Re: [netmod] augment and if-feature

On Thu, Mar 16, 2017 at 8:05 AM, Dale R. Worley <worley@ariadne.com> wrote:

> Robert Wilton <rwilton@cisco.com> writes:
> > 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.
>
> Yes, I would expect that.
>
> > 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.
>
> It's an interesting sort of out-of-sync, though, as nothing would be
> *incorrect*.  With some combinations of features, the augmented node
> would have the agumentation and with some, it would not.  But it seems
> to me that is quite OK, since whatever depends on the presence of the
> augmentation (e.g., client logic or XPath expressions in the
> augmentation) only expects the augmentation to be there if the
> additional feature is present.
>

YANG does not do a great job of separating conformance from conditional
schema nodes.
The if-feature-stmt does not say false means the conformance requirement is
removed.
It says the schema node is removed.  This means a server cannot pick some
nodes
from a YANG feature. The schema tree has all or none (of the nodes marked
with this if-feature).

This distinction may be important, especially if one tries to reconcile the
"always exists" property
of NP-containers with if-feature. If there are no semantics for an
NP-container then how
can it be included or excluded from the conformance model or the
implementation?
Maybe when-stmt and if-feature-stmt on an NP container really just impacts
the descendants,
not the NP-container itself.

> Dale
>

Andy

>
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