Re: [yang-doctors] [netmod] Clarification needed for YANG 1.1 XPATH context

Ladislav Lhotka <lhotka@nic.cz> Fri, 13 April 2018 15:00 UTC

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From: Ladislav Lhotka <lhotka@nic.cz>
To: Radek Krejčí <rkrejci@cesnet.cz>, Martin Bjorklund <mbj@tail-f.com>
Cc: FRANK RIMPLER <FRANK.RIMPLER@adtran.com>, YANG Doctors <yang-doctors@ietf.org>, netmod@ietf.org
Date: Fri, 13 Apr 2018 17:00:47 +0200
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Subject: Re: [yang-doctors] [netmod] Clarification needed for YANG 1.1 XPATH context
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On Fri, 2018-04-13 at 13:51 +0200, Radek Krejčí wrote:
> Hi,
> I'm refreshing an old thread to clarify specific use case, see below...
> 
> Dne 25.2.2016 v 16:31 Martin Bjorklund napsal(a):
> > William Ivory <wivory@Brocade.com> wrote:
> > > 
> > > -----Original Message-----
> > > From: Martin Bjorklund [mailto:mbj@tail-f.com] 
> > > Sent: 25 February 2016 15:17
> > > To: William Ivory <wivory@Brocade.com>
> > > Cc: netmod@ietf.org
> > > Subject: Re: [netmod] Clarification needed for YANG 1.1 XPATH context
> > > 
> > > William Ivory <wivory@Brocade.com> wrote:
> > > > Hi,
> > > > 
> > > > I'm looking for clarification on the meaning of the following 
> > > > paragraph in section 6.4.1 (XPATH context) in RFC6020bis:
> > > > 
> > > >    'If a node that exists in the accessible tree has a non-presence
> > > >    container as a child, then the non-presence container also exists in
> > > >    the tree.'
> > > > 
> > > > It's unclear to me what this is trying to say, and why - for example, 
> > > > does this mean that I need to validate any 'must' and 'when'
> > > > statements on the child container even when nothing within that child 
> > > > container is configured?
> > > 
> > > must expressions are always evaluated if the node where the must
> > > expression is defined exists, regardless of the number of children
> > > this node has.
> > > 
> > > [wivory] So in my example where the child container (non-presence) has
> > > NO children, then it doesn't exist, and any must statement on it
> > > should not be run.  Only when a non-presence container has a non-zero
> > > number of children should any 'must' statements on that container be
> > > run.
> > > 
> > > [wivory] If that's the case, then would it be correct to say that the
> > > intention of this paragraph is as a reminder that one must evaluate
> > > 'must' statements on nodes that have no inherent meaning and exist
> > > only because they contain child nodes?
> > 
> > No; section 7.5.3 says:
> > 
> >    When a datastore is validated, all "must" constraints are
> >    conceptually evaluated once for each node in the accessible tree (see
> >    Section 6.4.1).
> > 
> > And the quoted paragraph of 6.4.1 says that the NP-container
> > (conceptually) exists if its parent exists - regardless of number of
> > children.
> > 
> > So if the parent exists, any must expressions in the NP-container are
> > evaluated.
> > 
> 
> what about top-level NP-container with must constraint? Is a root node
> something which is always present in accessible tree (even in an empty tree)?
> Intuitively, I believe that it is, so even constraints on top-level NP-
> containers are supposed to be evaluated, but I cannot find something about it
> in RFC.

Yes, this follows already from XPath 1.0 spec, section 5.

Lada

> 
> Regards,
> Radek
> 
> 
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Ladislav Lhotka
Head, CZ.NIC Labs
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