Re: [netmod] xpath expressions in JSON

Martin Bjorklund <mbj@tail-f.com> writes:

> Ladislav Lhotka <lhotka@nic.cz> 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) <rrahman@cisco.com>
>> > wrote:
>> > > On 2018-10-10, 9:59 AM, "netmod on behalf of Martin Bjorklund" <
>> > > netmod-bounces@ietf.org on behalf of mbj@tail-f.com> wrote:
>> > > 
>> > >     Ladislav Lhotka <lhotka@nic.cz> wrote:
>> > >     > Martin Bjorklund <mbj@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.
>> 
>> 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.

Except some parts that don't apply. For example, siblings nodes in XPath
are ordered, whereas in YANG 'foo[1]' may give unpredictable results.

This is probably OK in YANG modules but not so much if XPath expressions
are used as configuration data.

Lada

>
>
> /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
>> > >     > > 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
>> > 
>> > 
>> -- 
>> Ladislav Lhotka
>> Head, CZ.NIC Labs
>> PGP Key ID: 0xB8F92B08A9F76C67
>> 

-- 
Ladislav Lhotka
Head, CZ.NIC Labs
PGP Key ID: 0xB8F92B08A9F76C67