Re: Conceptual, Logical, and Physical views of data

"dawn" <dawnwolthuis_at_gmail.com> wrote in message news:1125611358.454623.172590_at_g47g2000cwa.googlegroups.com...
>
> mAsterdam wrote:
> > David Cressey wrote:
> > > mAsterdam wrote:
> > [snip agreement]
> > >>Does it matter if the technical
> > >>architecture is given or not (say: we will use DBMS xyz)?
> > >
> > > Here's the way it works in practice for me:
> > >
> > > The conceptual model is implementation independent.
> >
> > Agreed.
>
> I would agree that is the idea, but it seems rare that it works out
> this way.

Not so rare in my experience. The difficulty is distinguishing a feature of the problem itself from a feature of some imaginary soultion to the problem. In other words, the difference between analysis and design. If you can keep analysis and design separate, you can create a conceptual model that is implementation independent.

> > 2. How do we make the conceptual model explicit?
>
> This is probably a naive response, but where I used to choose an early
> prototyping tool, possibly paper & pencil, even in the analysis phase
> of a project, I now use web pages. I can mock these up even in place
> of showing any end-user an erd (or uml class diagram) with any level of
> detail in it. If you model the conceptual data with web pages with
> foreign keys turned into links and sample data values, you can get a
> lot of bang for the buck.
>

Interesting. I would imagine that a collection of interlinked web pages could model
anything that can be modeled as a web of interlinked nodes.

> That seems close to what happens, although plenty of people seem to
> think that the logical model is db independent. --dawn

I'm one of them. The logical model should be DBMS independent... but not DBMS class independent.

The best success, in practice, came when two of us had finished the logical modeling for a DEC Rdb/VMS implementation, and it was decided to implement in Oracle instead. (That was about the time that Oracle bought the Rdb division from DEC).
We had zero changes to make to our logical model.

If we had decided to implement in a non relational DBMS, or even a non SQL DBMS, we might have had to undo and the redo some design decisions.

And, of course, all of our physical design decisions were worthless. We just tossed them, and started over. Received on Tue Sep 06 2005 - 23:53:02 CEST