Access Languages are Tied to Schemas

[Jonathan Robie]

At 12:40 PM 11/20/97 -0800, Mark L. Fussell wrote:
 
>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.

Our basic difference here is that I am thinking primarily in terms of the
network of objects available in object oriented systems at run-time, with
their metadata (if available), and you seem to be thinking of abstractions
used to create object oriented systems. For instance, the state of an object
is precisely equivalent to the set of attribute values associated with that
object. Either of these can be referred to as an object model, but they are
not the same thing. Also, you may be inferring that I am trying to say that
SGML can be a replacement for CORBA or other distributed object
architectures. No way.

In fact, at this point I am not advocating anything concrete, except that I
think there should be some kind of query language that SGML/XML systems can
use to access data in foreign systems like relational or object oriented
databases, and at present, it makes sense to me that such a query language
should be defined in terms of SGML/XML structure. And I think that SGML/XML
is probably powerful enough for that - at least, it is if we are using it
only for retrieval of information, and not for modification of information;
for instance, everything that is stored in an object oriented database can
be stored in SGML - the object ids can be turned into IDs, containers can be
expressed either through containment or sets of IDREFs, etc. As long as
access is read-only, you aren't losing much. However, you wouldn't want to
modify it through such an interface, since you have lost encapsulation,
polymorphic references, type safety of references, etc.

This is analogous, in some ways, to ODBC access for object oriented
databases, which allows a view on the data in the model, but does not
encompass the full semantics of the object database. Such interfaces are
great for read-only access, but certainly do not replace the need for an
object database, and are not really very good for write access.
 
Jonathan


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