Re: Evolution of Date's "Abstract Machine" (via CM)

"erk" <eric.kaun_at_gmail.com> wrote in message news:1117723989.613025.282900_at_z14g2000cwz.googlegroups.com...
> Pete, I don't know what you did to attract the degree of bile you did
> with this post, but I don't envy you. Thank you for posting new topics
> and keeping the discussion rolling.

It's a pleasure Eric. I have absolutely no idea either, except that I appear to have hit some sort of nerve using un-capitalised ascii.

In certain papers, Date uses the terms "abstract machine" and "data model" interchangeably, and in the following article he exchanges the use of these terms in his very definitions:

Quoting Date from here:
http://www.oreilly.com/catalog/databaseid/chapter/ch01.pdf

"DEFINITION: A data model is an abstract, self-contained, logical definition of the data structures, data operators, and so forth, that together make up the _abstract_machine_ with which the users interact."

>> <SNIP>
>>
>> 1) Is it clear/true that Date's "abstract machine" necessarily
>> evolves due to change management considerations over
>> time?
>
> Not exactly. The abstract machine, as I understand it (I don't have his
> source references), is the relational data model - not a specific data
> model, for a particular application. So while relational doesn't
> evolve, my database for my application does.

OK, thanks for this clarification. It is probably close to the mark. Date goes on (in the above ref) to define two meanings for the term "data model". The first and more fundamental meaning is given in the above definition.

The second meaning for data model is provided: "A data model (second sense) is a model of persistent data of some particular enterprise." (eg: a database design).

He then says this:

"A data model in the first sense is like a programming language whose constructs can be used to solve many specific problems, but in and of themselves have no direct connection with any such specific problem.

A data model in the second sense is like a specific program written in that language - it uses the facilities privided by the model, in the first sense of that term, to solve some specific problem."

>> 2) Does Date reference this issue?
>
> Not that I know of. I have his Temporal book on my shelf, but haven't
> read it yet - I think that may offer something, indirectly.

It is clear that Date would acknowledge that the data model in the seond sense obviously evolves, via change management.

He would probably acknowledge that the data model in the first sense (ie: the RM) only evolves though advance of "relational theory", such as a new database language.

So I think I have answered my own question with your assistance, and a little further reading.

>> Thanks for any comments,
>
> I'm not sure I'd thank people before the fact for "any" comments.

We do things differently in rural Australia, and ascii has never troubled me in my entire life, so I like to appear polite, even if I may not in fact be so all the time. ;-)

-- 
Pete Brown
IT Managers & Engineers
Falls Creek
Australia
www.mountainman.com.au/software