Use case for: min-version

[Matt Bruzek]

We wrote a charm that needed election logic, so we used the new Juju
feature "is_leader".  A user was interested in using a bundle that
contained this charm and it failed on them.  It was hard to track down the
cause of the problem.  It appears they were using an earlier version of
Juju that is available from universe and only the PPA had the more current
version.

Read more about the problem here:
https://bugs.launchpad.net/charms/+source/etcd/+bug/1483380

I heard the "min-version" feature discussed at previous Cloud sprints but
to my knowledge we do not have it implemented yet.  The idea was a charm
could specify in metadata.yaml what min-version of Juju they support.

There are a lot of new features that juju-core are cranking out (and that
is *awesome*)!  We have already run into this problem with a real user, and
will have the problem in the future.

Can we reopen the discussion of min-version?  Or some other method of
preventing this kind of problem in the future?

   - Matt Bruzek <matthew.bruzek at canonical.com>
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