Open Stack

Since forever [0], nova has gently recommended [1] that the placement
service be upgraded first.

However, we've not made any serious effort to test scenarios where this
isn't done.  For example, we don't have grenade tests running placement
at earlier levels.

After a(nother) discussion [2] which touched on the impacts - real and
imagined - of running new nova against old placement, we finally decided
to turn the recommendation into a hard requirement [3].

This gives admins a crystal clear guideline, this lets us simplify our
support statement, and also means we don't have to do 406 fallback code
anymore.  So we can do stuff like [4], and also avoid having to write
(and subsequently remove) code like that in the future.

Please direct any questions to #openstack-nova

Your Faithful Scribe,
efried

[0] Like, since upgrading placement was a thing.
[1]
https://docs.openstack.org/nova/latest/user/upgrade.html#rolling-upgrade-process
(#2, first bullet)
[2]
http://eavesdrop.openstack.org/irclogs/%23openstack-nova/%23openstack-nova.2018-03-26.log.html#t2018-03-26T17:35:11
[3] https://review.openstack.org/556631
[4] https://review.openstack.org/556633