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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
   supported;
   - 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
      behave.

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.

Thoughts?
Armando

[1]
http://specs.openstack.org/openstack/neutron-specs/specs/kilo/mtu-selection-and-advertisement.html
[2]
http://specs.openstack.org/openstack/neutron-specs/specs/kilo/nfv-vlan-trunks.html
[3]
https://review.openstack.org/#/q/project:openstack/neutron+branch:master+topic:bp/mtu-selection-and-advertisement,n,z
[4]
https://review.openstack.org/#/q/project:openstack/neutron+branch:master+topic:bp/nfv-vlan-trunks,n,z
[5] https://review.openstack.org/#/c/136760/

On 19 March 2015 at 12:01, Gary Kotton <gkotton at vmware.com> wrote:

>  With regards to the MTU can you please point me to where we validate
> that the MTU defined by the tenant is actually <= the supported MTU on the
> network. I did not see this in the code (maybe I missed something).
>
>
>   From: Ian Wells <ijw.ubuntu at cack.org.uk>
> Reply-To: OpenStack List <openstack-dev at lists.openstack.org>
> Date: Thursday, March 19, 2015 at 8:44 PM
> To: OpenStack List <openstack-dev at lists.openstack.org>
> Subject: Re: [openstack-dev] [Neutron] VLAN transparency support
>
>    Per the other discussion on attributes, I believe the change walks in
> historical footsteps and it's a matter of project policy choice.  That
> aside, you raised a couple of other issues on IRC:
>
> - backward compatibility with plugins that haven't adapted their API -
> this is addressed in the spec, which should have been implemented in the
> patches (otherwise I will downvote the patch myself) - behaviour should be
> as before with the additional feature that you can now tell more about what
> the plugin is thinking
>  - whether they should be core or an extension - this is a more personal
> opinion, but on the grounds that all networks are either trunks or not, and
> all networks have MTUs, I think they do want to be core.  I would like to
> see plugin developers strongly encouraged to consider what they can do on
> both elements, whereas an extension tends to sideline functionality from
> view so that plugin writers don't even know it's there for consideration.
>
>  Aside from that, I'd like to emphasise the value of these patches, so
> hopefully we can find a way to get them in in some form in this cycle.  I
> admit I'm interested in them because they make it easier to do NFV.  But
> they also help normal cloud users and operators, who otherwise have to do
> some really strange things [1].  I think it's maybe a little unfair to post
> reversion patches before discussion, particularly when the patch works,
> passes tests and implements an approved spec correctly.
>  --
>  Ian.
> [1] https://bugzilla.redhat.com/show_bug.cgi?id=1138958
> <https://urldefense.proofpoint.com/v2/url?u=https-3A__bugzilla.redhat.com_show-5Fbug.cgi-3Fid-3D1138958&d=AwMFaQ&c=Sqcl0Ez6M0X8aeM67LKIiDJAXVeAw-YihVMNtXt-uEs&r=VlZxHpZBmzzkWT5jqz9JYBk8YTeq9N3-diTlNj4GyNc&m=NzYY0bOpToH9ZNwzqI_SpQHiPFRXD_nfb1bM3qAw7Cs&s=FlF57GYJqeWgx5ivxnK5kfWlyTIc1ZFbdlXoi2cfdhw&e=>
> (admittedly first link I found, but there's no shortage of them)
>
> On 19 March 2015 at 05:32, Gary Kotton <gkotton at vmware.com> wrote:
>
>>  Hi,
>> This patch has the same addition too -
>> https://review.openstack.org/#/c/154921/. We should also revert that one.
>> Thanks
>> Gary
>>
>>   From: Gary Kotton <gkotton at vmware.com>
>> Reply-To: OpenStack List <openstack-dev at lists.openstack.org>
>> Date: Thursday, March 19, 2015 at 1:14 PM
>> To: OpenStack List <openstack-dev at lists.openstack.org>
>> Subject: [openstack-dev] [Neutron] VLAN transparency support
>>
>>   Hi,
>> It appears that https://review.openstack.org/#/c/158420/ update the base
>> attributes for the networks. Is there any reason why this was not added as
>> a separate extension like all others.
>> I do not think that this is the correct way to go and we should do this
>> as all other extensions have been maintained. I have posted a revert (
>> https://review.openstack.org/#/c/165776/) – please feel free to knack if
>> it is invalid.
>> Thanks
>> Gary
>>
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>
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