Open Stack

A change in nova [1] was approved which makes an assertion that we 
(nova? openstack?) do not support running nova from stein with cinder 
from queens, and I guess I'd like to know where that support statement 
is written down? Granted, I know we don't gate that way but I'm not 
aware of anything preventing it from working given we use microversions 
and nova, as the client, should be able to work with cinder from v1, v2 
or v3 assuming it's doing version discovery correctly (which we've been 
doing in nova since queens when we needed to start using the cinder 
v3.44 volume attachments API for multiattach - but it's not required).

In fact, nova-api still has compatibility code for older versions of 
cinder to determine what it should do about working with volume 
attachments [2]. I've been thinking lately about how to drop that code 
which would at the very least require a release note saying nova 
requires cinder >= queens, but nothing like that was requested in the 
change that drops support for cinder v1 from nova and asserts that nova 
in stein requires cinder >= queens.

Maybe I'm just yelling at kids to get off my lawn, but I don't really 
want this to be precedent without some discussion because I know at 
various times operators have complained about upgrades being hard 
because they assume all of the services must be upgraded to the same 
release at the same time, and I don't think that is true, or should be 
true, because if it is, we're doing a really bad job of defining 
versioned interfaces between the services.

[1] https://review.openstack.org/#/c/617927/
[2] 
https://github.com/openstack/nova/blob/7217e38bafb75e8a613763835b64e48e6b2c8ece/nova/compute/api.py#L4260-L4264

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Thanks,

Matt