Re: [netmod] augment and if-feature

Hi Dale,

On 15/03/2017 19:02, Dale R. Worley wrote:
> JOEY BOYD <joey.boyd@adtran.com> writes:
>> module base-module {
>>    prefix bmod;
>>
>>    feature do-things;
>>
>>    container things {
>>      if-feature do-things;
>>      ...
>>    }
>> }
>>
>> module augment-module {
>>    prefix amod;
>>
>>    augment "/bmod:do-things" {
>>      container other-things {
>>      }
>>    }
>> }
> First question:  I'm not expert in Yang, but as far as I can figure out,
> the augment statement is augmenting "container things", right?  So the
> augment statement should be 'augment "/bmod:things"' not 'augment
> "/bmod:do-things"'.
Yes, the augment should be to "things".

>
> But on the important question, I don't see it as at all unreasonable
> that the augment needs to be qualified by the same if-feature.  The
> reason is that if you're reading the text of module augment-module, it's
> helpful to have documented, right there, that the augmentation depends
> on the presence of a particular feature in the augmented module.  And
> it's helpful to know that the designer did, at least for one moment,
> think about the fact that the augmentation is conditional.

It isn't just any if-feature on the container that is being augmented 
that needs to be considered.  You would have to consider all if-feature 
statements by walking up the augmented node's ancestors to the top of 
the tree and combine them, or have multiple if-feature statements.

Further, the 7950 YANG update rules allow for the augmented module to be 
revised and some of those if-feature statements to be subsequently 
removed.  If the augmenting module had restated the if-feature 
conditions then this would probably leave the augmenting module 
unintentionally out of sync with the module that it is augmenting.

In short, I think that if-feature statements work better if they act on 
the given node and all descendant nodes, regardless of which module 
those descendants are defined in.

Rob

>
> Dale
>
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