Re: [netmod] xpath expressions in JSON

[Ladislav Lhotka <[e-mail]>]

On Mon, 2018-10-22 at 14:56 +0200, Martin Bjorklund wrote:
> Ladislav Lhotka <[e-mail]> wrote:
> > Martin Bjorklund <[e-mail]> 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.

Yes, every module that uses XPath in data will have to deal with this. There may
potentially be multiple independent prefix declarations (this is actually a
con). 

> 
> 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...)

Hmm, I think this is a bad idea because it departs even further from the
original XPath semantics. Such chameleon names should IMO be pretty rare, and if
they are needed, local-name() is always available.

Lada

> 
> 
> /martin
> 
> 
> 
> 
> 
> > 
> > Lada
> > 
> > >
> > > (This said, I would like to have a context-independent encoding of all
> > > YANG types in the future.  But not now.)
> > >
> > >
> > >
> > >
> > > /martin
> > >
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> > > [e-mail]
> > > 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