Re: [netmod] Deviations and augmentations

Robert Wilton <rwilton@cisco.com> wrote:
> In the Thursday Netmod meeting, it was interesting to hear Rob Shakir
> describe how deviations and augmentations are used in OpenConfig to
> add functionality into an older YANG model where the semver rules
> prevent the version number from being incremented.
> 
> Further, I think that someone (Martin?) stated on the audio bridge
> that this was an intended/allowed behavior for deviations.

I said that using augmentations (not deviations) was one idea we
originally had for solving the "branching problem".

I think that this works for OC b/c they don't branch their modules.
Hence I think it is important that we decide if branching is a
requirement or not.

/martin

> This surprised me, because I thought that RFC 7950 was quite explicit
> that this is not what deviations are intended for.  My reading of RFC
> 7950 is that the deviation statement represents the case where the
> server *implementation* does not match the *specification*.  However,
> the versioning issue that we are discussing are bug fixes/changes in
> the specification rather than the bug fixes in the implementation.
> 
> Personally, I'm really not keen on using deviations to represent bug
> fixes to older YANG models for three reasons:
> 
> (i) It is changing the meaning of deviation.  It is much cleaner to
> keep the meaning of deviation statements as they are defined today,
> and not conflate their semantics.
> (ii) A different mechanism is used to put a bug fix into an older
> branch rather than in the head of the development.
> (iii) For clients to track the lifecycle of modules they would not
> only need to know the module version number but would also need to
> find and track all associated deviation modules.  This seems
> significantly more complex for clients than the modified semver that
> was proposed.
> 
> ---
> 
> I think that has also been some suggestion that augmentations (or
> duplicate YANG modules with their major version number changed) can be
> used to make bug fixes in a completely backwards compatible way. 
> However, I still don't understand a robust scheme of how this works.
> 
> ---
> 
> Finally, there were some comments about using augmentation modules for
> enhancements.  This is fine, where appropriate (e.g. a non trivial
> number of data nodes are being added as an enhancement) then a
> separate module may be the right way to go. But here, I presume that
> the new functionality will always be tracked by that separate module. 
> If that functionality folds back into the original module at some
> point in the future, then obviously a non backwards compatible version
> change is being forced on to the client, along with additional work on
> the server as well.
> 
> I think that there are also many cases where the number of data nodes
> being added via an enhancement is small compared to the size of the
> module being updated.  In this case I believe that it better to add
> these data nodes into the module itself, perhaps predicated under
> if-feature if appropriate.
> 
> Thanks,
> Rob
> 
> 
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