Explicit image selection

[William Reade]

On Mon, 2013-04-22 at 16:55 +1000, Ian Booth wrote:
> Allowing the users to explicitly select the image id they want to run can of
> course lead to problems. But some users require that functionality.
> 
> Proposal:
> 
> Introduce a new config attribute called "user-image-id".
> If set, this short circuits the image id lookup from metadata.
> 
> The existing default-image-id attribute as implemented for Openstack only would
> remain unchanged, and is still used as a fallback when all else fails (if
> specified). The difference is that this new attribute overrides the image id
> lookup and would be documented with suitable warnings etc.
> 
> Sound reasonable?

"default-image-id" enables a real use case, but it enables it in a
profoundly broken way. I'm implacably -1 on the idea that it is sensible
to allow an explicit end-run around our series/arch selection: it will
break instance type selection on ec2, it will break arbitrary charms in
arbitrary ways, and it will probably have further consequences that I'm
not farsighted enough to see.

Counter-proposal: allow users to specify the clouddata URL for ec2 as
well as (as is planned) in openstack. There's still the possibility of
poisoned data, but we can't defend against that; we can however avoid
threading brokenness right through our model.

Cheers
William