[Nml-wg] XML syntax for NML relations

[Freek Dijkstra]

Comments follow original text:

Roman Łapacz wrote:

>> So far, we have seen these two proposals:
>>
>>    <nml:link id="urn:ogf:network:example.net:link_A-to-C">
>>      <nml:relation type="serialcompound">
>>         ...
>>      </nmlserialcompound:relation>
>>    </nml:link>
>>
>> and:
>>
>>    <nml:link id="urn:ogf:network:example.net:link_A-to-C">
>>      <nmlserialcompound:relation>
>>         ...
>>      </nmlserialcompound:relation>
>>    </nml:link>
>>
>> The advantage of the first syntax is that it is very easily extendable,
>> and it is still obvious for a parser to understand that it is some kind
>> of nml:relation, even if the particular type of relation is not known by
>> the parser.
>>
>> The advantage of the second syntax is that it is easy to create a
>> meaningful validator for each specific nml:relation.
>>
>>
>> I dislike both syntaxes, and was hoping for a syntax that would provide
>> both benefits.
>>
>> If I'm correct, the following syntax will do just that:
>>
>>    <nml:link id="urn:ogf:network:example.net:link_A-to-C">
>>      <nml:relations>
>>        <nmlserialcompound:relation>
>>           ...
>>        </nmlserialcompound:relation>
>>      </nml:relations>
>>    </nml:link>
>>
>> This adds a parent element to the relation elements, signifying that
>> <nmlserialcompound:relation>  is indeed a nml:relation. So even a parser
>> that has no knowledge about this particular nml:relation still knows
>> it's base syntax, while a parser that understands the details can still
>> use an meaningful syntax validator (such as XSD) to make sure the syntax
>> is correct.
> 
> The solution with namespaces gives you that (nmlserialcompound:relation 
> inherits from the base nml:relation). nml:relations only complicates the 
> xml structure without giving too much.

You mean you prefer the following?:

    <nml:link id="urn:ogf:network:example.net:link_A-to-C">
      <nmlserialcompound:relation>
         ...
      </nmlserialcompound:relation>
    </nml:link>

How should the parser know that nmlserialcompound:relation inherits from
the base nml:relation? I can think of two things:

- Because the parser has knowledge of the schema definition
- Because the parser assumes that all elements named "relation" are
subclasses of nml:relation.

The problem with the first is that it requires all parsers known all
schemas beforehand. I see a risk with backward compatibility after
future extensions if that is required.

The problem with the second is that this fails if some schema (for
whatever reason) includes a namespace where relation has a different
meaning. Eg: family:relation or work:relation.

For this reason, including the extra nesting with <nml:relations> seems
to me a relative simple solution to solve these problems.

Regards,
Freek