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>> My current feeling is that we got ourselves into our existing mess
>> of ugly, convoluted code when we tried to add these complex 
>> relationships into the resource tracker and the scheduler. We set
>> out to create the placement engine to bring some sanity back to how
>> we think about things we need to virtualize.
> 
> Sorry, I completely disagree with your assessment of why the
> placement engine exists. We didn't create it to bring some sanity
> back to how we think about things we need to virtualize. We created
> it to add consistency and structure to the representation of
> resources in the system.
> 
> I don't believe that exposing this structured representation of 
> resources is a bad thing or that it is leaking "implementation
> details" out of the placement API. It's not an implementation detail
> that a resource provider is a child of another or that a different
> resource provider is supplying some resource to a group of other
> providers. That's simply an accurate representation of the underlying
> data structures.

This ^.

With the proposal Jay has up, placement is merely exposing some of its
own data structures to a client that has declared what it wants. The
client has made a request for resources, and placement is returning some
allocations that would be valid. None of them are nova-specific at all
-- they're all data structures that you would pass to and/or retrieve
from placement already.

>> I don't know the answer. I'm hoping that we can have a discussion 
>> that might uncover a clear approach, or, at the very least, one
>> that is less murky than the others.
> 
> I really like Dan's idea of returning a list of HTTP request bodies
> for POST /allocations/{consumer_uuid} calls along with a list of
> provider information that the scheduler can use in its
> sorting/weighing algorithms.
> 
> We've put this straw-man proposal here:
> 
> https://review.openstack.org/#/c/471927/
> 
> I'm hoping to keep the conversation going there.

This is the most clear option that we have, in my opinion. It simplifies
what the scheduler has to do, it simplifies what conductor has to do
during a retry, and it minimizes the amount of work that something else
like cinder would have to do to use placement to schedule resources.
Without this, cinder/neutron/whatever has to know about things like
aggregates and hierarchical relationships between providers in order to
make *any* sane decision about selecting resources. If placement returns
valid options with that stuff figured out, then those services can look
at the bits they care about and make a decision.

I'd really like us to use the existing strawman spec as a place to
iterate on what that API would look like, assuming we're going to go
that route, and work on actual code in both placement and the scheduler
to use it. I'm hoping that doing so will help clarify whether this is
the right approach or not, and whether there are other gotchas that we
don't yet have on our radar. We're rapidly running out of runway for
pike here and I feel like we've got to get moving on this or we're going
to have to punt. Since several other things depend on this work, we need
to consider the impact to a lot of our pike commitments if we're not
able to get something merged.

--Dan