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
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