Re: [netmod] xpath expressions in JSON

Martin Bjorklund <mbj@tail-f.com> Tue, 23 October 2018 08:42 UTC

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Date: Tue, 23 Oct 2018 10:42:48 +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:
> On Tue, 2018-10-23 at 10:23 +0200, Martin Bjorklund wrote:
> > 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.
> 
> But this is an UI issue. I am not against any aid provided by client tools, what
> I am talking about is the "oficial" lexical representation of that data type,
> that is also exchanged in protocols.


> The above expression breaks XPath rules.

This I agree with!


/martin