Re: [netmod] xpath expressions in JSON

"Reshad Rahman (rrahman)" <rrahman@cisco.com> Thu, 11 October 2018 15:34 UTC

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From: "Reshad Rahman (rrahman)" <rrahman@cisco.com>
To: "Robert Wilton -X (rwilton - ENSOFT LIMITED at Cisco)" <rwilton@cisco.com>, Martin Bjorklund <mbj@tail-f.com>
CC: "netmod@ietf.org" <netmod@ietf.org>
Thread-Topic: [netmod] xpath expressions in JSON
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Date: Thu, 11 Oct 2018 15:34:34 +0000
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Subject: Re: [netmod] xpath expressions in JSON
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Hi Rob,

From: netmod <netmod-bounces@ietf.org> on behalf of "Robert Wilton -X (rwilton - ENSOFT LIMITED at Cisco)" <rwilton@cisco.com>
Date: Thursday, October 11, 2018 at 7:17 AM
To: Martin Bjorklund <mbj@tail-f.com>
Cc: "netmod@ietf.org" <netmod@ietf.org>
Subject: Re: [netmod] xpath expressions in JSON




On 11/10/2018 11:50, Martin Bjorklund wrote:

Robert Wilton <rwilton@cisco.com><mailto:rwilton@cisco.com> wrote:



On 11/10/2018 11:21, Martin Bjorklund wrote:

Andy Bierman <andy@yumaworks.com><mailto:andy@yumaworks.com> wrote:

On Wed, Oct 10, 2018 at 11:39 PM, Martin Bjorklund <mbj@tail-f.com><mailto:mbj@tail-f.com>

wrote:



Andy Bierman <andy@yumaworks.com><mailto:andy@yumaworks.com> wrote:

On Wed, Oct 10, 2018 at 6:59 PM, Reshad Rahman (rrahman) <

rrahman@cisco.com<mailto:rrahman@cisco.com>>

wrote:



On 2018-10-10, 9:59 AM, "netmod on behalf of Martin Bjorklund" <

netmod-bounces@ietf.org<mailto:netmod-bounces@ietf.org> on behalf of mbj@tail-f.com<mailto:mbj@tail-f.com>> wrote:



     Ladislav Lhotka <lhotka@nic.cz><mailto:lhotka@nic.cz> wrote:

     > Martin Bjorklund <mbj@tail-f.com><mailto:mbj@tail-f.com> writes:

     >

     > > Hi,

     > >

     > > While reviewing restconf-notif, I saw this example:

     > >

     > >    {

     > >       "ietf-subscribed-notifications:input": {

     > >          "stream": "NETCONF",

     > >          "stream-xpath-filter": "/ds:foo/",

     > >          "dscp": "10"

     > >       }

     > >    }

     > >

     > > Note the "stream-xpath-filter".  It has a prefix in the XPath

string.

     > > How are prefixes declared when JSON is used?

     > >

     > > The leaf "stream-xpath-filter" says:

     > >

     > >               o  The set of namespace declarations are those in

scope on

     > >                  the 'stream-xpath-filter' leaf element.

     > >

     > > (I think I provided that text...)

     > >

     > > This assumes that the encoding is XML, or at leas that the

encoding

     > > can somehow transfer namespace declarations.

     >

     > It can't. There are two options:

     >

     > 1. have different representations of this value in XML and JSON,

     >    analogically to instance indentifiers (sec. 6.11 in RFC 7951).

     >

     > 2. use a module name rather than a prefix in XML, too.

     >

     > I would suggest #2.

<RR> But that means making non-backwards compatible change to the XML

representation?



Not really. It means NETMOD WG would be creating its own special

variant

of

XPath.

Not at all.  What I propose is perfectly fine, legal XPath 1.0.



XPath 1.0 says that an XPath expression is evaluated in a context.

One item in the context is a set of mappings from <prefix> to <uri>,

where <prefix> is used to lookup prefixes used in the XPath

expression, e.g. in "/foo:interfaces" "foo" is the prefix.



It is perfectly fine to say that the prefix mapping set is this:



    "ietf-interfaces" -> "urn:ietf:params:xml:ns:yang:ietf-interfaces"

    "ietf-ip"         -> "urn:ietf:params:xml:ns:yang:ietf-ip"



and use that to evaluate the expression



   /ietf-interfaces:interfaces/ietf-interfaces:interface/ietf-ip/ipv4







The XPath expression is normally parsed within an XML instance

document.

There are "xmlns" attributes present that map the prefix to a

namespace URI.

These mappings will not be present in the JSON at all.



A custom XPath implementation is required to magically identify the

prefix

as a module name and magically find the namespace URI for the module

name.

I disagree.  You need an XPath implementation + custom code to set up

the environment.

This is OK, but can we just use the JSON encoding instance identifier

format exactly?  I.e .RFC 7951 section 6.11.



So "/ietf-interfaces:interfaces/interface/ietf-ip:ipv4/enabled"



can trivially be expanded to:



"/ietf-interfaces:interfaces/ietf-interfaces:interface/ietf-ip:ipv4/ietf-ip:enabled",



and then interpreted with the context:

     "ietf-interfaces" -> "urn:ietf:params:xml:ns:yang:ietf-interfaces"

   "ietf-ip"         -> "urn:ietf:params:xml:ns:yang:ietf-ip"

*this* would require a custom XPath implementation.
Why?  I.e. how is this different from stating "Custom code is needed to connect things together"?





and it is not obvious what the rules for the "auto-assignment" of

prefixes would be.  For example:



  /ietf-interfaces//ietf-ip:address[../foo]



what is the prefix for "foo"?
OK, so here the module for "../foo" would need to be specified.

Perhaps the rule that I'm looking for is the module name may be omitted when it matches the parent node module, and can easily be inferred.  I.e. so that for any XPath string, it is possible to trivially expand it without any additional schema context.

It just seems to be that requiring the long hand of "/ietf-interfaces:interfaces/ietf-interfaces:interface/ietf-ip:ipv4/ietf-ip:enabled" seems like it will get very verbose, and I wonder whether we are introducing yet another Xpath format to YANG.
<RR> I’m willing to live with verbosity if it avoids the need of another format.

Finally, I'm trying to figure out have RFC 8040 query parameter (sect 4.8.4), which also uses XPath expressions is meant to work.  That states:



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.

Yet the examples in section 8.3.6 don't seem to use namespace prefixes in very many places, e.g. why is it "/example-mod:event1/name='joe'" and not "/example-mod:event1/example-mod:name='joe'"?  Is the example wrong, or otherwise what am I missing? :-)

<RR> Section 8.3.6 of which document?

Regards,
Reshad.

Thanks,
Rob










/martin







Thanks,

Rob





There is no standard XPath implementation that can just take an XML

instance document + YANG module and figure out what to do.  Custom

code is needed to connect things together.  This proposal doesn't

change this.





/martin





A normal XPath implementation will not find any namespace mapping for

the

prefixes.



An XPath expression has no concept of the "current module" inherited

from

the parent

like the JSON encoding. This is problematic for predicates



    /ietf-interfaces:interfaces/interface[name='eth0']



XPath says the missing prefixes for 'interface' and 'name' are simply

missing (no namespace).

The JSON encoding says "ietf-interfaces" is used for 'interfaces'. and

'interface'.

There is no specification for the 'name' node inside a predicate.



So you must mean the full module name will be used at every node:



  /ietf-interfaces:interfaces/ietf-interfaces:interface[ietf-interfaces:name='eth0']







/martin





Andy





     Hmm, so you mean change the leaf "stream-xpath-filter" to say:



              o  The set of namespace declarations has one member for

each

                 YANG module supported by the server.  This member maps

                 from the YANG module name to the YANG module namespace.



                 This means that in the XPath expression, the module

name

                 serves as the prefix.



     .... and then also give an example of this.



     This is probably what we need to do in all places where

yang:xpath1.0

     is used, going forward.  Maybe even define a new type

     yang:xpath1.0-2 (name?) with the set of namespace declarations

     built-in.



We should avoid making off-the-shelf implementations of standards like

XPath unusable.

At the very least this should be only available if the server supports

it

(with a capability URI)







<RR> So we need an update to RFC7951?



Regards,

Reshad.





Andy





     /martin











     >

     > Lada

     >

     > >

     > > How is this supposed to work with JSON?

     > >

     > >

     > > /martin

     > >

     > > _______________________________________________

     > > netmod mailing list

     > > netmod@ietf.org<mailto:netmod@ietf.org>

     > > https://www.ietf.org/mailman/listinfo/netmod

     >

     > --

     > Ladislav Lhotka

     > Head, CZ.NIC Labs

     > PGP Key ID: 0xB8F92B08A9F76C67

     >



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