[openstack-dev] [Ironic] maintaining backwards compatibility within a cycle

Devananda van der Veen devananda.vdv at gmail.com
Thu Nov 20 17:19:03 UTC 2014

Let's get concrete for a moment, because it makes a difference which API
we're talking about.

We have to guarantee a fairly high degree of backwards compatibility within
the REST API. Adding new capabilities, and exposing them in a discoverable
way, is fine; a backwards-incompatible breaking change to the REST API is
definitely not OK without a version bump. We should (and do) make a strong
effort not to land any REST API change without appreciable thought and
testing of its impact. Changes here have an immediate effect on anyone
following trunk.

The RPC API is another area of compatibility, and perhaps the one most
clearly versioned today. We must continue supporting running disparate
versions of the RPC client and server (that is, rpcapi.py and manager.py)
so that operators can upgrade the API and Conductor services
asymmetrically. Changes to the RPC API are done in such a way that each
service can be upgraded independently of other services.

The driver API is the only purely-python API we support -- and we know that
there are downstream consumers of that API. OnMetal is one such; many other
spoke up at the recent summit. While the impact of a breaking change here
is less than in the REST API, it is not to be overlooked. There is a cost
associated with maintaining an out-of-tree driver and we should make a
best-effort to minimize that cost for folks who (for what ever reason) are
in that boat.


On Thu Nov 20 2014 at 8:28:56 AM Lucas Alvares Gomes <lucasagomes at gmail.com>

> Hi Ruby,
> Thank you for putting this up.
> I'm one of the ones think we should try hard (even really hard) to
> maintain the compatibility on every commit. I understand that it may
> sound naive because I'm sure that sometimes we will break things, but
> that doesn't means we shouldn't try.
> There may be people running Ironic in a continuous deployment
> environment, those are the users of the project and therefor the most
> important part of Ironic. Doesn't matter how well written Ironic code
> may be if nobody is using it. If we break that user workflow and he's
> unhappy that's the ultimate failure.
> I also understand that in the project POV we want to have fast
> interactions and shiny new features as quick as possible and trying to
> be backward compatibility all the time - on every commit - might slow
> that down. But in the user POV I believe that he doesn't care much
> about all the new features, he would mostly care about the things that
> used to work to continue to work for him.
> Also the backwards approach between releases and not commits might
> work fine in the non-opensource world where the code is kept indoors
> until the software is release, but in the opensource world where the
> code is out to people to use it all the time it doesn't seem to work
> that well.
> That's my 2 cents.
> Lucas
> On Thu, Nov 20, 2014 at 3:38 PM, Ruby Loo <rlooyahoo at gmail.com> wrote:
> > Hi, we had an interesting discussion on IRC about whether or not we
> should
> > be maintaining backwards compatibility within a release cycle. In this
> > particular case, we introduced a new decorator in this kilo cycle, and
> were
> > discussing the renaming of it, and whether it needed to be backwards
> > compatible to not break any out-of-tree driver using master.
> >
> > Some of us (ok, me or I) think it doesn't make sense to make sure that
> > everything we do is backwards compatible. Others disagree and think we
> > should, or at least strive for 'must be' backwards compatible with the
> > caveat that there will be cases where this isn't
> feasible/possible/whatever.
> > (I hope I captured that correctly.)
> >
> > Although I can see the merit (well, sort of) of trying our best, trying
> > doesn't mean 'must', and if it is 'must', who decides what can be
> exempted
> > from this, and how will we communicate what is exempted, etc?
> >
> > Thoughts?
> >
> > --ruby
> >
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