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Re: Impact of XML on Data Modeling

From: Anthony B. Coates (W3C Lists) <abcoatesecure-w3c@yahoo.co.uk>
Date: Tue, 29 Jan 2008 10:28:01 -0000
To: xmlschema-dev@w3.org
Message-ID: <op.t5osozucbeptyg@laptop01.mileywatts.com>

An XML schema is a physical data model.  It is physical because it  
includes significant amounts of information specific to a particular  
physical representation of the data, e.g. you have to worry about elements  
versus attributes in an XML schema.  By contrast, the equivalent UML  
logical data model (for the same information as the schema) would describe  
the same data, but not low-level issues like elements versus attributes.   
Sometimes UML models are annotated with this kind of low-level  
information, but when that happens, they also become physical models.

A logical model is a model of the solution, written with software  
implementers as the target audience.  By contrast, a conceptual model is  
typically a model of the problem, and is written with business experts as  
the target audience.  What does that difference mean?  Well, my usual  
quick example is that in a conceptual model, you might have "Employer" and  
"Employee", as these are business concepts.  However, you wouldn't  
normally expect to factor out "Person" as the common superclass; that's  
the kind of technical construct that is appropriate for a logical model,  
but often inappropriate for a conceptual model.

Cheers, Tony.

On Tue, 29 Jan 2008 09:32:27 -0000, Essam Mansour  
<essam.mansour@gmail.com> wrote:

> Dear Scott,
> This is an interesting point.
> I understand that the 3-level of abstractions for modeling data,  
> Conceptual,
> Logical, and Physical are difference as follow:
> Conceptual: the model at this level is platform-independent and also  
> could
> be logically modeled using any modeling technique, ERD, OO model, or XML
> Schema.
> Logical: is also platform-independent but it is based on specific  
> modeling
> technique.
> Physical: is platform dependent.
> These three concepts have been addressed very well in the area of DB data
> modeling. You will find the ER modeling tools, such as power designer,
> supporting the three levels, but the the context of RDB.
> However, in the area of XML, I did not come across any paper regarding  
> that.
> I agree with you that "I would say that an XML schema would correspond to
> the logical data model"
> Because, XML schema is based on a specific model and at the same time is
> platform-independent.
> from my point of view, the implementation of the XML Schema  is a  
> physical
> model that is platform dependent.
> I do believe that as the ER modeling tools, supported the three levels of
> abstractions,  an XML Schema modeling tool could support these three  
> level.
> Best Regards,
> Essam Mansour
> On Jan 29, 2008 4:20 AM, Tsao, Scott <scott.tsao@boeing.com> wrote:
>>  I recently read an article A Few Thoughts on Data Modeling and Kids'
>> Soccer - An Interview with William G.  
>> Smith<http://www.wilshireconferences.com/interviews/smith.htm>.
>> In this article Mr. Smith advocates a 3-schema architecture for data
>> modeling, i.e., Conceptual, Logical, and Physical data models.  I
>> understand that this architecture has been a popular approach by  
>> information
>> architects in the late 80's to early 90's, under the banner of Data or
>> Information Resource Management (DRM or IRM).
>> It seems to me that, other than the Conceptual data model, the Logical  
>> and
>> Physical data models no longer directly apply to the XML approach of  
>> data
>> modeling (e.g., using the W3C XML Schema).  For example, Mr. Smith talks
>> about a 3NF logical data model, which only applies when one is taking a
>> strictly 'relational' approach.  So, I was wondering if there is a  
>> parallel
>> sets of data models in the XML-based data modeling world.
>> Based on my (limited) understanding of the purpose and techniques
>> mentioned in the article, I would say that an XML schema would  
>> correspond to
>> the logical data model, and an XML binding (to a particular database or
>> programming language) would correspond to the physical data model.  In
>> addition, transformation or mapping between logical and physical data  
>> models
>> could be implemented by adopting a standard such as SQL/XML.
>> Could anyone on this list help me to gain a understanding on this issue?
>> Thanks,
>> *Scott Tsao*
>> Associate Technical Fellow
>> The Boeing Company

Anthony B. Coates
London, UK
+44 (79) 0543 9026
Received on Tuesday, 29 January 2008 11:27:13 UTC

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