[openstack-dev] [Nova][Neutron] Thoughts on the nova<->neutron interface

Ian Wells ijw.ubuntu at cack.org.uk
Mon Jan 26 02:30:51 UTC 2015

Lots of open questions in here, because I think we need a long conversation
on the subject.

On 23 January 2015 at 15:51, Kevin Benton <blak111 at gmail.com> wrote:

> It seems like a change to using internal RPC interfaces would be pretty
> unstable at this point.

> Can we start by identifying the shortcomings of the HTTP interface and see
> if we can address them before making the jump to using an interface which
> has been internal to Neutron so far?

I think the protocol being used is a distraction from the actual

Firstly, you'd have to explain to me why HTTP is so much slower than RPC.
If HTTP is incredibly slow, can be be sped up?  If RPC is moving the data
around using the same calls, what changes?  Secondly, the problem seems
more that we make too many roundtrips - which would be the same over RPC -
and if that's true, perhaps we should be doing bulk operations - which is
not transport-specific.

I absolutely do agree that Neutron should be doing more of the work, and
Nova less, when it comes to port binding.  (And, in fact, I'd like that we
stopped considering it 'Nova-Neutron' port binding, since in theory another
service attaching stuff to the network could request a port be bound; it
just happens at the moment that it's always Nova.)

One other problem, not yet raised,  is that Nova doesn't express its needs
when it asks for a port to be bound, and this is actually becoming a
problem for me right now.  At the moment, Neutron knows, almost
psychically, what binding type Nova will accept, and hands it over; Nova
then deals with whatever binding type it receives (optimisitically
expecting it's one it will support, and getting shirty if it isn't).  The
problem I'm seeing at the moment, and other people have mentioned, is that
certain forwarders can only bind a vhostuser port to a VM if the VM itself
has hugepages enabled.  They could fall back to another binding type but at
the moment that isn't an option: Nova doesn't tell Neutron anything about
what it supports, so there's no data on which to choose.  It should be
saying 'I will take these binding types in this preference order'.  I
think, in fact, that asking Neutron for bindings of a certain preference
type order, would give us much more flexibility - like, for instance, not
having to know exactly which binding type to deliver to which compute node
in multi-hypervisor environments, where at the moment the choice is made in

I scanned through the etherpad and I really like Salvatore's idea of adding
> a service plugin to Neutron that is designed specifically for interacting
> with Nova. All of the Nova notification interactions can be handled there
> and we can add new API components designed for Nova's use (e.g. syncing
> data, etc). Does anyone have any objections to that approach?

I think we should be leaning the other way, actually - working out what a
generic service - think a container management service, or an edge network
service - would want to ask when it wanted to connect to a virtual network,
and making an Neutron interface that supports that properly *without* being
tailored to Nova.  The requirements are similar in all cases, so it's not
clear that a generic interface would be any more complex.

Notifications on data changes in Neutron to prevent orphaning is another
example of a repeating pattern.  It's probably the same for any service
that binds to Neutron, but right now Neutron has Nova-specific code in it.
Broadening the scope, it's also likely the same in Cinder, and in fact it's
also pretty similar to the problem you get when you delete a project in
Keystone and all your resources get orphaned.  Is a Nova-Neutron specific
solution the right thing to do?
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