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Hi, so shortly after the HOT migration landed, we hit
https://bugs.launchpad.net/tripleo/+bug/1354305 which is that on even
quite recently deployed clouds, the migrated templates were just too
new. A partial revert (of just the list_join bit) fixes that, but a
deeper problem emerged which is that stack-update to get from a
non-HOT to HOT template appears broken
(https://bugs.launchpad.net/heat/+bug/1354962).

I think we need to revert the HOT migration today, as forcing a
scorched earth recreation of a cloud is not a great answer for folk
that have deployed versions - its a backwards compat issue.

Its true that our release as of icehouse isn't  really useable, so we
could try to wiggle our way past this one, but I think as the first
real test of our new backwards compat policy, that that would be a
mistake.

What we need to be able to land it again, is some way whereby an
existing cloud can upgrade their undercloud (via stack-update against
the old heat deployed in the seed [today, old heat in the undercloud
itself in future]) and then once that is deployed subsequent templates
can use the new features. We're likely to run into such lockstep
changes in future, so we also need to be able to recognise them in
review / design, and call them out so we can fix them early rather
than deep down the pike.

-Rob

-- 
Robert Collins <rbtcollins at hp.com>
Distinguished Technologist
HP Converged Cloud