Re: [netconf] XPath support in RESTCONF

[Andy Bierman <[e-mail]>]

On Wed, Dec 6, 2023 at 4:12 AM Per Andersson (perander) <[e-mail]>
wrote:

> Andy Bierman <[e-mail]>:
> >On Tue, Dec 5, 2023 at 9:50 AM Per Andersson (perander) <
> [e-mail]<mailto:[e-mail]>> wrote:
> >>Andy Bierman <[e-mail]<mailto:[e-mail]>>:
> >>>
> >>> The 'get' or 'get-config' operation can be used.
> >>> The filter element is part of the payload, not the request URI.
> >>> This supports a 'select' attribute which is set to the XPath
> expression.
> >>
> >> This is a very NETCONF way to use RESTCONF though. It would
> >> probably be more appreciated to actually use HTTP techniques and
> >> mechanics.
> >
> > No -- this is the way RFC 8040 defines to invoke a YANG rpc-stmt.
> >
> > IMO every RESTCONF server should provide access to all the RPC operations
> > that are advertised as implemented in the YANG library.
>
> I agree with this of course.
>
>
> > The syntax is very simple and it does not require a new RESTCONF
> protocol version.
> >
> >
> > POST /restconf/operations/ietf-netconf:get-config HTTP/1.1
> > Server: example.com<http://example.com>
> > Content-Type: application/yang-data+xml
> >
> > <input xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
> >   <filter type="xpath"
> >          xmlns:if="urn:ietf:params:xml:ns:yang:ietf-interfaces"
> >          select="/if:interfaces/if:interfaces[if:name='eno1']"/>
> > </input>
>
> Indeed.
>
>

Real use cases exist that are not trivial to replicate using a data
resource URI
E.g.

get all interfaces with names starting with 'eno'  [filter on regexp name
match]

     select="/if:interfaces/if:interface[starts-with(if:name,'eno')]"

get all interfaces that are not enabled [filter on non-key leaf]

     select="/if:interfaces/if:interface[if:enabled='false']"



Perhaps good enough.
>
> However this would not support e.g. pagination as the current
> list-pagination
> draft defines it. Which a XPath query draft could include/update. Another
> option
> could be to bolt on pagination support for arbitrary RPC invocation, which
> probably is a lot more work.
>
>
>
One would use the pagination operations and not get/get-config/get-data in
that case.
If the pagination APIs cannot even handle what XPath can do, then they may
not get used.



> >>> I think it is a hard sell to state that one should use HTTP more or
> less
> >>> as expected to fetch data, but if you want do advanced filtering it is
> >>> necessary to invoke an RPC.
> >
> > The query language is even more advanced and complex to implement.
> > IMO much more so than the standard POST already defined in RESTCONF.
>
> Even so, "exclude" HTTP query parameter is asked for instead of invoking
> a NETCONF get-config RPC.
>
>
IMO XPath is already heavyweight enough for NETCONF and RESTCONF.
Is there a specific document that specifies how QUERY would be used with
YANG datastores?

IMO issues like https://github.com/netconf-wg/netconf-next/issues/24 are
meaningless
without a detailed solution proposal.



--
> Per



Andy