Access Languages are Tied to Schemas

[Mark L. Fussell]

Jonathan Robie <jwrobie at mindspring.com> wrote
> The following properties of object models are easily represented in 
> SGML/XML:
> o       Identity
> o       State
> o       Type

I would disagree that even these items can be easily represented in 
SGML/XML (for example, State is more complicated than a particular set of 
attribute values).  I think it is more the other way around: SGML/XML has 
a particular model of Identity, State, and Type which an object model can 
easily represent.

But in any case, these items are (mostly) the core concept of OO (i.e. 
Objects) instead of being properties of object models.  Objects have 
Identity, State, and Behavior where the implementation of both state and 
behavior is encapsulated.  Object models describe the possible objects 
and structures that can exist in a system.  This will include describing[1]:

    Types:         The interfaces (methods, associations, and abstract 
                   state) that objects can have.  

    Associations:  The possible relationships between objects 
    Operations:    The messages an object can respond to
    State Models:  The possible state transitions for an object
    Attributes:    The simple associations (to basic value types) of an 
                   object

    Inheritance:   The similarities/relationships among types

DTDs can describe some of this modeling information, but not particularly 
well and really only for a limited set of object models.  Examples of 
weaknesses are: only one true association (content) which is a pure 
containment, all other attributes must be basic data types, limited 
cardinality control, likelihood of arbitrary ordering, inability (or 
difficulty) to express Type relationships, inability (or difficulty) for 
an Object to support more than one type.  These are weaknesses compared 
to the most basic modeling abilities of common modeling techniques (UML, 
Booch, HOOD, Syntropy, OORAM).

Thought about another way, DTDs are good models for textual input of 
information (what rules must be satisfied by the encoding) but this 
should be considered only a view onto the true information model.  
SGML/XML describes a construction view of an information model and 
provides the front-end to instantiating an Objectbase from that model.  

Using SGML/XML to try to describe any information model (via DTDs) will 
be over extending its abilities into areas where other tools/techniques 
are much better qualified.

--Mark
mark.fussell at chimu.com

[1] An implementation of an object model (or an implementation model 
developed from a conceptual model) also uses classes, methods, and 
instance variables to satisfy the above descriptions within a particular 
system.  I am trying to use the most established and main-stream 
definitions of all these terms, but you may also want to see the 
references at the MONDO site for possible different definitions (e.g. 
Dictionary of Object Technology [Fireside+E 95]).

  i   ChiMu Corporation      Architectures for Information
 h M   info at chimu.com         Object-Oriented Information Systems
C   u    www.chimu.com         Architecture, Frameworks, and Mentoring


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