Re: [netmod] yang-data-ext issues

Martin Bjorklund <mbj@tail-f.com> Mon, 16 April 2018 16:29 UTC

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Date: Mon, 16 Apr 2018 18:29:26 +0200 (CEST)
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From: Martin Bjorklund <mbj@tail-f.com>
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Subject: Re: [netmod] yang-data-ext issues
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Andy Bierman <andy@yumaworks.com>; wrote:
> On Mon, Apr 16, 2018 at 8:44 AM, Robert Wilton <rwilton@cisco.com>; wrote:
> 
> > Don't groupings have a somewhat similar concern?
> >
> >  E.g. if two groupings define the same data node name and are used at the
> > same point then you would get a namespace clash, but YANG does not disallow
> > the groupings:
> >
> >      grouping foo_widget {
> >        leaf name {
> >          type string;
> >          description "Name of my foo widget";
> >        }
> >      }
> >
> >      grouping bar_widget {
> >        leaf name {
> >          type string;
> >          description "Name of my bar widget";
> >        }
> >      }
> >
> >
> >      container all_widgets {
> >        uses foo_widget;
> >        uses bar_widget;
> >      }
> >
> >
> > The principal difference here, is that the compiler can easily check and
> > reject the conflict at the uses statements.
> >
> > Hence I think that it would be good if we could find a solution for
> > yang-data-ext that doesn't not require all root yang-data nodes to be
> > unique, since that feels somewhat clunky.  I.e. my preference is to keep
> > them less restrictive, as Martin has proposed, if this is feasible.
> >
> >
> 
> 
> It is not clunky that 2 top-level YANG data nodes in the same module
> have unique names.

If the yang-data structure is used nested within some other node, it
does not define a top-level node.  In this case it doesn't have to
have uniquely named nodes, and it doesn't have to define a single
container.

> This is simple and deterministic.
> This restriction has not been a problem so far.

yang-data is a new construct.  A more restrictive version was defined
in RFC 8040, and these restrictions *were* problematic.  I don't want
to make the same mistake again.

> The yang-data statement has to define the context or new abstract namespace,

Yes, agreed.  Regardless of the outcome of this thread, the yang-data
statement has to define where the structure being defined is supposed
to be used.

> or whatever this hack is called.  Every tool that implements yang-data has
> to be able
> to interpret a yang-data statement exactly the same way.
> 
> If you want to reinvent XSD substitutionGroup, then do it right.

This is not a new substitutionGroup.


/martin


> 
> 
> 
> 
> > Thanks,
> > Rob
> >
> >
> 
> Andy
> 
> 
> >
> > On 16/04/2018 15:36, Andy Bierman wrote:
> >
> > Hi,
> >
> > I am strongly opposed to this change because it breaks the rule in YANG 1.1
> > that there cannot be 2 sibling nodes defined in the same module namespace.
> >
> > IMO since any yang-data nodes are ALLOWED to be used at the top-level,
> > then these top-level nodes cannot have conflicting names.
> >
> > It is very important when parsing an instance document that the instance
> > data
> > can be associated with the correct schema.  This is not possible if the
> > same top-level node has multiple yang-data nodes defined.
> >
> > If one needs to define data that is not top-level, (1) use
> > augment-yang-data
> > or (2) use a different module.
> >
> >
> > Andy
> >
> >
> >
> > On Mon, Apr 16, 2018 at 5:56 AM, Martin Bjorklund <mbj@tail-f.com>; wrote:
> >
> >> Hi,
> >>
> >> While preparing draft-ietf-netmod-yang-data-ext-02, it turned out that
> >> it is not clear what, if any, restrictions should be enforced for
> >> yang-data structures.  Even among the authors we have different ideas
> >> for how this should work.
> >>
> >> Background:
> >>
> >> In 8040, the original yang-data extension had a restriction that said
> >> that a yang-data structure MUST have exactly one container, since it
> >> wouldn't be possible to have a yang-data structure in an XML instance
> >> document otherwise.
> >>
> >> Since people want to use yang-data structures in other places, this
> >> restriction was lifted in the new draft:
> >>
> >>    There is no longer an assumption that a yang data structure can
> >>    only be used as a top-level abstraction, instead of nested within
> >>    some other data structure.
> >>
> >>
> >> With this in mind, here's a use case that I think we ought to support:
> >>
> >>   rpc my-first-rpc {
> >>     description
> >>       "Bla bla...
> >>        If an error occurs, <error-info> will contain an instance of
> >>        the yang-data structure 'my-first-rpc-error-info'.";
> >>     ...
> >>   }
> >>
> >>   yang-data my-first-rpc-error-info {
> >>     leaf reason { ... }
> >>     container user-info { ... }
> >>   }
> >>
> >>   rpc my-second-rpc {
> >>     description
> >>       "Bla bla...
> >>        If an error occurs, <error-info> will contain an instance of
> >>        the yang-data structure 'my-second-rpc-error-info'.";
> >>     ...
> >>   }
> >>
> >>   yang-data my-second-rpc-error-info {
> >>     leaf reason { ... }
> >>     leaf important-url { ... }
> >>   }
> >>
> >> (maybe in the future we could even have a YANG extension statement to
> >> formalize the description:
> >>
> >>    rpc my-first-rpc {
> >>      ...
> >>      opx:error-info-structure my-first-rpc-error-info;
> >>    }
> >>
> >> but this is not point now.)
> >>
> >
> 
> 
> I see no reason to reinvent the grouping-stmt.
> You could easily say opx:error-info-structure argument is a grouping name
> as it is a yang-data name.
> 
> 
> 
> >
> >> In the example above, note that the leaf "reason" is present in both
> >> structures.  IMO this is not a problem, since these structures are
> >> used in different contexts.
> >>
> >> My point is that I think we should impose as few restrictions as
> >> possible to the yang-data extension.  It should be up to the user of
> >> yang-data to ensure that the structure is defined in such a way so
> >> that it can be used properly.  For example, a structure that is
> >> supposed to describe an XML instance document cannot define two leafs
> >> at the top level.
> >>
> >> If the WG agrees with what I wrote above, we need to change the
> >> augment-yang-data extension so that you would write for example:
> >>
> >>   yx:augment-yang-data /ex:my-first-rpc-error-info/ex:user-info {
> >>     ...
> >>   }
> >>
> >> Comments?
> >>
> >>
> >>
> >> /martin
> >>
> >> _______________________________________________
> >> netmod mailing list
> >> netmod@ietf.org
> >> https://www.ietf.org/mailman/listinfo/netmod
> >>
> >
> >
> >
> > _______________________________________________
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> >
> >
> >