Dear All,

I propose this paragraph to be added to the implementation guidelines for RDFS:

"About implementing multiple Instantiation

Knowledge representation models and more generally semantic networks differ fundamentally in one aspect from data structures, such as XML, Relational database schemata and data structures in all programming languages, including the object-oriented one:

·       Knowledge representation starts with an item in the real world regardless its nature, assigns an identifier to it in order to be able to make assertions about it, and then accumulates statements (assertions, propositions) about it.

·       Data structures start with a set of templates, a set of foreseen kinds of statements dedicated to a particular category each (class, entity), to be filled in by a user.

 

Consequently, knowledge representation may assign multiple classes to a given identifier without any problem. The associated processing software will then allow for asserting for this identifier all properties applicable to each assigned class. This process is called “multiple instantiation. For instance, the “weapon” with all its characteristics may also be a “ceremonial object”.

 

A system based on data structures must create a different instance of the respective templates for each class an item belongs to. It may later the link the different instances describing aspects of the same thing, in order to simulate the mechanism. In particular the very successful “encapsulation principle” of object-oriented programming languages requires dedicated data structures and constitutes a fundamental mismatch with the Open-World modeling of semantic relationships (see, for instance Schnase 1993). Fundamental to semantic data integration are also superproperties, which are not provided by data structures either.

 

The CRM as ontology relies heavily on multiple instantiation: Classes that use to co-occur on things simultaneously “incidentally”, without being associated with properties only applicable to the combination of such classes, are not modelled individually as subclasses of multiple parent classes. The latter would be called “multiple IsA”. To avoid multiple IsA in such cases is an important normalization principle to keep the ontology very compact and unambiguous.

 

Most implementations on top of RDF still use RDF as if it were a fixed schema and repeat in the UI code all the schema. Therefore, the promise of RDF and other semantic models to be able to accommodate dynamically new properties often does not work. It is still as if they were using Relational systems. Generic XML editors do adapt already to the schema, but usually the rendering paradigms they employ, without additional parameters, are too poor for good UI code. One can however write code that reads the RDF schema used at run-time and that extends data entry and display by the actual properties found. This functionality is foreseen by SPARQL, but most programmers still do not appreciate the utility of querying the schema. Even if fixed templates are used, the data entry system should foresee the same thing to be described by multiple templates, relatively freely selectable by the user.

 

In the specification modules of mapping software used to transform data into a CRM-compatible form, care must be taken to foresee and allow the user to combine RDF classes systematically. It may be useful to develop tools for specific guidance that show users how a valid path from a given domain class to a certain range class can be created by using multiple instantiation (and, by the way, also by using subclasses of the domain class), such as combining E41 Appellation with E33 Linguistic Object in order to reach E56 Language via P72 has language.

 

In a local system, another workaround for multiple instantiation can be the creation of classes that replace all candidate cases for multiple instantiation by subclasses using multiple IsA. For good reasons, the compatibility with the CRM is defined at the import/export/query level and not at the system internals. Therefore, such internal workarounds do not affect the interoperability: Whereas the query compatibility of this solution with the standard is immediate, the respective import/export system simply needs to make the trivial replacements of the respective class combinations with their multiple IsA counterparts and vice-versa.

 

So, partially, problems with multiple instantiation are a question of programming practice. On the other side, it is also a question of user training and extended good practice. Users may provide feedback about frequent cases where multiple instantiation is used, in order to guide users to these modelling cases. These could systematically be entered into the CRM RDF implementation, without requiring the CRM standard itself to repeat them."

John L. Schnase, (1993). "Semantic Data Modelling of Hypermedia Associations", in: ACM Transactions on Information Systems, Vol.11,No.1, January 1993, p 45.

Comments welcome!

Best,


Martin

-- 
------------------------------------
 Dr. Martin Doerr
              
 Honorary Head of the                                                                   
 Center for Cultural Informatics
 
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