Relation addresses

[John Meinel]

...


> In a nutshell:
>>  - There will be a new hook, relation-address-changed, and a new tool
>> called address-get.
>>
>
> This seems less than ideal, we already have standards ways of getting this
> data and being notified of its change. introducing non-orthogonal ways of
> doing the same lacks value afaics or at least any rationale in the document.
>

So maybe the spec isn't very clear, but the idea is that the new hook is
called on the unit when *its* private address might have changed, to give
it a chance to respond. After which, "relation-changed" is called on all
the associated units to let them know that the address they need to connect
to has changed.

It would be possible to just roll relation-address-changed into config
changed.

The reason it is called for each associated unit is because the network
model means we can actually have different addresses (be connected on a
different network) for different things related to me.

e.g. I have a postgres charm related to application on network A, but
related to my-statistics-aggregator on network B. The address it needs to
give to "application" should be different than the address given to
"my-statistics-aggregator". And, I believe, the config in pg_hba.conf would
actually be different.



> the two perspectives of addresses for self vs related also seem to be a
> bit muddled. a relation hook is called in notification of a remote unit
> change, but now we're introducing one that behaves in the opposite manner
> of every other, and we're calling it redundantly for every relation instead
> of once for the unit?
>
>
>>  - The hook will be called when the relation's address has changed, and
>> the tool can be called to obtain the address. If the hook is not
>> implemented, the private-address setting will be updated. Otherwise it is
>> down to you to decide how you want to react to address changs (e.g. for
>> proxy charms, probably just don't do anything.)
>>
>
> perhaps there is a  misunderstanding of proxies, but things that set their
> own address have taken responsibility for it. ie juju only updates private
> address if it provided it, else its the charms responsibility.
>
> fwiw, i think this could use some additional discussion.
>
>
So one of the reasons is that it takes some double handling of values to
know if the existing value was the one that was what we last set it. And
there is the possibility that it has changed 2 times, and it was the value
we set it to, but that was the address before this one and we just haven't
gotten to update it.
There was a proposal that we could effectively have 2 fields "this is the
private address you are sharing, which might be empty" and "this is the
private address we set" which is where we put our data. And we return the
second value if the first is still nil. Or we set it twice, and we only set
the first one if it matches what was in the second one, etc.
All these things are possible, but in the discussions we had it seemed
simpler to not have to track extra data for marginal benefit. Things which
are proxy charms know that they are, and they found the right address to
give in the past, and they simply do the same thing again when told that we
want to change their address.

John
=:->
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