Re: [netmod] xpath expressions in JSON

[Martin Bjorklund <[e-mail]>]

Ladislav Lhotka <[e-mail]> wrote:
> On Wed, 2018-10-10 at 19:23 -0700, Andy Bierman 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.
> 
> The thing is that XPath is "XML Path Language", so using it outside XML is
> problematic.

Not necessarily.  Section 5 of the XPath 1.0 spec defins the "data
model" where an XPath expression is applied.  We could make a formal
specification of how a YANG data tree maps to this data model (pretty
straightforward...).  I think experience from implementations over the
last 8 years show that this in fact works quite well.


/martin



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