Re: [netmod] xpath expressions in JSON

[Andy Bierman <[e-mail]>]

On Thu, Oct 18, 2018 at 12:58 PM, Martin Bjorklund <[e-mail]> wrote:

> Andy Bierman <[e-mail]> wrote:
> > Hi,
> >
> > I strongly agree that a new data type is needed (ypath1.0 or just ypath
> is
> > fine)
> > Adding new semantics or requirements to published data types is
> > unacceptable.
> >
> > Also, we must get the type and module containing the data type right on
> the
> > first try.
> > No moving it later because the import looks bad. That said, a "quick
> > 6991-bis" is unrealistic,
> > and a multi-year 6991-bis is unhelpful.
> >
> > Should there be a canonical format, based on module-names as prefixes?
> > Consider how to compare 2 values using this data type.
>
> Ok.  So which alternative do you prefer for stream-xpath-filter, which
> is supposed to work also for JSON?  The current definition doesn't
> work for JSON.
>
>

I don't like calling it xpath when it is not XPath any more.
It should be as clear as possible to readers and designers that xpath
means XPath and ypath is not XPath.

I would prefer a new encoding that allows the parent node module-name to be
used somehow,
consistent with the JSON encoding used now.

Perhaps each absolute-path expression starts with a module-name step and
relative-path
expressions assume the module name of the context node if there is no
module-name.

Neither alternative below is a good long-term solution.

The old problems come up again:

  Client A writes the /foo node in XML.
  Client B reads the /foo node returned in JSON

Which format is returned? Does the server magically convert the value for
each encoding type?
If we bother to fix this problem at all, then we should get rid of reliance
on prefix to namespace mappings.





> /martin
>
>

Andy


>
> >
> >
> > Andy
> >
> >
> > On Thu, Oct 18, 2018 at 10:42 AM, Vladimir Vassilev <
> > [e-mail]> wrote:
> >
> > > Hi,
> > >
> > > Seems this discussion affects 10 draft modules using the xpath1.0 type.
> > > The proposed boilerplate description text that was not added to some
> RFC
> > > modules like [e-mail]
> > >
> > > should be as consistent as possible (or skipped based on the
> > > ietf-netconf-monitoring precedent) until a better alternative is
> available.
> > > Here is an example of a better alternative.
> > >
> > >    typedef ypath1.0 {
> > >     type xpath1.0;
> > >     description
> > >      "This type represents subset of XPATH 1.0 expressions
> > >       that apply to a data tree conforming to a YANG model.
> > >
> > >       Each encoding should allow conversion to an encoding
> > >       independent lexical representation where data node
> > >       prefixes are resolved to and substituted with module
> > >       names.
> > >
> > >       When a schema node is defined that uses this type, the
> > >       description of the schema node MUST specify the
> > >       context in which the expression is evaluated if it
> > >       is different from the default context.
> > >
> > >       The default context is as follows:
> > >
> > >         o  The set of variable bindings is empty.
> > >
> > >         o  The function library is the core function library, and
> > >            the XPath functions defined in section 10 in RFC 7950.
> > >
> > >         o  The context node is the leaf node.
> > >
> > >       ";
> > >     reference
> > >      "XPATH: XML Path Language (XPath) Version 1.0";
> > >   }
> > >
> > > That said I do not object to short-term application of alternative A as
> > > long as a long-term solution is on its way for future modules.
> > >
> > > Vladimir
> > >
> > > On 10/18/18 12:30 PM, 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
> > >>
> > >>
> > >> 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
> > >>
> > >> _______________________________________________
> > >> netmod mailing list
> > >> [e-mail]
> > >> https://www.ietf.org/mailman/listinfo/netmod
> > >>
> > >
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