Re: [netmod] yang-data-ext issues

Andy Bierman <andy@yumaworks.com> Mon, 16 April 2018 17:05 UTC

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From: Andy Bierman <andy@yumaworks.com>
Date: Mon, 16 Apr 2018 10:05:15 -0700
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To: Robert Wilton <rwilton@cisco.com>
Cc: Martin Bjorklund <mbj@tail-f.com>, NetMod WG <netmod@ietf.org>
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Subject: Re: [netmod] yang-data-ext issues
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On Mon, Apr 16, 2018 at 9:46 AM, Robert Wilton <rwilton@cisco.com>; wrote:

>
>
> On 16/04/2018 17:07, Andy Bierman wrote:
>
>
>
> On Mon, Apr 16, 2018 at 8:44 AM, Robert Wilton <rwilton@cisco.com>; wrote:
>
>> Don't groupings have a somewhat similar concern?
>>
>>  E.g. if two groupings define the same data node name and are used at the
>> same point then you would get a namespace clash, but YANG does not disallow
>> the groupings:
>>
>>      grouping foo_widget {
>>        leaf name {
>>          type string;
>>          description "Name of my foo widget";
>>        }
>>      }
>>
>>      grouping bar_widget {
>>        leaf name {
>>          type string;
>>          description "Name of my bar widget";
>>        }
>>      }
>>
>>
>>      container all_widgets {
>>        uses foo_widget;
>>        uses bar_widget;
>>      }
>>
>>
>> The principal difference here, is that the compiler can easily check and
>> reject the conflict at the uses statements.
>>
>> Hence I think that it would be good if we could find a solution for
>> yang-data-ext that doesn't not require all root yang-data nodes to be
>> unique, since that feels somewhat clunky.  I.e. my preference is to keep
>> them less restrictive, as Martin has proposed, if this is feasible.
>>
>>
>
>
> It is not clunky that 2 top-level YANG data nodes in the same module
> have unique names. This is simple and deterministic.
> This restriction has not been a problem so far.
>
> I agree with the statements above.
>
> But it is not clear to me that yang-data-ext is really defining new top
> level data nodes that are part of the same tree/namespace as the
> configuration/state nodes.  In Martin's examples they were used within
> RPCs, and it the forcing the names to be unique in that context that I
> think would be clunky.  E.g. in Martin's example forcing different names
> for "reason" and "user-info" doesn't seem to be helpful.
>
>
>
> The yang-data statement has to define the context or new abstract
> namespace,
> or whatever this hack is called.
>
> Perhaps.  I think that this depends on how they are used.
>


The yang-data statement has to specify the expansion point, or
at least specify that it is or is not the top-level.

  yang-data top/name1 {
      container mydata;
  }

where context is something like "top" or "error-info", etc.

It is trivial to use groupings if the same set of nodes needs to be used in
different contexts:


  yang-data error-info/name1 {
      container mydata;
  }

Only the context named "top" is restricted to a resulting single-container
and cannot have duplicate names.

This is OK:

  x:yang-data error-info/my-error1 {
      leaf reason {}
  }


  x:yang-data error-info/my-error2 {
      leaf reason {}
  }




> Could a fix for this be something along the lines of:
>  - yang-data names must be unique amongst other top level data nodes
> within the module.
>  - if yang-data extensions are used at the top level then their name must
> be used as a single top level container.
>  - if a yang-data extension is used within another structure then the
> yang-data name is excluded, and the top level nodes defined in the
> yang-data definition are used ....
>
>   Every tool that implements yang-data has to be able
> to interpret a yang-data statement exactly the same way.
>
> If you want to reinvent XSD substitutionGroup, then do it right.
>
> I'm not familiar with them.  From a quick read, I don't see how they are
> related to the problem that we are trying to solve here.
>
>

A substitutionGroup allows a point int the schema to be identified by name.
Different elements can be defined that match this name, which then can be
used (like a YANG choice) at the specified schema point.
(e.g. error-info above is like a substitutionGroup)




> Thanks,
> Rob
>

Andy


>
>
>
>
>
>
>> Thanks,
>> Rob
>>
>>
>
> Andy
>
>
>>
>> On 16/04/2018 15:36, Andy Bierman wrote:
>>
>> Hi,
>>
>> I am strongly opposed to this change because it breaks the rule in YANG
>> 1.1
>> that there cannot be 2 sibling nodes defined in the same module namespace.
>>
>> IMO since any yang-data nodes are ALLOWED to be used at the top-level,
>> then these top-level nodes cannot have conflicting names.
>>
>> It is very important when parsing an instance document that the instance
>> data
>> can be associated with the correct schema.  This is not possible if the
>> same top-level node has multiple yang-data nodes defined.
>>
>> If one needs to define data that is not top-level, (1) use
>> augment-yang-data
>> or (2) use a different module.
>>
>>
>> Andy
>>
>>
>>
>> On Mon, Apr 16, 2018 at 5:56 AM, Martin Bjorklund <mbj@tail-f.com>; wrote:
>>
>>> Hi,
>>>
>>> While preparing draft-ietf-netmod-yang-data-ext-02, it turned out that
>>> it is not clear what, if any, restrictions should be enforced for
>>> yang-data structures.  Even among the authors we have different ideas
>>> for how this should work.
>>>
>>> Background:
>>>
>>> In 8040, the original yang-data extension had a restriction that said
>>> that a yang-data structure MUST have exactly one container, since it
>>> wouldn't be possible to have a yang-data structure in an XML instance
>>> document otherwise.
>>>
>>> Since people want to use yang-data structures in other places, this
>>> restriction was lifted in the new draft:
>>>
>>>    There is no longer an assumption that a yang data structure can
>>>    only be used as a top-level abstraction, instead of nested within
>>>    some other data structure.
>>>
>>>
>>> With this in mind, here's a use case that I think we ought to support:
>>>
>>>   rpc my-first-rpc {
>>>     description
>>>       "Bla bla...
>>>        If an error occurs, <error-info> will contain an instance of
>>>        the yang-data structure 'my-first-rpc-error-info'.";
>>>     ...
>>>   }
>>>
>>>   yang-data my-first-rpc-error-info {
>>>     leaf reason { ... }
>>>     container user-info { ... }
>>>   }
>>>
>>>   rpc my-second-rpc {
>>>     description
>>>       "Bla bla...
>>>        If an error occurs, <error-info> will contain an instance of
>>>        the yang-data structure 'my-second-rpc-error-info'.";
>>>     ...
>>>   }
>>>
>>>   yang-data my-second-rpc-error-info {
>>>     leaf reason { ... }
>>>     leaf important-url { ... }
>>>   }
>>>
>>> (maybe in the future we could even have a YANG extension statement to
>>> formalize the description:
>>>
>>>    rpc my-first-rpc {
>>>      ...
>>>      opx:error-info-structure my-first-rpc-error-info;
>>>    }
>>>
>>> but this is not point now.)
>>>
>>
>
>
> I see no reason to reinvent the grouping-stmt.
> You could easily say opx:error-info-structure argument is a grouping name
> as it is a yang-data name.
>
>
>
>>
>>> In the example above, note that the leaf "reason" is present in both
>>> structures.  IMO this is not a problem, since these structures are
>>> used in different contexts.
>>>
>>> My point is that I think we should impose as few restrictions as
>>> possible to the yang-data extension.  It should be up to the user of
>>> yang-data to ensure that the structure is defined in such a way so
>>> that it can be used properly.  For example, a structure that is
>>> supposed to describe an XML instance document cannot define two leafs
>>> at the top level.
>>>
>>> If the WG agrees with what I wrote above, we need to change the
>>> augment-yang-data extension so that you would write for example:
>>>
>>>   yx:augment-yang-data /ex:my-first-rpc-error-info/ex:user-info {
>>>     ...
>>>   }
>>>
>>> Comments?
>>>
>>>
>>>
>>> /martin
>>>
>>> _______________________________________________
>>> netmod mailing list
>>> netmod@ietf.org
>>> https://www.ietf.org/mailman/listinfo/netmod
>>>
>>
>>
>>
>> _______________________________________________
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>>
>>
>
>