Open Stack

Stackers,

Eric Fried and I are currently at an impasse regarding a decision that 
will have far-reaching (and end-user facing) impacts to the placement 
API and how nova interacts with the placement service from the nova 
scheduler.

We need to make a decision regarding the following question:

"By default, should resources/traits submitted in different numbered 
request groups be supplied by separate resource providers?"

There are two competing proposals right now (both being amendments to 
the original granular request groups spec [1]) which outline two 
different viewpoints.

Viewpoint A [2], from me, is that like resources listed in different 
granular request groups should mean that those resources will be sourced 
from *different* resource providers.

In other words, if I issue the following request:

GET /allocation_candidates?resources1=VCPU:1&resources2=VCPU:1

Then I am assured of getting allocation candidates that contain 2 
distinct resource providers consuming 1 VCPU from each provider.

Viewpoint B [3], from Eric, is that like resources listed in different 
granular request groups should not necessarily mean that those resources 
will be sourced from different resource providers. They *could* be 
sourced from different providers, or they could be sourced from the same 
provider.

Both proposals include ways to specify whether certain resources or 
whole request groups can be forced to be sources from either a single 
provider or from different providers.

In Viewpoint A, the proposal is to have a can_split=RESOURCE1,RESOURCE2 
query parameter that would indicate which resource classes in the 
unnumbered request group that may be split across multiple providers 
(remember that viewpoint A considers different request groups to 
explicitly mean different providers, so it doesn't make sense to have a 
can_split query parameter for numbered request groups).

In Viewpoint B, the proposal is to have a separate_providers=1,2 query 
parameter that would indicate that the identified request groups should 
be sourced from separate providers. Request groups that are not listed 
in the separate_providers query parameter are not guaranteed to be 
sourced from different providers.

I know this is a complex subject, but I thought it was worthwhile trying 
to explain the two proposals in as clear terms as I could muster.

I'm, quite frankly, a bit on the fence about the whole thing and would 
just like to have a clear path forward so that we can start landing the 
12+ patches that are queued up waiting for a decision on this.

Thoughts and opinions welcome.

Thanks,
-jay


[1] 
http://specs.openstack.org/openstack/nova-specs/specs/rocky/approved/granular-resource-requests.html

[2] https://review.openstack.org/#/c/560974/

[3] https://review.openstack.org/#/c/561717/