[openstack-dev] [Nova] Some thoughts on API microversions

Sean Dague sean at dague.net
Thu Aug 4 12:20:55 UTC 2016

On 08/03/2016 08:54 PM, Andrew Laski wrote:
> I've brought some of these thoughts up a few times in conversations
> where the Nova team is trying to decide if a particular change warrants
> a microversion. I'm sure I've annoyed some people by this point because
> it wasn't germane to those discussions. So I'll lay this out in it's own
> thread.
> I am a fan of microversions. I think they work wonderfully to express
> when a resource representation changes, or when different data is
> required in a request. This allows clients to make the same request
> across multiple clouds and expect the exact same response format,
> assuming those clouds support that particular microversion. I also think
> they work well to express that a new resource is available. However I do
> think think they have some shortcomings in expressing that a resource
> has been removed. But in short I think microversions work great for
> expressing that there have been changes to the structure and format of
> the API.
> I think microversions are being overused as a signal for other types of
> changes in the API because they are the only tool we have available. The
> most recent example is a proposal to allow the revert_resize API call to
> work when a resizing instance ends up in an error state. I consider
> microversions to be problematic for changes like that because we end up
> in one of two situations:
> 1. The microversion is a signal that the API now supports this action,
> but users can perform the action at any microversion. What this really
> indicates is that the deployment being queried has upgraded to a certain
> point and has a new capability. The structure and format of the API have
> not changed so an API microversion is the wrong tool here. And the
> expected use of a microversion, in my opinion, is to demarcate that the
> API is now different at this particular point.
> 2. The microversion is a signal that the API now supports this action,
> and users are restricted to using it only on or after that microversion.
> In many cases this is an artificial constraint placed just to satisfy
> the expectation that the API does not change before the microversion.
> But the reality is that if the API change was exposed to every
> microversion it does not affect the ability I lauded above of a client
> being able to send the same request and receive the same response from
> disparate clouds. In other words exposing the new action for all
> microversions does not affect the interoperability story of Nova which
> is the real use case for microversions. I do recognize that the
> situation may be more nuanced and constraining the action to specific
> microversions may be necessary, but that's not always true.
> In case 1 above I think we could find a better way to do this. And I
> don't think we should do case 2, though there may be special cases that
> warrant it.
> As possible alternate signalling methods I would like to propose the
> following for consideration:
> Exposing capabilities that a user is allowed to use. This has been
> discussed before and there is general agreement that this is something
> we would like in Nova. Capabilities will programatically inform users
> that a new action has been added or an existing action can be performed
> in more cases, like revert_resize. With that in place we can avoid the
> ambiguous use of microversions to do that. In the meantime I would like
> the team to consider not using microversions for this case. We have
> enough of them being added that I think for now we could just wait for
> the next microversion after a capability is added and document the new
> capability there.

The problem with this approach is that the capability add isn't on a
microversion boundary, as long as we continue to believe that we want to
support CD deployments this means people can deploy code with the
behavior change, that's not documented or signaled any way.

A microversion is communication of change that is accessible all the way
to the end user (and is currently our only communication channel for
that). There are definitely gray areas here, in which case you are
deciding whether you over communicate (put in a microversion just in
case it turns out to be significant from programming perspective) or
under communicate, bunch things up and figure no one will really mind.

When we have discoverable capabilities infrastructure, we can probably
move some of these to that. But currently we don't have that. And until
we do, version numbers are cheap.

> Secondly we could consider some indicator that exposes how new the code
> in a deployment is. Rather than using microversions as a proxy to
> indicate that a deployment has hit a certain point perhaps there could
> be a header that indicates the date of the last commit in that code.
> That's not an ideal way to implement it but hopefully it makes it clear
> what I'm suggesting. Some marker that a user can use to determine that a
> new behavior is to be expected, but not one that's more intended to
> signal structural API changes.
> Thoughts?

I think we're going to get a ton of push back from people on this. When
we first rolled out microversions I got a number of people asking if
they could hide the supported microversions, because it gave some
indication of the code level on the server (like people hide apache
version in production). Which entirely missed the point of the
infrastructure. I can't see folks allowing this in. Plus this is git,
and people have local patches, so I'm not sure there is any meaningful
concept here to expose.

I'm on board with a future where we have the monotonically increasing
microversions, as well as a side channel of discoverable capabilities.
But I think the moment you try to introduce a 3rd communication channel
for behavior, which has something looking like a version number, it
actually becomes way too confusing from a consumption point of view. And
it also breaks one of the things we were trying to do, which is
guaruntee old behavior (as much as possible) on old API versions.

I think that if we put some code version into place, we'll just assume
we can use that to signal changes, and stop realizing how disruptive it
is to make those changes for existing users.


Sean Dague

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