[openstack-dev] [Nova] Should we signal backwards incompatible changes in microversions?

Sean Dague sean at dague.net
Tue Feb 16 12:01:41 UTC 2016

On 02/16/2016 03:33 AM, Sylvain Bauza wrote:
> Le 16/02/2016 09:30, Sylvain Bauza a écrit :
>> Le 16/02/2016 04:09, Alex Xu a écrit :
>>> 2016-02-16 9:47 GMT+08:00 GHANSHYAM MANN <ghanshyammann at gmail.com
>>> <mailto:ghanshyammann at gmail.com>>:
>>>     Regards
>>>     Ghanshyam Mann
>>>     On Mon, Feb 15, 2016 at 12:07 PM, Alex Xu <soulxu at gmail.com
>>>     <mailto:soulxu at gmail.com>> wrote:
>>>     > If we support 2.x.y, when we bump 'x' is a problem. We didn't
>>>     order the API
>>>     > changes for now, the version of API change is just based on the
>>>     order of
>>>     > patch merge. For support 2.x.y, we need bump 'y' first for
>>>     back-compatible
>>>     > changes I guess.
>>>     >
>>>     > As I remember, we said before, the new feature is the
>>>     motivation of user
>>>     > upgrade their client to support new version API, whatever the
>>>     new version is
>>>     > backward compatible or incompatible. So I guess the initial
>>>     thinking we hope
>>>     > user always upgrade their code than always stop at old version?
>>>     If we bump
>>>     > 'x' after a lot of 'y', will that lead to user always stop at
>>>     'x' version?
>>>     > And the evolution of api will slow down.
>>>     >
>>>     > Or we limit to each release cycle. In each release, we bump 'y'
>>>     first, and
>>>     > then bump 'x'. Even there isn't any back-incompatible change in
>>>     the release.
>>>     > We still bump 'x' when released. Then we can encourage user
>>>     upgrade their
>>>     > code. But I still think the back-incompatible API change will
>>>     be slow down
>>>     > in development, as it need always merged after back-compatible
>>>     API change
>>>     > patches.
>>>     Yea that true and will be more complicated from development
>>>     perspective which leads to slow down the evolution of API changes.
>>>     But if we support x.y then still we can change x at any time back
>>>     in-comp changes happens(i mean before y also)? Or I may not be
>>>     getting
>>>     the issue you mentioned about always bump y before x.
>>> If the back-incompatible change merged before back-compatible change,
>>> then 'y' become useless. For example, the initial version is 2.1.0,
>>> then we have 3 back-comp and 3 in-comp changes, and we are unlucky,
>>> in-comp changes merged first, then we get version 2.4.3, then if user
>>> want to use those back-comp changes, it still need upgrade those 3
>>> in-comp changes.
>>>     I like the idea of distinguish the backward comp and in-comp changes
>>>     with x and y which always gives clear perspective about changes.
>>>     But it should not lead users to ignore y. I mean some backward comp
>>>     changes which are really good gets ignored by users as they start
>>>     look
>>>     at the x only.
>>>     For example- "adding attribute in resource representation" is back
>>>     comp change (if so) and if that is added as y then, it might get
>>>     ignored by users.
>>>     Another way to clearly distinguish backward comp and in-comp changes
>>>     is through documentation which was initially discussed during
>>>     microversion specs. Currently doc has good description about each
>>>     changes but not much clear way about backward comp or not.
>>>     Which we can do by adding a clear flag [Backward Compatible/
>>>     Incompatible] for each version in doc [1]-
>>> +1 for doc the change is backward comp or not.
>> I'm not usually good at thinking API references, but something pinged
>> my brain so lemme know if that's terrible or not.
>> Why not semantically say that :
>>  - if the API microversion is a ten, then it's a non-backwards
>> compatible change
>>  - if not, it's backwards-compatible
>> If you are like with the version #29 and add a new
>> backwards-compatible version, then it would be #31 (and not #30).
>> That way, you would still have a monotonic increase, which I think was
>> an agreement when discussing about microversioning, but it would help
>> the users which would know the semantics and just look whether a ten
>> is between the version they use and the version they want (and if so,
>> if it was implemented).
>> Call me dumb, it's just a thought.
>> -Sylvain
> One slight improvement could be to consider hundreds and not tens for
> major versions. That would leave 99 'minor' versions between majors,
> which I think is doable.

No, please no.

https://dague.net/2015/06/05/the-nova-api-in-kilo-and-beyond-2/ - the
backwards compatibility fallacy. We've been here before during the
design of the system. Declaring additional semantic meaning to version
numbers is just complexity for very little gain.

It also means we're going to have another hard to figure out thing with
every change. Is this backwards compatible or not? Remember, we have
strict json schema checking, so adding a parameter is not backwards

Everyone thinks they know what's a compatible change, until they
release, then break people using the interface in ways that were
unexpected (see: weekly breaks from pypi).

The expectation is that client programs are going to do the following:


... write lots of nova code.

When the team wants a new feature from Nova, they'll bump that explicitly.


Sean Dague

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