RelationList

[William Reade]

On Sun, Aug 25, 2013 at 11:52 PM, Mike Sam <mikesam460 at gmail.com> wrote:

> I have a question around how RelationList cmd works specially on a peer
> relation. I have seen it that although you may deploy multiple units
> together (with -n on deploy), the relationlist may not return all of the
> deployed units on the first calls on its relation change hook. I am
> guessing maybe the units who are not showing up have not processed their
> join hook yet or something in that nature.
>

In narrow terms, there shouldn't be any significant differences in
different kinds of relations: the set of units that join and depart depend
on the relation kind, but everything I say below applies as much to
provider/requirer relations as to peer relations.

1> What does relationlist actually return? Are they the relations set at
> the deploy step? Are they the relations which have at least executed their
> join hook?
>

Relation-list lists the remote units known to the local unit. This is
determined purely by what hooks have been run locally: a remote unit
becomes known when the local unit runs its -joined hook, and becomes
unknown again after the local unit runs its -departed hook. The fact of a
remote unit's presence in relation-list implies *nothing* about what
relation hooks that unit may itself have run.


> 2> How can you guarantee at relation change hook step that relationlist
> can get a complete list of the units participating in the relation and have
> already processed their join hook before processing their the
> relation-changed hook?
>

You can't. But you *can* guarantee that each unit will eventually run a
changed hook in which all remote units are known and all their settings are
up to date. So if the charm you're developing needs (or is configured to
need) N units, it would be reasonable to write a -changed hook that did the
following:

  * if local settings (`relation-get $JUJU_UNIT_NAME`) are incomplete, fill
them in (`relation-set key=value ...`)
  * if less than N-1 remote units are returned from relation-list, exit
without error
  * collect the units' settings with relation-get; if any are incomplete,
exit without error
  * configure the software to use the collected unit settings


> 3> You need the above in situation where you need to do automatic master
> election among your units. Any better way to do this without relationlist?
>

relation-list is the expected way to list known remote units. You may find
https://juju.ubuntu.com/docs/authors-charms-in-action.html useful in
building your mental model of what hooks run when.

Cheers
William
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