Re: [netmod] xpath expressions in JSON

[Martin Bjorklund <[e-mail]>]

Qin Wu <[e-mail]> wrote:
> -----邮件原件-----
> 发件人: netmod [mailto:[e-mail]] 代表 Ladislav Lhotka
> 发送时间: 2018年10月22日 21:12
> 收件人: Martin Bjorklund
> 抄送: [e-mail]
> 主题: Re: [netmod] xpath expressions in JSON
> 
> 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.
> 
> [Qin]: Agree with Lada, Referencing RFC8407, section 4.6.2, I think the below guideline is relevant.
> "
> The "local-name" function SHOULD NOT be used to reference local names
>    outside of the YANG module that defines the must or when expression
>    containing the "local-name" function.  Example of a "local-name"
>    function that should not be used:
> 
>       /*[local-name()='foo']

This guideline is for must/when expressions *within* YANG modules.

I'm talking about a different use case, namely filtering.  It is
pretty convenient for users to send a filter:

  /interfaces/interface[name='eth0'/ipv4

and get back what they expect.  Even in the rare case of local name
clashes, this filter works and gives back what was expected (+
additional nodes).

I have no plans on writing up this as a proposal; I'm just pointing
out that when XPath is used in filters, this is convenient.


/martin