Re: [netmod] yang-data-ext issues

Martin Bjorklund <mbj@tail-f.com> Wed, 18 April 2018 17:43 UTC

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Date: Wed, 18 Apr 2018 19:43:35 +0200 (CEST)
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Subject: Re: [netmod] yang-data-ext issues
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Hi,

[Kent, your email program has messed up the quoting in this thread.
It becomes quite difficult to follow.  And no, please don't invent a
new quoting style in every email thread...]

Kent Watsen <kwatsen@juniper.net>; wrote:
> I like Andy's proposal below, for the argument of the 'yang-data'
> statement to encode some meta-information regarding the
> context/namespace in which it's used, but I wonder how it really
> works.

I *think* that what Andy proposes is a way to formally specify the
context in which the yang-data structure will be used.

My proposal was to simply define the context in plain text.  A formal
way of defining the context would of course be useful -- if we can
find agreement on how that would work.  I don't think we should
hard-code a few contexts in this draft; the mechanism must be
extensible.

An alternative syntax could be to introduce a new substatement:

  yd:yang-data foo {
    yd:context ...;
    description "...";
  }


/martin



> For instance, would "top" and "error-info" be the only allowed
> base-path values for the argument? and what is the value of the
> remainder of the path?  are we expecting for there to be some kind us
> 'uses' statement that can refer to just the base-path component to
> implement substitution-group like behavior?
> 
> Kent // contributor
> 
> 
> On 4/16/18, 1:05 PM, "netmod on behalf of Andy Bierman"
> <netmod-bounces@ietf.org<mailto:netmod-bounces@ietf.org> on behalf of
> andy@yumaworks.com<mailto:andy@yumaworks.com>> wrote:
> 
> 
> 
> On Mon, Apr 16, 2018 at 9:46 AM, Robert Wilton
> <rwilton@cisco.com<mailto: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<mailto: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<mailto: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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