Charm Store Series and Ubuntu Release

[Clint Byrum]

At UDS-Q, we had a pretty interesting discussion of how we should set
charm store release policy. To recap, many of us came into the discussion
thinking we would rubber stamp a really simple idea: never freeze.

But as we discussed it, it became clear that our desire for a laissez
faire policy was misguided. While its fantastic to be on the bleeding
edge, users also need to be able to build their applications on top of
the infrastructure that the charm store provides. If we ship MySQL 5.5
in the precise mysql charm, we can't just make the next 'upgrade-charm'
ship 5.6, which will most likely include "breaking" changes. We also can't
go re-arranging the relations, as that might break a deployed service.

We have a bit of an issue with regard to the inter-LTS releases as well.
Our focus is on stable deployments, and quite a large percentage of those
come on LTS releases. But we'd like to also make sure charms can take
advantage of the latest releases of Ubuntu (such as the upcoming quantal)
as well.

So, I'd like to get a discussion started about what to do about this issue.

One idea I have is to use an overlay system to reduce cross-series overhead.

Basically it should work something like this:

* Current LTS release of Ubuntu is the base.

* Newer releases assume everything from base, and apply delta on top

* New LTS is copied from the overlay of old LTS + newest stable

* Adding charms to current LTS requires testing on newest stable (so next LTS
has a higher probability of working).

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Implementation Details

* In doing this, we would not run the 'branch-distro' script on launchpad.
development on quantal. Instead, we would simply change it from
"experimental" to "development focus". We would perhaps need a variation on
branch-distro to copy precise+delta -> "T".

* We would need to change charm-tools to overlay based on the series
requested. Currently charm-tools assumes the current dev focus will have
all charms.

* Charm store and charm world code would also need to be changed to do
the overlay logic