On Fri, Jun 26, 2015 at 3:46 PM, Dmitry Tantsur <divius.ins...@gmail.com> wrote:
>
> 26 июня 2015 г. 2:48 пользователь "GHANSHYAM MANN" <ghanshyamm...@gmail.com>
> написал:
>>
>> On Thu, Jun 25, 2015 at 5:18 PM, Ken'ichi Ohmichi <ken1ohmi...@gmail.com>
>> wrote:
>> > Sorry for late response here,
>> >
>> > 2015-06-20 9:14 GMT+09:00 Devananda van der Veen
>> > <devananda....@gmail.com>:
>> >>
>> >> Long version...
>> >> Every HTTP response from Ironic today includes three headers: min, max,
>> >> and
>> >> version. The service can present an older API version, as long as it is
>> >> greater-than-or-equal-to the minimum supported version, even if that
>> >> version
>> >> is incompatible with the maximum supported version.  It does this by
>> >> rewriting responses to match what was expected in the requested (older)
>> >> version.
>> >>
>> >> When the newer version is identical *for all interfaces present* in the
>> >> older version, this can be called compatible. Dmitry's point is that we
>> >> don't need to hide newer interfaces from users who request an older API
>> >> version, because the client won't know or care about things that
>> >> weren't in
>> >> the version it requested.
>> >>
>> >> However, we *do* need to signal their presence, and we don't have a
>> >> good
>> >> means for that right now. We also need to signal to the client that the
>> >> response given is "compatible with" a certain "age" of API, even if
>> >> it's not
>> >> exactly the same. And we don't have any means for that, either.
>> >>
>> >> Time for an example....
>> >>
>> >> Let's say that an incompatible change was made in v1.3. Let's also say
>> >> that
>> >> a change was made in v1.5 that added a new endpoint. Today, this is
>> >> what the
>> >> response headers would look like when calling a server running v1.5.
>> >>
>> >> a) client requests v1.2, receives headers (min: 1.1, max: 1.5, current:
>> >> 1.2)
>> >> b) client requests v1.4, receives headers (min: 1.1, max: 1.5, current
>> >> 1.4)
>> >> c) client requests v1.5, receives headers (min: 1.1, max: 1.5, current:
>> >> 1.5)
>> >>
>> >> What we have implemented today is that in case (b), the service will
>> >> *hide*
>> >> any changes that we introduced in v1.5. But those changes did not
>> >> affect any
>> >> functionality of the v1.4 API, so Dmitry objects to this. So do I.
>> >>
>> >> The issue at hand is the response in case (b) ... though after spending
>> >> the
>> >> last several months working on api versioning, I actually think all of
>> >> those
>> >> are poor responses.
>> >>
>> >> What I believe we should have:
>> >> a) client requests v1.2, receives headers (min: 1.1, max: 1.5,
>> >> compatible-with: 1.1)
>> >> b) client requests v1.4, receives headers  (min: 1.1, max: 1.5,
>> >> compatible-with: 1.3)
>> >> b) client requests v1.5, receives headers  (min: 1.1, max: 1.5,
>> >> compatible-with: 1.3)
>> >>
>> >> Yes -- (b) and (c) are identical responses.
>> >>
>> >> Discuss.
>> >
>> > I think it is good that backwards compatible changes(new features) are
>> > available on older microversion also *only* if the clouds which are
>> > used by users continue upgrading.
>> >
>> > I think Sophia's role on "The Backwards Compatibility Fallacy" of
>> > Sean's blog[1] has answered to this question, but I'd like to try
>> > explaining it here for considering Ironic situation.
>> > As the example, there are multiple public clouds which provide
>> > different max microversions like:
>> >
>> > Cloud A: Max microversion: v1.5
>> > Cloud B: Max microversion: v1.1
>> >
>> > A user wrote his own application for running on cloud A and specifying
>> > v1.1 on the first application implementation.
>> > The first application used small number of APIs, and he wanted to
>> > extend the application.
>> > If all backwards compatible changes(v1.2 - v1.5) appear on lower
>> > microversion(in this case v1.1), he can use all new features even if
>> > specifying v1.1.
>> > That seemed really great for users at this time, and he extended the
>> > application for using all features as possible.
>> > but the specified microversion still is v1.1 because his application
>> > worked fine even if using newer features.
>> >
>> > After that, he needed to switch to the other cloud because of cost
>> > merit or something.
>> > The specified microversion was v1.1, so he did think his application
>> > can work fine on cloud B also because of cloud B's max microversion.
>> > But yes, his application could not work because his application had
>> > already used newer features which are implemented on v1.2+.
>> > In the real world, there are a lot of clouds and it is easy to imagine
>> > this situation.
>
> By the way, feature hiding did not change the situation: the application
> using new features won't work on older cloud, period.

But application can control the new feature usage with version (with current
design of hiding new features in older versions). With that its going to work
on both new or older cloud.

If older cloud is not upgraded, app knows their max supported version,
and can behave/control new feature usage in their app.

>
>> >
>>
>> This is really nice point. We should give power to users to decide whether
>> they
>> need/want to use/get the new features introduced in new microversion
>> irrespective
>> of it is backward compatible or not.
>
> I'm already tried of asking why, but I'll try again: why? Did you even read
> this thread? It's not power, it's limitation, and a pretty nasty one.
>

I am sorry if you my just in between reply and may be less wording
feels you i did not read whole thread :).

IMO, It could have been limitation where user/operators etc do not
have any control over what their cloud
or cloud they based on, returns. In that case, they theoretically
provide version control over software services
they provide but practically do not as new features automatically gets
added even they requesting
specific OpenStack API version where those were not present before upgrade.

Power here I meant, App based on OpenStack cloud has control to not
make them backward incompatible in case
of interoperatibility.

>>
>>
>> > Current microversion implementation of Nova is blocking this situation
>> > by making backwards every compatible change appear on each
>> > microversion.
>> > Nova team needs to consider interoperability between clouds so well
>> > because Nova API is one of general external interfaces for end users.
>> >
>> > On the other hand, Ironic API is for administrators, not for end users.
>> > I am imaging that:
>> > * Some administrator wrote his application for using Ironic API.
>> > * From the viewpoint of administrator, the switching destination cloud
>> > in newer in most cases.
>> > * The application can continue working on newer clouds even after
>> > switching many times.
>> >
>> > So I feel the above interoperable issue example would not happen on
>> > Ironic in most cases unless hiding backwards compatible changes on
>> > lower microversion.
>> > I guess this is the difference between Nova and Ironic on
>> > interoperability discussion.
>> >
>> > I cannot/don't want to enforce Ironic way at all, and it's fine to
>> > find the best way on each project as OSS projects.
>> > But only my concern here is that we cannot use "Microversions" as a
>> > perfect keyword for OpenStack interoperability on whole OpenStack
>> > projects if Ironic goes to the other way.
>> >
>> > Thanks
>> > Ken Ohmichi
>> >
>> > ---
>> > [1]: https://dague.net/2015/06/05/the-nova-api-in-kilo-and-beyond-2/
>> >
>> >
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>>
>>
>> --
>> Thanks & Regards
>> Ghanshyam Mann
>>
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-- 
Thanks & Regards
Ghanshyam Mann

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