in favor of allowing a fragment in a URI for metadata for an attribute type

Dick Hardt dick at sxip.com
Wed Apr 11 15:38:17 UTC 2007


btw: my main driver in stating +1 is that I was concerned with how it  
would be implemented, and given that Mark has the one working parser  
and is ok with it, then my concern has disappeared!

On 10-Apr-07, at 5:52 PM, Dick Hardt wrote:

> Good argument Mark, I concur. +1
>
> -- Dick
>
> On 10-Apr-07, at 4:52 PM, Mark Wahl wrote:
>
>>
>> Section 4.3 of
>> http://openid.net/specs/openid-attribute-types-1_0-02.html
>> suggests that in URIs defined for attributes for OpenID AX,
>> "URI fragment specifiers should not be used."
>>
>> Now I'm no RDF expert, but I'm in favor of allowing fragments,
>> and perhaps even encouraging them. I'd prefer this statement
>> be removed from subsequent versions of the OpenID AX, in order
>> to not dissuade other schema developers from using fragments.
>> Here are some points for discussion on that topic, I'd be
>> interested in hearing feedback esp. from other RDF implementors.
>>
>> 0. Some servers will have but a single, small, fixed schema.  I'd
>> rather those servers be able to reference and serve a single RDF
>> file with their complete schema, instead of needing to break that
>> schema into a bunch of little schemas.
>>
>> For example, suppose a server only supports FOAF.  Now FOAF does not
>> use fragments for the property definitions for its attribute types,
>> but the attribute types defined in FOAF are not currently resolvable
>> to an RDF document that describes those attribute types.
>>
>> If xmlns.com where the FOAF RDF is hosted were to implement having
>> these
>> attribute type URIs used in FOAF be resolvable, they
>> would either need to
>>   - create a few dozen little RDF files that together mirror the
>> content of
>>     foaf.rdf, or
>>   - implement a URI rule that http://xmlns.com/foaf/0.1/*
>>     returns foaf.rdf
>>
>> If I were redefining FOAF, I'd have its attributes be defined as
>> fragments,
>> so that there is only one base URI for the FOAF schema.  (Also to  
>> give
>> them RDF class definitions, see below).
>>
>> 1. It appears to be current practice for RDF representations of
>> metadata
>> for attributes in Higgins to use fragments.
>>
>> In OWL-based systems, the RDF object at the base URI of the document
>> is an OWL Ontology.
>>
>> In Higgins, which uses OWL, the object at the base URI is an OWL
>> Ontology that 'imports' the Higgins Ontology.  The RDF file for
>> an attribute contains an OWL Class for the attribute named by a
>> fragment,e.g., #Firstname, and several related OWL properties and
>> RDF instances in that same file that add structure to that class.
>>
>> 2. In our 'schemat' implementations which attempt to generate RDF for
>> existing schemas of 'legacy'/'installed base' protocols, it is
>> desirable to
>> be able to have additional, named OWL classes, RDF objects, and other
>> modelling and descriptive data definitions that are shared across
>> multiple attributes of a single schema. For example, a schema may
>> define its own value syntax and matching rules, and wish to share
>> those syntaxes and matching rules across the attributes of that
>> schema.
>> It would be desirable if there could be a single RDF file which
>> contains
>> the attribute type metadata, the syntax definition and matching rule
>> definition, rather than needing to have the attribute type metadata
>> in a set of files that are separate from the syntax definitions and
>> matching rule definitions, or are duplicated in those files.
>>
>> 3. I find that in our implementation 'schemat' of identity metaschema
>> attribute metadata retrieval that is a definite performance gain if
>> all the schema elements for a particular schema can be retrieved in
>> a single HTTP GET.  It is likely that an implementation interested
>> in an attribute Firstname of a particular schema would also be
>> seeing a few other attributes Lastname, Middlename etc of the same
>> schema, and it would be good if a GET that retrieves the data for
>> Firstname also gives the implementation the rest of the schema so
>> that it does not need to keep going back and GETing for each
>> attribute type.
>>
>> 4. Requiring that each be in a separate document would likely lead to
>> duplication of metadata, particularly Dublin Core metadata that
>> describes "the schema as a whole".  I feel it would be better if the
>> RDF object at the base URI has the Dublin Core metadata for the
>> schema as a whole, and that the Attribute Type Metadata is a class
>> named by a fragment below that base URI.
>>
>> 5. (appeal to authority) http://www.w3.org/DesignIssues/Fragment.html
>> "This means that identifiers for arbitrary RDF concepts should have
>> fragment identifiers. "
>>
>>
>> Mark Wahl
>> Informed Control Inc.
>>
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