On 10/20/2014 07:38 PM, Jay Pipes wrote:
> 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/*
> 

I think that we should have both in some cases, and although it makes
sense to have them only as objects in some cases - having them as
separate classes for some and not others may be confusing.

So when does it make sense to have them as separate classes? Well
basically whenever there is a need for driver-agnostic logic that will
be used outside of the driver (scheduler/claims/API/). Can this stuff go
in objects? Technically yes, but objects are really not a good place for
such logic as they may already be trying to solve too much (data
versioning and downgrading when there is a multi version cloud running,
database access for compute, and there are at least 2 more features
considered to be part of objects - cells integration and schema data
migrations).

Take CPU pinning as an example [1] - none of that logic would benefit
from living in the NovaObject child class itself, and will make it quite
bloated. Having it in the separate module objects can call into is
definitely beneficial, while we definitely should stay with objects for
versioning/backporting support. So I say in a number of cases we need both.

Both is exactly what I did for NUMA, with the exception of the compute
node side (we are hopping to start the json blob cleanup in K so I did
not concern myself with it for the sake of getting things done, but we
will need it). This is what I am doing now with CPU pinning.

The question I did not touch upon is what kind of interface does that
leave poor Nova developers with. Having everything as objects would
allow us to write things like (in the CPU pinning case):

  instance.cpu_pinning = compute.cpu_pinning.get_pinning_for_instance(
     instance)

Pretty slick, no? While keeping it completely separate would make us do
things like

  cpu_pinning = compute.cpu_pinning.topology_from_obj()
  if cpu_pinning:
    instance_pinning = cpu_pinning.get_pinning_for_instance(
        instance.cpu_pinning.topology_from_obj())
    instance.cpu_pinning = objects.InstanceCPUPinning.obj_from_topology(
        instance_pinning)

Way less slick, but can be easily fixed with a level of indirection.
Note that the above holds only when we are objectified everywhere -
until then - we pretty much *have* to have both.

So to sum up - what I think we should do is:

1) Don't bloat the object code with low level stuff
2) Do have objects for versioning everything
3) Make nice APIs that developers can enjoy (after we've converted all
the code to use objects).

N.

[1] https://review.openstack.org/#/c/128738/4/nova/virt/hardware.py

> 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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