[openstack-dev] [nova] nova hooks - document & test or deprecate?

Sean Dague sean at dague.net
Tue Mar 1 12:14:34 UTC 2016

On 02/29/2016 05:23 PM, Matt Riedemann wrote:
> On 2/29/2016 2:54 PM, Sean Dague wrote:
>> On 02/29/2016 11:59 AM, Sean Dague wrote:
>>> The nova/hooks.py infrastructure has been with us since early Nova. It's
>>> currently only annotated on a few locations - 'build_instance',
>>> 'create_instance', 'delete_instance', and 'instance_network_info'. It's
>>> got a couple of unit tests on it, but nothing that actually tests real
>>> behavior of the hooks we have specified.
>>> It does get used in the wild, and we do break it with changes we didn't
>>> ever anticipate would impact it -
>>> https://bugs.launchpad.net/nova/+bug/1518321
>>> However, when you look into how that is used, it's really really odd and
>>> fragile -
>>> https://github.com/richm/rdo-vm-factory/blob/master/rdo-ipa-nova/novahooks.py#L248
>>>      def pre(self, *args, **kwargs):
>>>          # args[7] is the injected_files parameter array
>>>          # the value is ('filename', 'base64 encoded contents')
>>>          ipaotp = str(uuid.uuid4())
>>>          ipainject = ('/tmp/ipaotp', base64.b64encode(ipaotp))
>>>          args[7].extend(self.inject_files)
>>>          args[7].append(ipainject)
>>> In our continued quest on being more explicit about plug points it feels
>>> like we should other document the interface (which means creating
>>> stability on the hook parameters) or we should deprecate this construct
>>> as part of a bygone era.
>>> I lean on deprecation because it feels like a thing we don't really want
>>> to support going forward, but I can go either way.
>>>     -Sean
>>> P.S. I'm starting to look at in tree functional testing for all of this,
>>> in the event that we decide not to deprecate it. It's definitely made a
>>> little hard by the way all exceptions are caught when hooks go wrong.
>> As there seemed to be some early enthusiasm for this from the core team,
>> the deprecation patch is proposed here -
>> https://review.openstack.org/#/c/286276/
>> Unless there is a big reversal of sentiment I'd suggest we get that
>> landed in Mitaka so that this is signalled to people going forward, and
>> we can collect use cases for things that are currently using hooks that
>> we may want to support in other ways in Newton.
>>     -Sean
> I think you should probably also post this to the operators mailing list
> before we actually deprecate hooks.

I'm not sure that's really true. As a team we have to decide we're going
to deprecate or document this facility. The status quo is really not a

So unless we've got folks that believe we should crystalize this
interface, and are signing up for doing that (which means you can't
change build_instance internal function parameters ever), then I think
it's fine to deprecate. We can measure our removal timeframe based on
reaction to that.


Sean Dague

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