-----邮件原件-----
发件人: netmod [mailto:netmod-boun...@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 <lho...@nic.cz> wrote:
> > Martin Bjorklund <m...@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']
"
Lada
>
>
> /martin
>
>
>
>
>
> >
> > Lada
> >
> > >
> > > (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
> > > netmod@ietf.org
> > > https://www.ietf.org/mailman/listinfo/netmod
> >
> > --
> > Ladislav Lhotka
> > Head, CZ.NIC Labs
> > PGP Key ID: 0xB8F92B08A9F76C67
> >
--
Ladislav Lhotka
Head, CZ.NIC Labs
PGP Key ID: 0xB8F92B08A9F76C67
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