Re: Newbie object design questions

From: Bruce Wood (brucewood_at_canada.com)
Date: 01/27/05 

Date: 26 Jan 2005 17:13:16 -0800

>> Or do you have to take care to isolate the data sets used as the
>> Business Entity from the actual structure of the database and the
Data
>> Access Layer has to mediate?

> Can that be done effectively, from the standpoint of both costs and
> benefits? I guess it really depends on the frequency of change--we do

> what's required to meet project goals and requirements.

One of the problems I've battled over the years is the tendency of
applications, which multiply like rabbits, to know too much about how
the data is structured, essentially casting your database schema in
stone. When you come along to change things in the database, kaboom! a
bunch of (usually business-critical) applications blow up because they
assumed such-and-so about the data.

Another problem is that things aren't necessarily stored in the
database in the same way that the business thinks of them. This can be
by accident (original programmers didn't understand the business well
enough) or by design (you can make more effective use of the database
by folding two business entities into one table, or having multiple
tables storing information about one business entity).

I agree that modern databases have come a long way in solving the first
problem. Changing the length of a field no longer trickles out to the
application and blows it up (particularly in .NET where you have
dynamically sized strings). You can add columns to a table without
affecting applications that don't care about those columns. To that
extent, it's a fair question to ask how much more engineering against
change is cost-effective.

However, as an example, what happens if I normalize a database table
into two tables (a lookup and what's left of the original table)? Oh, I
know: we're all supposed to design database schemas with everything
normalized from the outset... and we all do, right? :*) You have to
have some way of allowing old applications to see the data as if it
were still denormalized, or you have a lot of rewriting to do just for
an internal reorg of your data. Ugly. Or, even more ugly, you don't
normalize the data because it would cause too much code churn, so you
just live with it the way it is and it gets worse and worse....

The second problem is more difficult, I think: for business reasons,
technical reasons, or poor design, your database tables don't map
one-to-one onto business objects. Now what? For example, where I work,
a lift, or pallet is a stock item, but from the business's point of
view it's a special stock item with special rules and operations
associated with it. In object terms that sounds like a subclass to me,
but in the database it's just another row in the inventory master.
Mediating between these two points of view is what a business layer is
supposed to do, but I'm having trouble imagining how you do that if
your business entities are auto-generated from database tables.

As I said, my impression is that Microsoft is looking to Web Services
to save the day here: the business layer lives on the server side, and
delivers data to the client organized in business terms, no matter how
it happens to be stored in the database. The client can then accept the
data as an ADO.NET dataset, secure in the knowledge that the schema of
the dataset depends upon the wsdl (the Web Service contract), not the
organization of the raw data. Of course, the contract can always
change, but that's a business-level, logical change, not a data
reorganization.

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