Robert Wilton <rwil...@cisco.com> wrote:
>
>
> On 11/10/2018 11:21, Martin Bjorklund wrote:
> > Andy Bierman <a...@yumaworks.com> wrote:
> >> On Wed, Oct 10, 2018 at 11:39 PM, Martin Bjorklund <m...@tail-f.com>
> >> wrote:
> >>
> >>> Andy Bierman <a...@yumaworks.com> wrote:
> >>>> On Wed, Oct 10, 2018 at 6:59 PM, Reshad Rahman (rrahman) <
> >>> rrah...@cisco.com>
> >>>> wrote:
> >>>>
> >>>>> On 2018-10-10, 9:59 AM, "netmod on behalf of Martin Bjorklund" <
> >>>>> netmod-boun...@ietf.org on behalf of m...@tail-f.com> wrote:
> >>>>>
> >>>>> Ladislav Lhotka <lho...@nic.cz> wrote:
> >>>>> > Martin Bjorklund <m...@tail-f.com> writes:
> >>>>> >
> >>>>> > > Hi,
> >>>>> > >
> >>>>> > > While reviewing restconf-notif, I saw this example:
> >>>>> > >
> >>>>> > > {
> >>>>> > > "ietf-subscribed-notifications:input": {
> >>>>> > > "stream": "NETCONF",
> >>>>> > > "stream-xpath-filter": "/ds:foo/",
> >>>>> > > "dscp": "10"
> >>>>> > > }
> >>>>> > > }
> >>>>> > >
> >>>>> > > Note the "stream-xpath-filter". It has a prefix in the XPath
> >>>>> string.
> >>>>> > > How are prefixes declared when JSON is used?
> >>>>> > >
> >>>>> > > The leaf "stream-xpath-filter" says:
> >>>>> > >
> >>>>> > > o The set of namespace declarations are those in
> >>>>> scope on
> >>>>> > > the 'stream-xpath-filter' leaf element.
> >>>>> > >
> >>>>> > > (I think I provided that text...)
> >>>>> > >
> >>>>> > > This assumes that the encoding is XML, or at leas that the
> >>> encoding
> >>>>> > > can somehow transfer namespace declarations.
> >>>>> >
> >>>>> > It can't. There are two options:
> >>>>> >
> >>>>> > 1. have different representations of this value in XML and JSON,
> >>>>> > analogically to instance indentifiers (sec. 6.11 in RFC 7951).
> >>>>> >
> >>>>> > 2. use a module name rather than a prefix in XML, too.
> >>>>> >
> >>>>> > I would suggest #2.
> >>>>> <RR> But that means making non-backwards compatible change to the XML
> >>>>> representation?
> >>>>>
> >>>> Not really. It means NETMOD WG would be creating its own special
> >>>> variant
> >>> of
> >>>> XPath.
> >>> Not at all. What I propose is perfectly fine, legal XPath 1.0.
> >>>
> >>> XPath 1.0 says that an XPath expression is evaluated in a context.
> >>> One item in the context is a set of mappings from <prefix> to <uri>,
> >>> where <prefix> is used to lookup prefixes used in the XPath
> >>> expression, e.g. in "/foo:interfaces" "foo" is the prefix.
> >>>
> >>> It is perfectly fine to say that the prefix mapping set is this:
> >>>
> >>> "ietf-interfaces" -> "urn:ietf:params:xml:ns:yang:ietf-interfaces"
> >>> "ietf-ip" -> "urn:ietf:params:xml:ns:yang:ietf-ip"
> >>>
> >>> and use that to evaluate the expression
> >>>
> >>> /ietf-interfaces:interfaces/ietf-interfaces:interface/ietf-ip/ipv4
> >>>
> >>>
> >>>
> >> The XPath expression is normally parsed within an XML instance
> >> document.
> >> There are "xmlns" attributes present that map the prefix to a
> >> namespace URI.
> >> These mappings will not be present in the JSON at all.
> >>
> >> A custom XPath implementation is required to magically identify the
> >> prefix
> >> as a module name and magically find the namespace URI for the module
> >> name.
> > I disagree. You need an XPath implementation + custom code to set up
> > the environment.
> This is OK, but can we just use the JSON encoding instance identifier
> format exactly? I.e .RFC 7951 section 6.11.
>
> So "/ietf-interfaces:interfaces/interface/ietf-ip:ipv4/enabled"
>
> can trivially be expanded to:
>
> "/ietf-interfaces:interfaces/ietf-interfaces:interface/ietf-ip:ipv4/ietf-ip:enabled",
>
> and then interpreted with the context:
> "ietf-interfaces" -> "urn:ietf:params:xml:ns:yang:ietf-interfaces"
> "ietf-ip" -> "urn:ietf:params:xml:ns:yang:ietf-ip"
*this* would require a custom XPath implementation.
and it is not obvious what the rules for the "auto-assignment" of
prefixes would be. For example:
/ietf-interfaces//ietf-ip:address[../foo]
what is the prefix for "foo"?
/martin
>
> Thanks,
> Rob
>
>
> >
> > There is no standard XPath implementation that can just take an XML
> > instance document + YANG module and figure out what to do. Custom
> > code is needed to connect things together. This proposal doesn't
> > change this.
> >
> >
> > /martin
> >
> >
> >> A normal XPath implementation will not find any namespace mapping for
> >> the
> >> prefixes.
> >>
> >> An XPath expression has no concept of the "current module" inherited
> >> from
> >> the parent
> >> like the JSON encoding. This is problematic for predicates
> >>
> >> /ietf-interfaces:interfaces/interface[name='eth0']
> >>
> >> XPath says the missing prefixes for 'interface' and 'name' are simply
> >> missing (no namespace).
> >> The JSON encoding says "ietf-interfaces" is used for 'interfaces'. and
> >> 'interface'.
> >> There is no specification for the 'name' node inside a predicate.
> >>
> >> So you must mean the full module name will be used at every node:
> >>
> >>
> >> /ietf-interfaces:interfaces/ietf-interfaces:interface[ietf-interfaces:name='eth0']
> >>
> >>
> >>
> >>> /martin
> >>>
> >>>
> >> Andy
> >>
> >>
> >>>>
> >>>>> Hmm, so you mean change the leaf "stream-xpath-filter" to say:
> >>>>>
> >>>>> o The set of namespace declarations has one member for
> >>> each
> >>>>> YANG module supported by the server. This member maps
> >>>>> from the YANG module name to the YANG module namespace.
> >>>>>
> >>>>> This means that in the XPath expression, the module
> >>> name
> >>>>> serves as the prefix.
> >>>>>
> >>>>> .... and then also give an example of this.
> >>>>>
> >>>>> This is probably what we need to do in all places where
> >>> yang:xpath1.0
> >>>>> is used, going forward. Maybe even define a new type
> >>>>> yang:xpath1.0-2 (name?) with the set of namespace declarations
> >>>>> built-in.
> >>>>>
> >>>>
> >>>> We should avoid making off-the-shelf implementations of standards like
> >>>> XPath unusable.
> >>>> At the very least this should be only available if the server supports
> >>>> it
> >>>> (with a capability URI)
> >>>>
> >>>>
> >>>>
> >>>>> <RR> So we need an update to RFC7951?
> >>>>>
> >>>>> Regards,
> >>>>> Reshad.
> >>>>>
> >>>>>
> >>>> Andy
> >>>>
> >>>>
> >>>>> /martin
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>> >
> >>>>> > Lada
> >>>>> >
> >>>>> > >
> >>>>> > > How is this supposed to work with JSON?
> >>>>> > >
> >>>>> > >
> >>>>> > > /martin
> >>>>> > >
> >>>>> > > _______________________________________________
> >>>>> > > netmod mailing list
> >>>>> > > netmod@ietf.org
> >>>>> > > https://www.ietf.org/mailman/listinfo/netmod
> >>>>> >
> >>>>> > --
> >>>>> > Ladislav Lhotka
> >>>>> > Head, CZ.NIC Labs
> >>>>> > PGP Key ID: 0xB8F92B08A9F76C67
> >>>>> >
> >>>>>
> >>>>> _______________________________________________
> >>>>> netmod mailing list
> >>>>> netmod@ietf.org
> >>>>> https://www.ietf.org/mailman/listinfo/netmod
> >>>>>
> >>>>>
> >>>>> _______________________________________________
> >>>>> netmod mailing list
> >>>>> netmod@ietf.org
> >>>>> https://www.ietf.org/mailman/listinfo/netmod
> >>>>>
> > _______________________________________________
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> > netmod@ietf.org
> > https://www.ietf.org/mailman/listinfo/netmod
> > .
> >
>
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