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Sylvain and I were talking about how he's going to work placement 
microversion requests into his filter scheduler patch [1]. He needs to 
make requests to the placement API with microversion 1.4 [2] or later 
for resource provider filtering on specific resource classes like VCPU 
and MEMORY_MB.

The question was what happens if microversion 1.4 isn't available in the 
placement API, i.e. the nova-scheduler is running Ocata code now but the 
placement service is running Newton still.

Our rolling upgrades doc [3] says:

"It is safest to start nova-conductor first and nova-api last."

But since placement is bundled with n-api that would cause issues since 
n-sch now depends on the n-api code.

If you package the placement service separately from the nova-api 
service then this is probably not an issue. You can still roll out n-api 
last and restart it last (for control services), and just make sure that 
placement is upgraded before nova-scheduler (we need to be clear about 
that in [3]).

But do we have any other issues if they are not packaged separately? Is 
it possible to install the new code, but still only restart the 
placement service before nova-api? I believe it is, but want to ask this 
out loud.

I think we're probably OK here but I wanted to ask this out loud and 
make sure everyone is aware and can think about this as we're a week 
from feature freeze. We also need to look into devstack/grenade because 
I'm fairly certain that we upgrade n-sch *before* placement in a grenade 
run which will make any issues here very obvious in [1].

[1] https://review.openstack.org/#/c/417961/
[2] 
http://docs.openstack.org/developer/nova/placement.html#filter-resource-providers-having-requested-resource-capacity
[3] 
http://docs.openstack.org/developer/nova/upgrade.html#rolling-upgrade-process

-- 

Thanks,

Matt Riedemann