.net/juju/+bug/1417874

[pmatulis]

@dshcherb Yes, the "latter" was intended to refer to relation-departed. The rest will require some digging.

[pmatulis]

I asked about this on the Juju Discourse channel.

[pmatulis]

@dshcherb Here is a response from Discourse.