[Nml-wg] XML syntax for NML relations

[Jason Zurawski]

Hi Freek;

On 7/16/11 4:39 PM, thus spake Freek Dijkstra:
> Hi all,
>
> Let me follow up on a technical part of yesterday's discussion.
>
>> At the end of the session, we had a discussion on how to encode the
>> relation name (here: "serialcompound") in XML. E.g. each relation as
>> a different element, a different attribute, or a different value of a
>> fixed attribute.
>
> So far, the relation type is encoded in the value of a type attribute
> (e.g.<nml:relation type="serialcompound">). Freek pointed out that this
> makes it harder to create a meaningfull schema for validation. For
> example, there is a difference syntax associated with next relations and
> serialcompound relations.<nml:relation type="serialcompound">  may
> contain 0 or more children, while<nml:relation type="next">  must
> contain exactly one child. The RNC schema does provide a means to
> specify this difference. Jeff confirmed this and pointed out that it is
> of course still possible to check the syntax using other logic, it is
> just not possible to create a detailed schema. (Freek pointed out that
> NMC has the same problem, which is the reason that no non-trivial WSDL
> was created for perfsonar, and no software used a SOAP library, despite
> that most NMC messages are embedded in a SOAP envelope).
>
> The encoding of the type of relation in a element (e.g.
> <nml:serialcompound>) allows more meaningful validation. However, this
> has another drawback: if a client does not know about a particular
> relation, it does not know if the unknown element is subclass of a
> relation and can or can not be ignored. In fact, it can not even do
> basic syntax checking (as if it was just a unknown NML relation), since
> it does not know it is a NML relation without parsing the schema
> definition (which will list the new element as a subclass of the base
> element).


Just to confirm ... do you mean to say '<serialcompound:relation>' where 
the 'serialcompound' namespace is something defined in addition to the 
nml base namespace?  This would foster use of the same element 
'relation' in both the base and subsequent namespaces (for reference the 
concept is described here: 
http://books.xmlschemata.org/relaxng/relax-CHP-11-SECT-5.html).  In your 
example above you have created a new element, which would imply a 
special case instead of using the relation element at all.

Since this is a complicated construct I do want to be sure we are all on 
the same page.

Thanks;

-jason


> So there seem to conflicting requirements:
> * ability to do extended validation
> * know from just looking at XML (but not the schema) that something is a
> NML relation
>
> It may be possible to find a solution that caters to both requirements.
> For example, in NM, subclasses use the same name (a trick known as
> chameleon namespace in RNC). E.g. A subclass of e.g.
> http://schema.ogf.org/nm/base/port is also called "port", e.g.
> http://schema.ogf.org/nm/layer2/port. A similar trick is enforce that
> all relations must always be defined in the same namespace (which may
> only contain relations). E.g. everything that starts with
> http://schema.ogf.org/nml/relation/ is a relation.
>
> I think we should discuss this further. I know the above is a difficult
> technical discussion, so I appreciate that you read so far. Please
> provide feedback if this was clear, and if you have any opinion or
> thoughts on this topic.
>
> Regards,
> Freek