Subject: Re: WG Review: NETCONF Data Modeling Language (netmod)

From: Chris Newman <[email-masked]>
Date: Mon, 21 Apr 2008 01:20:29 -0600

--On April 15, 2008 13:30:01 -0700 IESG Secretary <[email-masked]> 
wrote:
> NETCONF Data Modeling Language (netmod)

I support the creation of this WG.

> 2. The YANG data modeling language and semantics (proposed
> standard)
...
> 5. Mapping rules of YANG to DSDL data modeling framework (ISO/IEC
> 19757), including annotations for DSDL to preserve top-level
> semantics during translation (proposed standard).

A great deal of effort has been put into designing standard XML data 
modeling languages over many years and given that both DTD and XML Schema 
have significant weaknesses (particularly in the area of extensibility), a 
DML for XML is clearly difficult and requires special expertise.  (5) is 
critical to demonstrating that YANG has learned from the mistakes of past 
XML-DMLs with respect to extensibility and other areas.  The simpler (5) 
happens to be, the more confident I will become that YANG is following best 
practices for XML DMLs.

> 4. YIN, a semantically equivalent fully reversible mapping to an
> XML-based syntax for YANG.  YIN is simply the data model in an XML syntax
> that can be manipulated using existing XML tools (e.g., XSLT) (proposed
> standard)

If 5 is as simple as I think it should be, then I suspect there will be 
little semantic difference between 4 & 5 and much additional utility in 5. 
I'd prefer if the WG was free to drop work item 4 in the event I'm correct. 
If 2 provides the human-friendly form and 5 provides the form that best 
leverages existing standard XML tools and parsers then I see no value in 4 
which is both less human-friendly than 2 and less XML-tool-friendly than 5. 
In the event I'm wrong and there are significant semantic differences 
between 2/4 and 5 that are well justified, then I don't object to continued 
work on 4.

I suggest adding a sentence to the charter:

  In the event work items 4 and 5 are semantically similar, the WG may 
choose to omit
  work item 4.

I'm interested in other opinions on this topic.

		- Chris

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---

From: Phil Shafer <[email-masked]>
Date: Mon, 21 Apr 2008 08:41:13 -0400

Chris Newman writes:
>The simpler (5) 

My guess is the opposite:  many of the more useful features of XSD
and DSDL require distinct and uncomfortable layout of the schema
material.  For example, the XSD substitution group mechanism allows
for extensibility, but requires the schema to include substitution
group information pervasively throughout the schema and to make a
very shallow hierarchy through the use of types and indirection.
This gives a format that I believe non-validating consumers of the
schema will find difficult to read and use.

By contrast, YIN is a straight-forward conversion of the textual
data from a YANG module into an XML format that can be easily and
directly used by the consumer.  The conversion is trivia and the
information is in a state identical to the YANG module's layout.
The encoding is changed (to XML) but the content is untouched.

So if (5) is simple, we've either chosen not to use significant but
uncomfortable features in the low-level output language, or we've
lost the conciseness, hierarchical view, and other high-level features
that makes YANG worthwhile.

This isn't to say that (5) shouldn't be fairly mechanical, something
that a perl or xslt script could handle, but it shouldn't be called
simple, nor should the complexity of that transformation a basis
for judging YANG.

Thanks,
 Phil