Re: [netmod] xpath expressions in JSON

Martin Bjorklund <mbj@tail-f.com> Mon, 22 October 2018 12:56 UTC

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Date: Mon, 22 Oct 2018 14:56:05 +0200 (CEST)
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From: Martin Bjorklund <mbj@tail-f.com>
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Subject: Re: [netmod] xpath expressions in JSON
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Ladislav Lhotka <lhotka@nic.cz> wrote:
> Martin Bjorklund <mbj@tail-f.com> writes:
> 
> > Hi,
> >
> > Going back to the most urgent issue, what is this WG's recommendation
> > for the subscribed-notifications draft in NETCONF wrt/ their usage of
> > yang:xpath1.0 in filters?
> >
> > To summarize:
> >
> > We already have
> >
> >   o  instance-identifier in XML uses prefixes from the XML document
> >   o  instance-identifier in JSON uses module names as prefixes
> >   o  XPath in NETCONF filter uses prefixes from the XML document
> >   o  XPath in JSON query filter uses module names as prefixes
> >
> >
> > Alternative A:
> > --------------
> >
> > Use different encodings for "stream-xpath-filter" as well, depending
> > on if it is XML or JSON.
> >
> > We would do in SN:
> >
> >     o  If the node is encoded in XML, the set of namespace
> >        declarations are those in scope on the
> >        'stream-xpath-filter' leaf element.
> >
> >     o  If the node is encoded in JSON, the set of namespace
> >        declarations is the set of prefix and namespace pairs
> >        for all supported YANG modules, where the prefix is
> 
> Is "supported" the same as "implemented", or something else?

It should be "implemented".

> >        the YANG module name and the namespace is as defined
> >        by the "namespace" statement in the YANG module.
> >
> > Pro: the format is consistent within each encoding.
> >
> > Con: unclear how to handle other encodings.
> > Con: we keep using context-depending encodings.
> 
>   Con: XPath expressions in JSON can get pretty long (I assume it's not
>   just an instance identifier but may contain predicates etc.). We
>   cannot use the trick with the default namespace as in YANG, so all
>   data node names will have to carry the prefix.

Yes.

> > We could probably add that CBOR uses the same representation as JSON.
> >
> > Example in XML:
> >
> >   <stream-xpath-filter
> >       xmlns:if="urn:ietf:params:xml:ns:yang:ietf-interfaces"
> >       xmlns:ip="urn:ietf:params:xml:ns:yang:ietf-ip">
> >     /if:interfaces/if:interface/ip:ipv4
> >   </stream-xpath-filter>
> >
> > Example in JSON:
> >
> >   "stream-xpath-filter":
> >     "/ietf-interfaces:interfaces/ietf-interfaces:interface/ietf-ip:ipv4"
> >
> >
> >
> > Alternative B:
> > --------------
> >
> > Use a non-context depending encoding, with the module name as prefix.
> >
> > We would do in SN:
> >
> >     o  The set of namespace
> >        declarations is the set of prefix and namespace pairs
> >        for all supported YANG modules, where the prefix is
> >        the YANG module name and the namespace is as defined
> >        by the "namespace" statement in the YANG module.
> >
> > Pro: the format is independent from the protocol encoding
> >
> > Con: in XML, this leaf is treated differently from other XPath
> >      expressions, such as get-config filter and nacm rules.
> >
> > Example in XML:
> >
> >   <stream-xpath-filter>
> >     /ietf-interfaces:interfaces/ietf-interfaces:interface/ietf-ip:ipv4
> >   </stream-xpath-filter>
> >
> > Example in JSON:
> >
> >   "stream-xpath-filter":
> >     "/ietf-interfaces:interfaces/ietf-interfaces:interface/ietf-ip:ipv4"
> >
> >
> > My proposal is A.  I think it is more important with consistency
> > within each encoding than across encodings.
> 
> I would suggest to consider declaring prefixes & namespaces explicitly
> in the data, as in the schema mount document. It is independent of
> encoding and the expressions can be kept short. In fact, one of the
> namespaces can be declared as default, so this use of XPath would then
> be very similar to YANG.

Ok, so this is another alternative that works today, and achieves the
goal of being encoding-independent.  It is still context-dependent
though.

BTW, when used in filters, it is nice to let an unprefixed name to
match any namespace; i.e., treat "foo" as syntactic sugar for
"local-name(.) = 'foo'".  ("*:foo" is not legal...)


/martin





> 
> Lada
> 
> >
> > (This said, I would like to have a context-independent encoding of all
> > YANG types in the future.  But not now.)
> >
> >
> >
> >
> > /martin
> >
> > _______________________________________________
> > netmod mailing list
> > netmod@ietf.org
> > https://www.ietf.org/mailman/listinfo/netmod
> 
> -- 
> Ladislav Lhotka
> Head, CZ.NIC Labs
> PGP Key ID: 0xB8F92B08A9F76C67
>