[openstack-dev] [Ironic] When to bump the microversion?

Dmitry Tantsur dtantsur at redhat.com
Thu Jun 4 15:27:23 UTC 2015

On 06/04/2015 05:03 PM, Sean Dague wrote:
> On 06/04/2015 10:50 AM, Dmitry Tantsur wrote:
>> On 06/04/2015 04:40 PM, Ruby Loo wrote:
>>> Hi,
>>> In Kilo, we introduced microversions but it seems to be a
>>> work-in-progress. There is an effort now to add microversion into the
>>> API-WG's guidelines, to provide a consistent way of using microversions
>>> across OpenStack projects [1]. Specifically, in the context of this
>>> email, there is a proposed guideline for when to bump the microversion
>>> [2].
>> As I understand this guideline tells to bump microversion on every
>> change which I strongly -1 as usual. Reason: it's bump for the sake of
>> bump, without any direct benefit for users (no, API discoverability is
>> not one, because microversion do not solve it).
>> I'll post the same comment to the guideline.
> Backwards compatible API adds with no user signaling is a fallacy
> because it assumes the arrow of time flows only one way.
> If at version 1.5 you have a resource that is
> foo {
>    "bar": ...
> }
> And then you decide you want to add another attribute
> foo {
>    "bar": ...
>    "baz": ...
> }
> And you don't bump the version, you'll get a set of users that use a
> cloud with baz, and incorrectly assume that version 1.5 of the API means
> that baz will always be there. Except, there are lots of clouds out
> there, including ones that might be at the code commit before it was
> added. Because there are lots of deploys in the world, your users can
> effectively go back in time.
> So now your API definition for version 1.5 is:
> "foo, may or may not contain baz, and there is no way of you knowing if
> it will until you try. good luck."
> Which is pretty aweful.

Which is not very different from your definition. "Version 1.5 contains 
feature xyz, unless it's disabled by the configuration or patched out 
downstream. Well, 1.4 can also contain the feature, if downstream 
backported it. So good luck again."

If you allow to group features under one microversion, that becomes even 
worse - you can have deployment that got microversion only partially.

For example, that's what I would call API discoverability:

  $ ironic has-capability foobar

and that's how it would play with versioning:

  $ ironic --ironic-api-version 1.2 has-capability foobar
  $ ironic --ironic-api-version 1.6 has-capability foobar

On the contrary, the only thing that microversion tells me is that the 
server installation is based on a particular upstream commit.

To me these are orthogonal problems, and I believe they should be solved 
differently. Our disagreement is due to seeing them as one problem.

> Looking at your comments in the WG repo you also seem to only be
> considering projects shipped at Major release versions (Kilo, Liberty).
> Which might be true of Red Hat's product policy, but it's not generally
> true that all clouds are at a release boundary. Continous Deployment of
> OpenStack has been a value from Day 1, and many public clouds are not
> using releases, but are using arbitrary points off of master.

I don't know why you decided I don't know it, but you're wrong.

 > A
> microversion describes when a changes happens so that applications
> writers have a very firm contract about what they are talking to.

No, they don't. Too many things can modify behavior - see above.

> 	-Sean

More information about the OpenStack-dev mailing list