Re: [netconf] [yang-doctors] Yangdoctors last call review of draft-ietf-netconf-subscribed-notifications-21

Martin Bjorklund <mbj@tail-f.com> Mon, 21 January 2019 09:49 UTC

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Date: Mon, 21 Jan 2019 10:49:51 +0100 (CET)
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Subject: Re: [netconf] [yang-doctors] Yangdoctors last call review of draft-ietf-netconf-subscribed-notifications-21
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Andy Bierman <andy@yumaworks.com>; wrote:
> On Wed, Jan 16, 2019 at 10:35 AM Eric Voit (evoit) <evoit@cisco.com>; wrote:
> 
> > Hi Andy,
> >
> > Thanks for the review.  Some thoughts....
> >
> > > From: Andy Bierman, January 14, 2019 8:08 PM

[...]

> > > I7) leaf stream-xpath-filter: [multiple uses]
> > >
> > >            The expression is evaluated in the following XPath context:
> > >
> > >              o   The set of namespace declarations is the set of prefix
> > >                  and namespace pairs for all YANG modules implemented
> > >                  by the server, where the prefix is the YANG module
> > >                  name and the namespace is as defined by the
> > >                  'namespace' statement in the YANG module.
> > >
> > >   -- This prefix processing is not done anywhere else in NETCONF
> > >      or RESTCONF.  IMO a bad precedent.  Only the XML prefixes
> > >      should be required for processing of XML encoding.  YANG
> > >      module prefixes are not required to be unique, unlike
> > >      the prefix mappings in XML
> >
> > This text was proposed by Martin as a result of the "xpath expressions in
> > JSON" thread last October in NETMOD.
> >
> > I am happy to incorporate whatever text is appropriate.  I was hoping that
> > the suggested text was sufficient for now.  Kent has already incorporated
> > this as an issue for yang-next
> > https://github.com/netmod-wg/yang-next/issues/55
> > So hopefully there is no final precedent being claimed.
> >
> >
> I do not agree that this YANG module should define a new way to encode XPath
> into XML instance documents.

This mechanism was first used in RFC 8040, section 4.8.4, in order to
have a transport-agnostic encoding of XPath expressions (same for XML
and JSON).

There was long debate on the ML for this issue, and this was the
outcome that the WG decided.

> This will require significant changes to server
> implementations.  YANG module prefixes are not even required to be unique
> so the set of prefixes used by the server in XML instance documents may be
> different,
> since it must be unique.

YANG module prefixes are not used here.  The module *name* is used as
the identifier prefix in XPath expressions:

  /ietf-interfaces:interfaces/ietf-interfaces:interface/...



/martin