Re: [netmod] yang-data-ext issues

Martin Bjorklund <mbj@tail-f.com> Wed, 18 April 2018 17:47 UTC

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Date: Wed, 18 Apr 2018 19:47:44 +0200
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Subject: Re: [netmod] yang-data-ext issues
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Hi,

Andy Bierman <andy@yumaworks.com> wrote:
> On Wed, Apr 18, 2018 at 10:26 AM, 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.  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?
> >
> >
> >
> 
> 
> If we want to avoid defining these contexts, then we could just define root
> vs. nonroot.
> 
> e,g:
> 
> x:yang-data /mydef1 {
>   container foo;
> }
> 
> x:yang-data mydef2 {
>   leaf x;
>   leaf y;
>   container z;
> }
> 
> 
> Only an argument starting with '/' would be treated as a top-level data
> node.
> 
> All other yang-data definitions are not allowed to appear as a root node.
> The context where this yang-data is used is completely proprietary.
> The mechanism used to expand this yang-data as if it was a grouping
> is completely proprietary.
> 
> The augment-yang-data extension only applies to top-level yang-data
> definitions.
> 
> However, my preference is to only standardize top-level yang-data.

What is "top-level" yang-data?


/martin


> I do not see any need for the other form since all functionality can be
> achieved with a grouping and a proprietary YANG extension.
> 
> Kent // contributor
> >
> 
> Andy
> 
> 
> >
> >
> >
> >
> > On 4/16/18, 1:05 PM, "netmod on behalf of Andy Bierman" <
> > netmod-bounces@ietf.org on behalf of andy@yumaworks.com> wrote:
> >
> >
> >
> >
> >
> >
> >
> > 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
> >
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