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

[Martin Bjorklund <[e-mail]>]

Ladislav Lhotka <[e-mail]> wrote:
> 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.

Yes, this is correct.  The quoted text needs to be rewritten to make
this more clear.  Altough the path refers to a (potential) node in the
data tree, that node obviously has a node in the schema tree, and its
value space restricts the value space of the leafref node.

> 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?

Yes.

I've been told that people don't read documentation or specifications
and just look at examples.



/martin