Re: [netmod] xpath expressions in JSON

[Ladislav Lhotka <[e-mail]>]

On Thu, 2018-10-18 at 12:30 +0200, Martin Bjorklund wrote:
> 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

Actually, schema mount uses yet another approach - prefix/namespace mapping is a
part of the data itself:

        +--ro namespace* [prefix]
        |  +--ro prefix    yang:yang-identifier
        |  +--ro uri?      inet:uri

It could work here, too.

Lada

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