Re: [netmod] xpath expressions in JSON

[Ladislav Lhotka <[e-mail]>]

Martin Bjorklund <[e-mail]> writes:

> 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

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.

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