Re: [netmod] Deviations and augmentations

[Robert Wilton <[e-mail]>]

On 12/11/2018 16:33, Martin Bjorklund wrote:
> Robert Wilton <[e-mail]> 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".
Ah, OK. I agree that makes sense.

>
> 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.
So, I think that this probably works for adding enhancements, but not 
for the (arguably more important) bug fix case, unless there is a 
reasonable solution to having two config data nodes both modifying the 
same underlying property.  Perhaps under some reasonable constraints 
this could be made to work - but I don't know.

Of course, even for enhancements it is not necessarily a perfect 
solution.  E.g. backporting some subset of a module already 
coded/implemented in latest to an older release.  And yes, we really do 
get asked to do this sometimes, although it is relatively rare.

Thanks,
Rob

>
>
> /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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