Re: [netmod] Adding a pre-existing leaf into a new 'choice' - NBC change?

[Ladislav Lhotka <[e-mail]>]

On Thu, 2018-11-22 at 15:00 +0100, Martin Bjorklund wrote:
> Andy Bierman <[e-mail]> wrote:
> > On Thu, Nov 22, 2018 at 5:39 AM Martin Bjorklund <[e-mail]> wrote:
> > 
> > > Hi,
> > >
> > > Ladislav Lhotka <[e-mail]> wrote:
> > > > Andy Bierman <[e-mail]> writes:
> > > >
> > > > > On Mon, Nov 19, 2018 at 12:32 PM Sterne, Jason (Nokia - CA/Ottawa) <
> > > > > [e-mail]> wrote:
> > > > >
> > > > >> Hi all,
> > > > >>
> > > > >>
> > > > >>
> > > > >> If we have a YANG model with a leaf:
> > > > >>
> > > > >>
> > > > >>
> > > > >> MODEL VERSION 1:
> > > > >>
> > > > >> container my-model {
> > > > >>
> > > > >>     leaf a { type string; }
> > > > >>
> > > > >> }
> > > > >>
> > > > >>
> > > > >>
> > > > >> And then later we produce another version of the model where that
> > > leaf is
> > > > >> placed into a choice construct:
> > > > >>
> > > > >>
> > > > >>
> > > > >> MODEL VERSION 2:
> > > > >>
> > > > >> container my-model {
> > > > >>
> > > > >>     choice some-choice {
> > > > >>
> > > > >>         case x {
> > > > >>
> > > > >>             leaf a { type string; }
> > > > >>
> > > > >>         }
> > > > >>
> > > > >>     }
> > > > >>
> > > > >> }
> > > > >>
> > > > >>
> > > > >>
> > > > >> Is that considered a non-backwards-compatible change?
> > > > >>
> > > > >
> > > > >
> > > > > yes -- even though the data node /my-model/x did not change,
> > > > > the schema node /my-model/a changed to /my-model/some-choice/x/a.
> > > > > Any leafref path pointing at this leaf will break.
> > > >
> > > > This is not correct. A leafref path is a special XPath, and as such
> > > > includes only data nodes, i.e. NOT choice and case nodes.
> > > >
> > > > What does change are schema node identifier. This could be significant
> > > > in an augment statement, but not ini this example because a leaf cannot
> > > > be augmented anyway.
> > > >
> > > > I don't see anything else that could break, so Jason's change seems
> > > > backward compatible to me.
> > >
> > > Since it does change the schema tree, this is not legal according to
> > > 7950.  So in that sense it is not backwards compatible.  The rules in
> > > 7950 protect both clients and other modules that import the module.
> > >
> > >
> > This text is confusing wrt/ schema tree vs data tree:
> > 
> > 
> > 9.9 <https://tools.ietf.org/html/rfc7950#section-9.9>;.  The leafref
> > Built-In Type
> > 
> >    The leafref built-in type is restricted to the value space of some
> >    leaf or leaf-list node in the schema tree and optionally further
> >    restricted by corresponding instance nodes in the data tree.  The
> >    "path" substatement (Section 9.9.2
> > <https://tools.ietf.org/html/rfc7950#section-9.9.2>;) is used to
> > identify the referred
> >    leaf or leaf-list node in the schema tree.  The value space of the
> >    referring node is the value space of the referred node.
> 
> Yes, it should be "data tree" in both occurrences.

I tend to disagree. The values of a leafref are first restricted according to
the *schema*, i.e. even before any leaf instance exists in the data tree that
the leafref can point to. Consider this example:

list map {
  key name;
  leaf name {
    type string;
  }
  leaf value {
    type uint8;
  }
}
leaf link {
  type leafref {
    path "../map[name='quux']/value";
    default "foo";
  }
}

We had a long discussion about this, maybe I could find it, and the conclusion
was that a YANG parser should flag the default "foo" value as incorrect even
before any instance data are in sight.

I wasn't exactly happy with this conclusion because it assumes that we can use
the XPath from the argument of "path" to locate the *schema node* and check its
type. Although it looks appealing (everybody sees what the type of "value" is,
right?), I think this is just another unfortunate example of mixing up the
schema and data instances.

Let me ask: can we expect a newcomer to understand what's going on if even
seasoned YANG doctors get confused?

Lada 

> 
> 
> 
> /martin
> 
> 
> > 
> > 
> > 
> > 
> > 
> > >
> > > /martin
> > >
> > >
> > 
> > Andy
> > 
> > 
> > >
> > >
> > > >
> > > > Lada
> > > >
> > > > >
> > > > >
> > > > > Andy
> > > > >
> > > > >
> > > > >>
> > > > >> Does the answer depend on whether the choice contains other cases (or
> > > > >> other cases that are the default case)?
> > > > >>
> > > > >>
> > > > >>
> > > > >
> > > > > no
> > > > >
> > > > >
> > > > >> MODEL VERSION 3:
> > > > >>
> > > > >> container my-model {
> > > > >>
> > > > >>     choice some-choice {
> > > > >>
> > > > >>         case x {
> > > > >>
> > > > >>             leaf a { type string; }
> > > > >>
> > > > >>         }
> > > > >>
> > > > >>         case y {
> > > > >>
> > > > >>             leaf b { type string; }
> > > > >>
> > > > >>         }
> > > > >>
> > > > >>     }
> > > > >>
> > > > >> }
> > > > >>
> > > > >>
> > > > >>
> > > > >> A client 'foo' using VERSION 1 would still be able to set & read back
> > > leaf
> > > > >> a in the same way as it always did.
> > > > >>
> > > > >>
> > > > >>
> > > > >> But if another client 'bar' (using VERSION 3) sets leaf 'b', then
> > > leaf 'a'
> > > > >> would disappear. That could be surprising to client 'foo' although
> > > perhaps
> > > > >> no more surprising than if another client simply deletes leaf 'a'
> > > (using
> > > > >> VERSION 1).
> > > > >>
> > > > >>
> > > > >>
> > > > >> Jason
> > > > >>
> > > > >>
> > > > >>
> > > > >>
> > > > >>
> > > > >>
> > > > >> _______________________________________________
> > > > >> netmod mailing list
> > > > >> [e-mail]
> > > > >> https://www.ietf.org/mailman/listinfo/netmod
> > > > >>
> > > > > _______________________________________________
> > > > > netmod mailing list
> > > > > [e-mail]
> > > > > https://www.ietf.org/mailman/listinfo/netmod
> > > >
> > > > --
> > > > Ladislav Lhotka
> > > > Head, CZ.NIC Labs
> > > > PGP Key ID: 0xB8F92B08A9F76C67
> > > >
> > > > _______________________________________________
> > > > netmod mailing list
> > > > [e-mail]
> > > > https://www.ietf.org/mailman/listinfo/netmod
> > > >
> > >
-- 
Ladislav Lhotka
Head, CZ.NIC Labs
PGP Key ID: 0xB8F92B08A9F76C67