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

[Ladislav Lhotka <[e-mail]>]

On Thu, 2018-11-22 at 15:01 +0100, Martin Bjorklund wrote:
> Ladislav Lhotka <[e-mail]> wrote:
> > On Thu, 2018-11-22 at 14:39 +0100, Martin Bjorklund 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.
> > 
> > Section 11 does not explicitly forbid changes that change schema node
> > identifiers. On the other hand, it contains this item:
> > 
> >    o  Any set of data definition nodes may be replaced with another set
> >       of syntactically and semantically equivalent nodes.  For example,
> >       a set of leafs may be replaced by a "uses" statement of a grouping
> >       with the same leafs.
> 
> But moving nodes into a grouping that is used doesn't change the
> schema tree.

The grouping is mentioned there as an example.

> 
> 
> > I think this applies nicely to the current case: the modified schema
> > is arguably
> > semantically equivalent to the old one.
> 
> But not syntactically (in the schema tree).

Well, "syntactically equivalent" is not defined in RFC 7950, and intuitively it
is not clear at all that it means "the same schema tree". In fact, the example
with a grouping leads to a different abstract syntax tree for the YANG module,
so one could also argue that it is not syntactically equivalent.

"Semantically equivalent" makes more sense: schemas A and B are semantically
equivalent if all data trees valid against A are also valid against B, and vice
versa.

Lada 

> 
> 
> /martin
> 
> 
> > 
> > Lada
> > 
> > 
> > > 
> > > 
> > > /martin
> > > 
> > > 
> > > 
> > > > 
> > > > 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
> > 
-- 
Ladislav Lhotka
Head, CZ.NIC Labs
PGP Key ID: 0xB8F92B08A9F76C67