Re: [netmod] yang-data-ext issues

Martin Bjorklund <mbj@tail-f.com> Mon, 16 April 2018 15:02 UTC

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To: andy@yumaworks.com
Cc: jclarke@cisco.com, netmod@ietf.org
From: Martin Bjorklund <mbj@tail-f.com>
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Subject: Re: [netmod] yang-data-ext issues
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Hi,

Andy Bierman <andy@yumaworks.com>; wrote:
> On Mon, Apr 16, 2018 at 6:33 AM, Joe Clarke <jclarke@cisco.com>; wrote:
> 
> > On 4/16/18 08:56, Martin Bjorklund 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.)
> > >
> > > 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?
> >
> > I found the "single container only" policy to be too restricting.  I was
> > modeling data that would [typically] be serialized to something other
> > than XML, and this just made me jump through more hoops than I wanted to
> > use rc:yang-data.
> >
> > I agree with your laissez-faire proposal here.  I think there should be
> > some text to the same effect about how it is up to the author of the
> > yd:yang-data elements to make sure the modeled data can be encoded as
> > they require.
> >
> >
> 
> How will your tool handle this?
> 
> 
> module foo {
>    x:yang-data fake-namespace1 {
>       container top { ... }
>    }
> 
>    x:yang-data fake-namespace2 {
>       container top { ... }
>    }
> 
>    container top { ... }
> 
> }
> 
> 
> You parse an artifact file:
> 
> <top  xmlns="module-foo-namespace">
>    <next> ... </next>
> </top>

If the yang-data structures are supposed be used in instance documents
w/ no additional context information, then the designer should not use
two nodes with the same name in different strcuctures, just like he
wouldn't use two top-level containers.

But as my original email demonstrated, there are cases where the
context provides the information which structure to use.n

> Current YANG says that the <toip> node can only be defined once.
> This makes it possible for a parser to pick the correct schema.
> If 2 or or more yang-data definitions have this name (foo:top)
> then how does your tool pick the right one.?
> 
> This existing restriction in YANG serves a useful purpose.
> Removing it would be unwise.
> 
> There is no restriction to limit the yang-data to a container.
> This has already been removed.
> 
> 
> My counter-proposal is to remove the argument for yang-data
> since it servers no purpose:
> 
> 
>   x:yang-data  {
>      container top { ... }
>   }
> 
>   x:yang-data {
>       list bar { ... }
>    }

I think this would be a mistake.  It wouldn't even be possible to talk
about the different structures in a module.  For example, the
subscribed notification draft has this:


   These are the following three yang-data
   structures for failed event stream subscriptions:

   1.  "establish-subscription-stream-error-info": This MUST be returned
        ...

And it would be impossible to add an extension like:

    opx:error-info-structure my-first-rpc-error-info;

(see my original email for details)



/martin




> 
> Joe
> >
> >
> 
> Andy
> 
> 
> 
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> >