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

Martin Bjorklund <mbj@tail-f.com> Thu, 22 November 2018 15:14 UTC

Return-Path: <mbj@tail-f.com>
X-Original-To: netmod@ietfa.amsl.com
Delivered-To: netmod@ietfa.amsl.com
Received: from localhost (localhost [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id EB52112DD85 for <netmod@ietfa.amsl.com>; Thu, 22 Nov 2018 07:14:41 -0800 (PST)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -1.9
X-Spam-Level:
X-Spam-Status: No, score=-1.9 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, SPF_PASS=-0.001, URIBL_BLOCKED=0.001] autolearn=ham autolearn_force=no
Received: from mail.ietf.org ([4.31.198.44]) by localhost (ietfa.amsl.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id H1-ACvd06mD1 for <netmod@ietfa.amsl.com>; Thu, 22 Nov 2018 07:14:40 -0800 (PST)
Received: from mail.tail-f.com (mail.tail-f.com [46.21.102.45]) by ietfa.amsl.com (Postfix) with ESMTP id 52B4C126CB6 for <netmod@ietf.org>; Thu, 22 Nov 2018 07:14:39 -0800 (PST)
Received: from localhost (h-39-108.A165.priv.bahnhof.se [213.136.39.108]) by mail.tail-f.com (Postfix) with ESMTPSA id 86E7F1AE018C; Thu, 22 Nov 2018 16:14:38 +0100 (CET)
Date: Thu, 22 Nov 2018 16:14:38 +0100 (CET)
Message-Id: <20181122.161438.975515366125603770.mbj@tail-f.com>
To: lhotka@nic.cz
Cc: andy@yumaworks.com, jason.sterne@nokia.com, netmod@ietf.org
From: Martin Bjorklund <mbj@tail-f.com>
In-Reply-To: <adedb81ce97abf16bafa47118349287954d4d410.camel@nic.cz>
References: <CABCOCHS18StYKGC4f7cPWFraKNHRsC9cWfrmfZ0j773awdicvQ@mail.gmail.com> <20181122.150027.823800945772964674.mbj@tail-f.com> <adedb81ce97abf16bafa47118349287954d4d410.camel@nic.cz>
X-Mailer: Mew version 6.7 on Emacs 25.2 / Mule 6.0 (HANACHIRUSATO)
Mime-Version: 1.0
Content-Type: Text/Plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Archived-At: <https://mailarchive.ietf.org/arch/msg/netmod/GWTNP9BOzDT1Oye4lqotjdWQpmc>
Subject: Re: [netmod] Adding a pre-existing leaf into a new 'choice' - NBC change?
X-BeenThere: netmod@ietf.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: NETMOD WG list <netmod.ietf.org>
List-Unsubscribe: <https://www.ietf.org/mailman/options/netmod>, <mailto:netmod-request@ietf.org?subject=unsubscribe>
List-Archive: <https://mailarchive.ietf.org/arch/browse/netmod/>
List-Post: <mailto:netmod@ietf.org>
List-Help: <mailto:netmod-request@ietf.org?subject=help>
List-Subscribe: <https://www.ietf.org/mailman/listinfo/netmod>, <mailto:netmod-request@ietf.org?subject=subscribe>
X-List-Received-Date: Thu, 22 Nov 2018 15:14:42 -0000

Ladislav Lhotka <lhotka@nic.cz> wrote:
> On Thu, 2018-11-22 at 15:00 +0100, Martin Bjorklund wrote:
> > Andy Bierman <andy@yumaworks.com> wrote:
> > > On Thu, Nov 22, 2018 at 5:39 AM Martin Bjorklund <mbj@tail-f.com> wrote:
> > > 
> > > > Hi,
> > > >
> > > > Ladislav Lhotka <lhotka@nic.cz> wrote:
> > > > > Andy Bierman <andy@yumaworks.com> writes:
> > > > >
> > > > > > On Mon, Nov 19, 2018 at 12:32 PM Sterne, Jason (Nokia - CA/Ottawa) <
> > > > > > jason.sterne@nokia.com> 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