Re: [netmod] uses and augment

[Juergen Schoenwaelder <[e-mail]>]

On Thu, Mar 23, 2017 at 08:50:48PM -0400, Dale R. Worley wrote:
> Martin Bjorklund <[e-mail]> writes:
> >> I notice that "augment" is not allowed to target a "grouping", despite
> >> that naively seems to be an operation that a module designer might like
> >> to do.  I expect that there is a reason why this is not allowed.
> >
> > There were lots of debate over this one when we first designed YANG.
> > The main reason for not allowing this is that it can easily have
> > unintended consequences.  Module A uses a grouping G b/c it fits the
> > purpose.  Later someone augments G with some nodes; at this point it
> > is not at all clear that the additional nodes are suitable for module
> > A.
> 
> True...  But assuming that the grouping G has clean semantics, it
> corresponds to some facility in the device, which in some way or another
> appears in multiple places in the device's data model.  And a module
> that augments G adds semantics about that facility, and would only be
> implemented by a device for which the facility uniformly has that
> additional semantics.  So it would be suitable for every place where the
> grouping is used.

But this is an assumption and it is not generally true.

[...]
 
> But what I'm considering is a modification of
> the grouping which implicitly applies to all "uses" of that grouping --
> because you don't want to have duplicate declarations of the added nodes
> in every place the grouping is used.

There may be cases where this may appear useful but I see also cases
where people will get largely suprised by the result if such wildcard
augmentations would be possible.

> > Augments are restricted to things that have a well defined name in the
> > data tree because this makes it clear what is being augmented. One
> > would have to create additional language constructs to make
> > augmentations of groupings work.
> 
> It's clear that *groupings* have well-defined names, because "uses"
> statements can refer to them.

I wrote 'name in the data tree' and I meant 'name in the schema tree'
(sorry). Augments apply to something that has a name in the schema
tree, groupings do not have that.

   The "grouping" statement is not a data definition statement and, as
   such, does not define any nodes in the schema tree.

> RFC 7950 section 7.13 isn't particularly
> clear about how the argument string of the statement is to be
> interpreted, but going back over 7950, I'm getting the idea that the
> names of groupings are not descendant-schema-nodeid's, that is, named
> based on where the grouping statement sits in the syntactic hierarchy,
> but are in a separate namespace which is flat regarding equality and
> inequality comparisons, but has elaborate scoping rules regarding which
> groupings are visible in which locations.

Yes.

> OK, that clarifies why you can't apply "augment" to a grouping --
> groupings (and thus the things defined within them) don't have names
> that can be expressed by descendant-schema-nodeid's.

Yes. This is how things were defined.

/js

-- 
Juergen Schoenwaelder           Jacobs University Bremen gGmbH
Phone: [phone]         Campus Ring 1 | 28759 Bremen | Germany
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