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Hi Dan, Dan, Nikola, all Nova devs,

OK, so in reviewing Dan B's patch series that refactors the virt 
driver's get_available_resource() method [1], I am stuck between two 
concerns. I like (love even) much of the refactoring work involved in 
Dan's patches. They replace a whole bunch of our nested dicts that are 
used in the resource tracker with real objects -- and this is something 
I've been harping on for months that really hinders developer's 
understanding of Nova's internals.

However, all of the object classes that Dan B has introduced have been 
unversioned objects -- i.e. they have not derived from 
nova.objects.base.NovaObject. This means that these objects cannot be 
sent over the wire via an RPC API call. In practical terms, this issue 
has not yet reared its head, because the resource tracker still sends a 
dictified JSON representation of the object's fields directly over the 
wire, in the same format as Icehouse, therefore there have been no 
breakages in RPC API compatibility.

The problems with having all these objects not modelled by deriving from 
nova.objects.base.NovaObject are two-fold:

  * The object's fields/schema cannot be changed -- or rather, cannot be 
changed without introducing upgrade problems.
  * The objects introduce a different way of serializing the object 
contents than is used in nova/objects -- it's not that much different, 
but it's different, and only has not caused a problem because the 
serialization routines are not yet being used to transfer data over the wire

So, what to do? Clearly, I think the nova/virt/hardware.py objects are 
badly needed. However, one of (the top?) priorities of the Nova project 
is upgradeability, and by not deriving from 
nova.objects.base.NovaObject, these nova.virt.hardware objects are 
putting that mission in jeopardy, IMO.

My proposal is that before we go and approve any BPs or patches that add 
to nova/virt/hardware.py, we first put together a patch series that 
moves the object models in nova/virt/hardware.py to being full-fledged 
objects in nova/objects/*

Thoughts?

-jay

[1] 
https://review.openstack.org/#/q/status:open+project:openstack/nova+branch:master+topic:bp/virt-driver-get-available-resources-object,n,z