Access Languages are Tied to Schemas
Mark L. Fussell
fussellm at alumni.caltech.edu
Thu Nov 20 20:40:39 GMT 1997
Jonathan Robie <jwrobie at mindspring.com> wrote
> The following properties of object models are easily represented in
> 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
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:
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
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.fussell at chimu.com
 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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