Heat and requirements for all-it-does-is-docker cloud image

[Matthew Miller]

This is a three part question. No, wait, one clarifying statement and
two questions. Okay, one clarifying statement, one main question with
many sub-questions, and then a bonus question. :)

-----------------------------------------------------------------------

First, again, the primary Fedora Cloud image is going to continue to
contain heat-cfntools, and python, and we won't migrate away from
cloud-init without a replacement which covers what heat needs. So, this
is just about a specialized image tailored for docker.

-----------------------------------------------------------------------

So, the main question: given that Docker upstream is recommending Heat
as the best way to do Docker in OpenStack, what are the minimal
requirements on the image for doing that?

Is there a way to preconfigure the image so that heat-cfntools aren't
required? In that case, do we need #cloud-config formatted userdata or
can we get away with the #! shell-script style? If it is #cloud-config,
is a _subset_ of that syntax sufficient (as for example CoreOS does)?

If we choose to go with rpm-ostree for the image itself, will Heat be
foiled by a surprise lack of ability to install packages? (Not just no
yum, but actually read-only /usr?) I know that CloudFormation can do
all sorts of things, but which will it need to do in this case? And is
there a way to present these limitations to users in a way that won't
be confusing? (Including, I assume, pointing them to the
general-purpose image if the Docker one isn't flexible enough.)

-----------------------------------------------------------------------

And the bonus question: if in the F23 or F24 timeframe, we go full-on
rpm-ostree for the primary image (while still keeping it basically
general purpose), will the "pkglayer" concept (where groups of packages
can be added on top) be sufficient? I assume that it is _required_, and
the basic idea is that we would need to add support for it to
cfn_helper.py -- and possibly to Amazon's CloudFormation stuff too.
Would going this direction be _workable_ for Heat?

-----------------------------------------------------------------------

Thanks!

-- 
Matthew Miller    --   Fedora Project    --    <mattdm at fedoraproject.org>