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On 11/24/15 at 10:26am, Balázs Gibizer wrote:
>> From: Ryan Rossiter [mailto:rlrossit at linux.vnet.ibm.com]
>> Sent: November 23, 2015 22:33
>> On 11/23/2015 2:23 PM, Andrew Laski wrote:
>> > On 11/23/15 at 04:43pm, Balázs Gibizer wrote:
>> >>> From: Andrew Laski [mailto:andrew at lascii.com]
>> >>> Sent: November 23, 2015 17:03
>> >>>
>> >>> On 11/23/15 at 08:54am, Ryan Rossiter wrote:
>> >>> >
>> >>> >
>> >>> >On 11/23/2015 5:33 AM, John Garbutt wrote:
>> >>> >>On 20 November 2015 at 09:37, Balázs Gibizer
>> >>> >><balazs.gibizer at ericsson.com> wrote:
>> >>> >>><snip>
>> >>> >>><snip>
>> >> >>
>> >>> >>There is a bit I am conflicted/worried about, and thats when we
>> >>> >>start including verbatim, DB objects into the notifications. At
>> >>> >>least you can now quickly detect if that blob is something
>> >>> >>compatible with your current parsing code. My preference is really
>> >>> >>to keep the Notifications as a totally separate object tree, but I
>> >>> >>am sure there are many cases where that ends up being seemingly
>> >>> >>stupid duplicate work. I am not expressing this well in text form
>> >>> >>:(
>> >>> >Are you saying we don't want to be willy-nilly tossing DB objects
>> >>> >across the wire? Yeah that was part of the rug-pulling of just
>> >>> >having the payload contain an object. We're automatically tossing
>> >>> >everything with the object then, whether or not some of that was
>> >>> >supposed to be a secret. We could add some sort of property to the
>> >>> >field like dont_put_me_on_the_wire=True (or I guess a
>> >>> >notification_ready() function that helps an object sanitize
>> >>> >itself?) that the notifications will look at to know if it puts
>> >>> >that on the wire-serialized dict, but that's adding a lot more
>> >>> >complexity and work to a pile that's already growing rapidly.
>> >>>
>> >>> I don't want to be tossing db objects across the wire.  But I also
>> >>> am not convinced that we should be tossing the current objects over
>> >>> the wire either.
>> >>> You make the point that there may be things in the object that
>> >>> shouldn't be exposed, and I think object version bumps is another
>> >>> thing to watch out for.
>> >>> So far the only object that has been bumped is Instance but in doing
>> >>> so no notifications needed to change.  I think if we just put
>> >>> objects into notifications we're coupling the notification versions
>> >>> to db or RPC changes unnecessarily.  Some times they'll move
>> >>> together but other times, like moving flavor into instance_extra,
>> >>> there's no reason to bump notifications.
>> >>
>> >>
>> >> Sanitizing existing versioned objects before putting them to the wire
>> >> is not hard to do.
>> >> You can see an example of doing it in
>> >> https://review.openstack.org/#/c/245678/8/nova/objects/service.py,cm
>> >> L382.
>> >> We don't need extra effort to take care of minor version bumps
>> >> because that does not break a well written consumer. We do have to
>> >> take care of the major version bumps but that is a rare event and
>> >> therefore can be handled one by one in a way John suggested, by keep
>> >> sending the previous major version for a while too.
>> >
>> > That review is doing much of what I was suggesting.  There is a
>> > separate notification and payload object.  The issue I have is that
>> > within the ServiceStatusPayload the raw Service object and version is
>> > being dumped, with the filter you point out.  But I don't think that
>> > consumers really care about tracking Service object versions and
>> > dealing with compatibility there, it would be easier for them to track
>> > the ServiceStatusPayload version which can remain relatively stable
>> > even if Service is changing to adapt to db/RPC changes.
>> Not only do they not really care about tracking the Service object versions,
>> they probably also don't care about what's in that filter list.
>>
>> But I think you're getting on the right track as to where this needs to go. We
>> can integrate the filtering into the versioning of the payload.
>> But instead of a blacklist, we turn the filter into a white list. If the underlying
>> object adds a new field that we don't want/care if people know about, the
>> payload version doesn't have to change. But if we add something (or if we're
>> changing the existing fields) that we want to expose, we then assert that we
>> need to update the version of the payload, so the consumer can look at the
>> payload and say "oh, in 1.x, now I get _______" and can add the appropriate
>> checks/compat. Granted with this you can get into rebase nightmares ([1]
>> still haunts me in my sleep), but I don't see us frantically changing the
>> exposed fields all too often. This way gives us some form of pseudo-pinning
>> of the subobject. Heck, in this method, we could even pass the whitelist on
>> the wire right? That way we tell the consumer explicitly what's available to
>> them (kinda like a fake schema).
>
>I think see your point, and it seems like a good way forward. Let's turn the black list to
>a white list. Now I'm thinking about creating a new Field type something like
>WhiteListedObjectField which get a type name (as the ObjectField) but also get
>a white_list that describes which fields needs to be used from the original type.
>Then this new field serializes only the white listed fields from the original type
>and only forces a version bump on the parent object if one of the white_listed field
>changed or a new field added to the white_list.
>What it does not solve out of the box is the transitive dependency. If today we
>Have an o.vo object having a filed to another o.vo object and we want to put
>the first object into a notification payload but want to white_list fields from
>the second o.vo then our white list needs to be able to handle not just first
>level fields but subfields too. I guess this is doable but I'm wondering if we
>can avoid inventing a syntax expressing something like 'field.subfield.subsubfield'
>in the white list.

Rather than a whitelist/blacklist why not just define the schema of the 
notification within the notification object and then have the object 
code handle pulling the appropriate fields, converting formats if 
necessary, from contained objects.  Something like:

class ServicePayloadObject(NovaObject):
     SCHEMA = {'host': ('service', 'host'),
               'binary': ('service', 'binary'),
               'compute_node_foo': ('compute_node', 'foo'),
              }

     fields = {
         'service': fields.ObjectField('Service'),
         'compute_node': fields.ObjectField('ComputeNode'),
     }

     def populate_schema(self):
         self.compute_node = self.service.compute_node
         notification = {}
         for key, (obj, field) in schema.iteritems():
             notification[key] = getattr(getattr(self, obj), field)

Then object changes have no effect on the notifications unless there's a 
major version bump in which case a SCHEMA_VNEXT could be defined if 
necessary.


>
>Cheers,
>Gibi
>
>>
>> I think I can whip a PoC up for this (including the tests, since I'm so intimately
>> familiar with them now that I'm THE nova-objects guy) if we want to see
>> where this goes.
>>
>> <super snip>
>>
>> [1] https://review.openstack.org/#/c/198730/
>>
>> --
>> Thanks,
>>
>> Ryan Rossiter (rlrossit)
>>
>>
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