Re: [netmod] yang-data-ext issues

Andy Bierman <andy@yumaworks.com> Mon, 16 April 2018 16:07 UTC

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From: Andy Bierman <andy@yumaworks.com>
Date: Mon, 16 Apr 2018 09:07:04 -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 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.


The yang-data statement has to define the context or new abstract namespace,
or whatever this hack is called.  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.




> 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
>>
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