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

Sylvain Bauza sbauza at redhat.com
Mon Mar 13 13:33:15 UTC 2017

Le 13/03/2017 14:21, Sean Dague a écrit :
> On 03/13/2017 09:16 AM, Sylvain Bauza wrote:
>> Le 13/03/2017 13:41, Chris Dent a écrit :
>>> 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."
>> Please don't.
>> Having a separate repository would mean that deployers would need to
>> implement a separate package for placement plus discussing about
>> how/when to use it.
>> For the moment, I'd rather prefer to leave operators using the placement
>> API by using Nova first and then after like 3 or 4 cycles, possibly
>> discussing with them how to cut it.
>> At the moment, I think that we already have a good priority for
>> placement in Nova, so I don't think it's a problem to still have it in Nova.
> Given that the design was always to split (eventually), and part of that
> means that we get to start building up a dedicated core team, I'm not
> sure why waiting 3 or 4 additional cycles makes sense here.
> I get that Pike is probably the wrong release to do this cut, given that
> it only *just* became mandatory. But It feels like saying this would be
> a Queens goal, and getting things structured in such a way that the
> split is easy (like any renaming of binaries, any things that should
> deprecate), would seem to be good goals for Pike.

We are close to the first milestone in Pike, right ? We also have
priorities for Placement that we discussed at the PTG and we never
discussed about how to cut placement during the PTG.

Also, we haven't discussed yet with operators about how they would like
to see Placement being cut. At least, we should wait for the Forum about

For the moment, only operators using Ocata are using the placement API
and we know that most of them had problems when using it. Running for
cutting Placement in Queens would then mean that they would only have
one stable cycle after Ocata for using it.
Also, discussing at the above would then mean that we could punt other
disucssions. For example, I'd prefer to discuss how we could fix the
main problem we have with the scheduler about scheduler claims *before*
trying to think on how to cut Placement.


> 	-Sean

More information about the OpenStack-dev mailing list