Re: [netmod] xpath expressions in JSON

[Martin Bjorklund <[e-mail]>]

Andy Bierman <[e-mail]> wrote:
> On Wed, Oct 10, 2018 at 6:59 PM, Reshad Rahman (rrahman) <[e-mail]>
> wrote:
> 
> > On 2018-10-10, 9:59 AM, "netmod on behalf of Martin Bjorklund" <
> > [e-mail] on behalf of [e-mail]> wrote:
> >
> >     Ladislav Lhotka <[e-mail]> wrote:
> >     > Martin Bjorklund <[e-mail]> 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




/martin


> 
> 
> >     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
> >     > > [e-mail]
> >     > > https://www.ietf.org/mailman/listinfo/netmod
> >     >
> >     > --
> >     > Ladislav Lhotka
> >     > Head, CZ.NIC Labs
> >     > PGP Key ID: 0xB8F92B08A9F76C67
> >     >
> >
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