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

Joshua Harlow harlowja at fastmail.com
Tue Mar 1 18:15:27 UTC 2016

Sorry for the top posting, but seems like doing so is ok,

So an idea, feel free to use it or not...

But if a in-memory notification mechanism is desired then a library that 
I have been using and extracted from taskflow could be helpful here,

It doesn't bring in the full oslo.messaging notifications (since its 
only for in-memory after-all); but it could be just the thing needed to 
hook into observation points (with a more concrete definition/schema of 
what is/can be being observed being done by the nova folks):



A really simple example:

 >>> import notifier
 >>> import time
 >>> def watcher(event_type, details):
...    print("%s happened with details: %s" % (event_type, details))
 >>> n = notifier.Notifier()
 >>> n.register('vm-booted', watcher)
<Listener object at 0x7f1e6b738b90 calling into '<function watcher at 
 >>> n.notify('vm-booted', details={'when': time.time()})
vm-booted happened with details: {'when': 1456855953.650803}
<Future at 0x7f1e6b738c10 state=finished returned _Notified>

Already in global-requirements btw,


On 02/29/2016 08:59 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.

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