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Forgive me for thinking out loud, but I'm trying to sort out how nova 
would use a microversion in the nova API for the get-me-a-network 
feature recently added to neutron [1] and planned to be leveraged in 
nova (there isn't a spec yet for nova, I'm trying to sort this out for a 
draft).

Originally I was thinking that a network is required for nova boot, so 
we'd simply check for a microversion and allow not specifying a network, 
easy peasy.

Turns out you can boot an instance in nova (with neutron as the network 
backend) without a network. All you get is a measly debug log message in 
the compute logs [2]. That's kind of useless though and seems silly.

I haven't tested this out yet to confirm, but I suspect that if you 
create a nova instance w/o a network, you can latter try to attach a 
network using the os-attach-interfaces API as long as you either provide 
a network ID *or* there is a public shared network or the tenant has a 
network at that point (nova looks those up if a specific network ID 
isn't provided).

The high-level plan for get-me-a-network in nova was simply going to be 
if the user tries to boot an instance and doesn't provide a network, and 
there isn't a tenant network or public shared network to default to, 
then nova would call neutron's new auto-allocated-topology API to get a 
network. This, however, is a behavior change.

So I guess the question now is how do we handle that behavior change in 
the nova API?

We could add an auto-create-net boolean to the boot server request which 
would only be available in a microversion, then we could check that 
boolean in the compute API when we're doing network validation.

Today if you don't specify a network and don't have a network available, 
then the validation in the API is basically just quota checking that you 
can get at least one port in your tenant [3]. With a flag on a 
microversion, we could also validate some other things about 
auto-creating a network (if we know that's going to be the case once we 
hit the compute).

Anyway, this is mostly me getting thoughts out of my head before the 
weekend so I don't forget it and am looking for other ideas here or 
things I might be missing.

[1] https://blueprints.launchpad.net/neutron/+spec/get-me-a-network
[2] 
https://github.com/openstack/nova/blob/30ba0c5eb19a9c9628957ac8e617ae78c0c1fa84/nova/network/neutronv2/api.py#L594-L595
[3] 
https://github.com/openstack/nova/blob/30ba0c5eb19a9c9628957ac8e617ae78c0c1fa84/nova/network/neutronv2/api.py#L1107

-- 

Thanks,

Matt Riedemann