Re: [netmod] xpath expressions in JSON

Martin Bjorklund <mbj@tail-f.com> Tue, 23 October 2018 08:30 UTC

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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:
> 
> > Qin Wu <bill.wu@huawei.com> wrote:
> >> -----邮件原件-----
> >> 发件人: netmod [mailto:netmod-bounces@ietf.org] 代表 Ladislav Lhotka
> >> 发送时间: 2018年10月22日 21:12
> >> 收件人: Martin Bjorklund
> >> 抄送: netmod@ietf.org
> >> 主题: Re: [netmod] xpath expressions in JSON
> >> 
> >> On Mon, 2018-10-22 at 14:56 +0200, Martin Bjorklund wrote:
> >> > 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.
> >> 
> >> 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
> 
> This is impossible if we want to call it XPath. With an explicit
> namespace/prefix declaration, for example
> 
>   "namespace": [
>     {
>       "prefix": "if",
>       "uri": "urn:ietf:params:xml:ns:yang:ietf-interfaces",
>       "default": true
>     },
>     {
>       "prefix": "ip",
>       "uri": "urn:ietf:params:xml:ns:yang:ietf-ip"
>     }
>   ]
> 
> it would be
> 
>   /interfaces/interface[name='eth0']/ip:ipv4
> 
> which is not too bad either.

Well, "ncc myhost --get -x /interfaces/interface[name='eth0']/ipv4" is
a one-liner; if I also have to specify the namespace list for every
filter, it suddenly is not that convenient.


/martin


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