[openstack-dev] [nova] [placement] experimenting with extracting placement

Sylvain Bauza sbauza at redhat.com
Mon Mar 13 14:14:42 UTC 2017

Le 13/03/2017 14:59, Jay Pipes a écrit :
> On 03/13/2017 08:41 AM, Chris Dent wrote:
>> From the start we've been saying that it is probably right for the
>> placement service to have its own repository. This is aligned with
>> the long term goal of placement being useful to many services, not
>> just nova, and also helps to keep placement contained and
>> comprehensible and thus maintainable.
>> I've been worried for some time that the longer we put this off, the
>> more complicated an extraction becomes. Rather than carry on
>> worrying about it, I took some time over the weekend to experiment
>> with a slapdash extraction to see if I could identify what would be
>> the sticking points. The results are here
>>     https://github.com/cdent/placement
>> My methodology was to lay in the basics for being able to run the
>> functional (gabbi) tests and then using the failures to fix the
>> code. If you read the commit log (there's only 16 commits) in
>> reverse it tells a little story of what was required.
>> All the gabbi tests are now passing (without them being changed)
>> except for four that verify the response strings from exceptions. I
>> didn't copy in exceptions, I created them anew to avoid copying
>> unnecessary nova-isms, and didn't bother (for now) with replicating
>> keyword handling.
>> Unit tests and non-gabbi functional tests were not transferred over
>> (as that would have been something more than "slapdash").
>> Some observations or things to think about:
>> * Since there's only one database and all the db query code is in
>>   the objects, the database handling is simplified. olso_db setup
>>   can be used more directly.
>> * The objects being oslo versioned objects is kind of overkill in
>>   this context but doesn't get too much in the way.
>> * I collapsed the fields.ResourceClass and objects.ResourceClass
>>   into the same file so the latter was renamed. Doing this
>>   exploration made a lot of the ResourceClass handling look pretty
>>   complicated. Much of that complexity is because we had to deal
>>   with evolving through different functionality. If we built this
>>   functionality in a greenfield repo it could probably be more
>>   simple.
>> * The FaultWrapper middleware is turned off in the WSGI stack
>>   because copying it over from nova would require dealing with a
>>   hierarchy of classes. A simplified version of it would probably
>>   need to be stuck back in (and apparently a gabbi test to exercise
>>   it, as there's not one now).
>> * The number of requirements in the two requirements files is nicely
>>   small.
>> * The scheduler report client in nova, and to a minor degree the
>>   filter scheduler, use some of the same exceptions and ovo.objects
>>   that placement uses, which presents a bit of blechiness with
>>   regards to code duplication. I suppose long term we could consider
>>   a placement-lib or something like that, except that the
>>   functionality provided by the same-named objects and exceptions
>>   are not entirely congruent. From the point of view of the external
>>   part of the placement API what matters are not objects, but JSON
>>   structures.
>> * I've done nothing here with regard to how devstack would choose
>>   between the old and new placement code locations but that will be
>>   something to solve. It seems like it ought to be possible for two
>>   different sources of the placement-code to exist; just register
>>   one endpoint. Since we've declared that service discovery is the
>>   correctly and only way to find placement, this ought to be okay.
>> I'm not sure how or if we want to proceed with this topic, but I
>> think this at least allows us to talk about it with less guessing.
>> My generally summary is "yeah, this is doable, without huge amounts
>> of work."
> Chris, great work on this over the weekend. It gives us some valuable
> data points and information to consider about the split out of the
> placement API. Really appreciate the effort.
> A few things:
> 1) Definitely agree on the need to have the Nova-side stuff *not*
> reference ovo objects for resource providers. We want the Nova side to
> use JSON/dict representations within the resource tracker and scheduler.
> This work can be done right now and isn't dependent on anything AFAIK.
> 2) The FaultWrapper stuff can also be handled relatively free of
> dependencies. In fact, there is a spec around error reporting using
> codes in addition to messages [1] that we could tack on the FaultWrapper
> cleanup items. Basically, make that spec into a "fix up error handling
> in placement API" general work item list...
> 3) While the split of the placement API is not the highest priority
> placement item in Pike (we are focused on traits, ironic integration,
> shared pools and then nested providers, in that order), I do think it's
> worthwhile splitting the placement service out from Nova in Queens. I
> don't believe that doing claims in the placement API is something that
> needs to be completed before splitting out. I'll respond to Sylvain's
> thread about this separately.
> Thanks again for your efforts this weekend,
> -jay

To be honest, one of the things I think we're missing yet is a separate
client that deployers would package so that Nova and other customer
projects would use for calling the Placement API.
At the moment, we have a huge amount of code in nova.scheduler.report
module that does smart things and I'd love to see that being in a
separate python package (maybe in the novaclient repo, or something
else) so we could ask deployers to package *that only*

The interest in that is that it wouldn't be a separate service project,
just a pure client package at a first try, and we could see how to cut
placement separately the cycle after that.


> [1] https://review.openstack.org/#/c/418393/
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