[openstack-dev] [Neutron] Neutron extenstions

Armando M. armamig at gmail.com
Thu Mar 19 23:23:15 UTC 2015

Forwarding my reply to the other thread here:


If my memory does not fail me, changes to the API (new resources, new
resource attributes or new operations allowed to resources) have always
been done according to these criteria:

   - an opt-in approach: this means we know the expected behavior of the
   plugin as someone has coded the plugin in such a way that the API change is
   - an opt-out approach: if the API change does not require explicit
   backend support, and hence can be deemed supported by all plugins.
   - a 'core' extension (ones available in neutron/extensions) should be
   implemented at least by the reference implementation;

Now, there might have been examples in the past where criteria were not
met, but these should be seen as exceptions rather than the rule, and as
such, fixed as defects so that an attribute/resource/operation that is
accidentally exposed to a plugin will either be honored as expected or an
appropriate failure is propagated to the user. Bottom line, the server must
avoid to fail silently, because failing silently is bad for the user.

Now both features [1] and [2] violated the opt-in criterion above: they
introduced resources attributes in the core models, forcing an undetermined
behavior on plugins.

I think that keeping [3,4] as is can lead to a poor user experience; IMO
it's unacceptable to let a user specify the attribute, and see that
ultimately the plugin does not support it. I'd be fine if this was an
accident, but doing this by design is a bit evil. So, I'd suggest the
following, in order to keep the features in Kilo:

   - Patches [3, 4] did introduce config flags to control the plugin
   behavior, but it looks like they were not applied correctly; for instance,
   the vlan_transparent case was only applied to ML2. Similarly the MTU config
   flag was not processed server side to ensure that plugins that do not
   support advertisement do not fail silently. This needs to be rectified.
   - As for VLAN transparency, we'd need to implement work item 5 (of 6) of
   spec [2], as this extension without at least a backend able to let tagged
   traffic pass doesn't seem right.
   - Ensure we sort out the API tests so that we know how the features

Now granted that controlling the API via config flags is not the best
solution, as this was always handled through the extension mechanism, but
since we've been talking about moving away from extension attributes with
[5], it does sound like a reasonable stop-gap solution.


[5] https://review.openstack.org/#/c/136760/

On 19 March 2015 at 14:56, Ian Wells <ijw.ubuntu at cack.org.uk> wrote:

> On 19 March 2015 at 11:44, Gary Kotton <gkotton at vmware.com> wrote:
>> Hi,
>> Just the fact that we did this does not make it right. But I guess that we
>> are starting to bend the rules. I think that we really need to be far more
>> diligent about this kind of stuff. Having said that we decided the
>> following on IRC:
>> 1. Mtu will be left in the core (all plugins should be aware of this and
>> treat it if necessary)
>> 2. Vlan-transparency will be moved to an extension. Pritesh is working on
>> this.
> The spec started out as an extension, and in its public review people
> requested that it not be an extension and that it should instead be core.
> I accept that we can change our minds, but I believe there should be a good
> reason for doing so.  You haven't given that reason here and you haven't
> even said who the 'we' is that decided this.  Also, as the spec author, I
> had a conversation with you all but there was no decision at the end of it
> (I presume that came afterward) and I feel that I have a reasonable right
> to be involved.  Could you at least summarise your reasoning here?
> I admit that I prefer this to be in core, but I'm not terribly choosy and
> that's not why I'm asking.  I'm more concerned that this is changing our
> mind at literally the last moment, and in turn wasting a developer's time,
> when there was a perfectly good process to debate this before coding was
> begun, and again when the code was up for review, both of which apparently
> failed.  I'd like to understand how we avoid getting here again in the
> future.  I'd also like to be certain we are not simply reversing a choice
> on a whim.
> --
> Ian.
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