Re: OOD/OOP and databases

From: Nick Malik (nickmalik_at_hotmail.nospam.com)
Date: 09/14/04 

Date: Tue, 14 Sep 2004 08:07:44 GMT

Buschmann: Pattern Oriented Software Architecture
http://www.amazon.com/exec/obidos/tg/detail/-/0471958697/qid=1095148535/sr=1-1/ref=sr_1_1/104-0692560-9975161?v=glance&s=books

Microsoft Patterns and Practices: discussion of the Layers pattern (Not an
application block!)
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnpatterns/html/ArcLayeredApplication.asp

GoF is never easy. But it is much more comprehensive than the DP Explained
book.

Now: for the follow-up:
If you view the interaction with the domain model as a series of messages,
then it's not too difficult to seperate the View from the Model.

In other words, you wouldn't need to warn the user that they cannot add
contact information on the unsaved user record, since the act of adding
contact information would necessarily trigger a request to send the user
record to the domain model. Either that, or you don't prevent the user from
adding contact info to an unsaved record. (Why NOT add contact info to an
unsaved user? The most common reason I hear is when folks are using
Identity columns as their primary keys in the Database... but that's easy to
work around... use GUIDs. Since the app can generate a unique id, there is
no need to create a transaction to write a record just to get the joining
data value back. Just create the Guid, use it as the primary key in one
message, and the foreign key in another. No problems and no need to
round-trip one transaction before generating another).

That goes back to my comment about creating a services interface between the
front end and the domain model. It really forces you to decouple your
system.

Good luck and I hope this helps,

--- Nick

"Thomas Jespersen" <tje@nospam.mentum.dk> wrote in message
news:eTrwrSdmEHA.3520@TK2MSFTNGP11.phx.gbl...
> Hello Nick
>
> Thanks for you input.
>
> First... Yes, you are right, I haven't read the GoF book yet. I thought I
> start with something a bit lighter which Design Patterns Explained was (a
> great introduction to OOD though). Is it readable for a .NET guy?
>
> Buschmann... never heard of him (embarrassed smiley here). I took a look
on
> Amazon... can you recommend a specific book?
>
> I read a lot from the Microsoft Patterns & Practices group. I like the
> Application Blocks, although I don't really see them as a design guide....
> they are just... well Application Blocks.
>
> Regarding my problem about separating the Business and the UI. I agree
that
> business rules is much more than validation. But I often find that all
kind
> of rules are need in the UI. E.g. which menu items is shown, warnings and
> other things to make sure that exceptions doesn't occurs when calling the
> Domain Model. This might sound odd, but e.g. I might have a rule that I
> can't call Customer.AddContactInformation, if the customer is inactive or
> unsaved. So I'd make a Property on the Customer called
> AddContactInformationAllowed. I use this property to decide whether a
> certain menu option is available, and/or I call this before
> AddContactInformation, to make sure that it doesn't throw an exception.
>
> For me this kind of interaction is a must to make a usable client
> application (which is what's count in the end... at least for me), but I
> don't se much of this kind in the OO books. But I will consider splitting
up
> the Domain Model in two (on for validation like things, and one for hiding
> business rules)... I'm just not quit sure that this is doable, if related
> responsibilities is to be grouped on the same object.
>
> Thomas
>
> "Nick Malik" <nickmalik@hotmail.nospam.com> wrote in message
> news:7MG0d.417794$%_6.111013@attbi_s01...
> > Yes... that's a lot for a newsgroup.
> >
> > Start here:
> > http://www.microsoft.com/resources/practices/default.mspx
> >
> > I, too, have read many of the same books. You didn't mention the Design
> > Patterns book itself (Gang of Four: Gamma, Helm, Johnson, and Vlissides)
> but
> > it, too, is an excellent volume. There are also a number of patterns
> books
> > that describe code in C# that have been coming out in the past two
years.
> >
> > The books do not focus on a particular tier. The Design Patterns books,
> > specifically, focus on a specific level of abstraction. The Fowler book
> > focuses on a different level of abstraction. Buschmann takes on the
> > architectural level. Honestly, to answer your questions, Buschmann is
the
> > best of the group.
> >
> > The Microsoft site has some very specific advice for the .NET platform
> with
> > regards to optimizations... worth looking into.
> >
> > As for me: my middle tier is normally tailored to one thing: hiding the
> > business rules. That requires real analysis: what are the business
> rules...
> > what will change... how will they change... how will we maintain them.
> >
> > Config files are not good for business rule changes... they are too
> > tactical. You can change the name of a stored proc in the config file.
> To
> > change the business rules, you need to encapsulate logic, not just
> > constants. Using pluggable components will only get you so far.
> >
> > So, if the business layer has very few business rules, it's really thin.
> > For many purposes, especially for domain tables (lookup tables), I don't
> > have any logic at all in the middle tier. In that case, I often simply
> pass
> > the data through directly from the DAL to the UI layer as an array of
> > DataRows. I'll even occasionally bind directly to a control (not often,
> but
> > for drop-downs, it's good). On the other hand, if the business layer
has
> > business rules to encapsulate, then I will organize the objects so that
> > managing those rules makes sense. This is where we start to deviate
from
> > "business objects that mirror the database". This is where the design
> > patterns come in handy. You really have to map your data into objects
> that
> > isolate change and minimize complexity.
> >
> > I've yet to find a tool or framework that does more than simply
> > copy-and-reflect operations from the database through the middle tier.
I
> > don't use them. WHat a massive waste of time and processing power.
> Heck...
> > the DataSet object does a better job than most of these, and it doesn't
> > require a huge amount of effort to use! (I don't use it either... I'm
> just
> > making a point).
> >
> > As for copying the validation rules: For a thick client app, I'd have
the
> > same validation dll on both the client and the server. That way, you
can
> > insure common rules. Note: business rules are much more than
validation.
> > I'd have basic validation types that I can use in both places, but the
> rest
> > of the rules are quite different.
> >
> > As for use of remoting: depends on the kind and style of scalability you
> > want to achieve. For my preference, remoting is too tightly coupled for
> the
> > kinds of things that I do... I tend toward web services... but that may
> not
> > be advice that you can use... depends on your app.
> >
> > I hope this info is helpful.
> > --- Nick Malik
> > Application Architect
> > MCSD, CFPS, Certified Scrummaster
> > "Thomas Jespersen" <tje@nospam.mentum.dk> wrote in message
> > news:ecFEdt%23lEHA.2880@TK2MSFTNGP14.phx.gbl...
> > > Hello
> > >
> > > I've been reading a lot of great OOD/OOP books lately (e.g.. Martin
> > Fowlers
> > > UML Distilled, Patterns of Enterprise Application Architecture,
> > Refactoring,
> > > Kent Becks's Test Driven Development, and also a book called Design
> > Patterns
> > > Explained.)... al of them with the focus on Java, because I think the
> best
> > > books are targeted Java. But the all fit very well to the .NET
platform.
> > >
> > > I have one big problem when reading these great books, and that is
that
> > they
> > > focus almost entirely on the Domain Model (that is the Middle Layer in
a
> 3
> > > layered architecture). They hardly mention that the code you write is
to
> > be
> > > used in a UI interface and committed to a database ([PoEAA] might be
an
> > > exception). I find the separation of Business Login and UI to be the
> > hardest
> > > thing. And also I find the performance issues when talking to the
> Database
> > > to a very tricky thing, that calls for a few hack in the Domain Layer
> (I'm
> > > almost only doing thick clients... that is no web).
> > >
> > > So I would like to know how you OO Guys really work (references to
books
> > or
> > > articles would be great).
> > >
> > > Don't you design your database the same way that you design you
Business
> > > Model if you are responsible for all 3 layers/tiers?
> > >
> > > How much time does you spend on writing the DAL (the mapping between
the
> > OO
> > > and the Database)? Do you use any kind of O/R Mappers to help you
write
> > the
> > > DAL?
> > >
> > > Where do you place the tiers in a Smart Client configuration? The way
I
> > see
> > > it the UI and Domain Model must be on the client. Because you Domain
> Model
> > > contains a lot of information e.g. about validation the input, and I
> don't
> > > want a Server Round Trip every time the users enters something in a
> > textbox.
> > > But I'm also thinking about putting the same domain objects on the
> server
> > > aswell and use some kind of serialization using .NET Remoting or Web
> > > Services. Anyone else placing the Domain Model on both the Client and
> > > Server?
> > >
> > > Can you recommend any third party frameworks, which addresses any of
> these
> > > problems? I find many of the MS Reference application to be to exotic.
> > >
> > > I know these are big questions for a thread in a newsgroup... but
never
> > the
> > > less, I haven't found any books or articles that really deals with
these
> > > issues.
> > >
> > > Best regards,
> > > Thomas
> > > MCSD, MCSE+I, MCP+SB
> > >
> > >
> >
> >
>
>

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