Re: beginner - stuck on data storage
From: Universe (no email)
Date: 10/30/04
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Date: Sat, 30 Oct 2004 03:52:18 -0400
"Kavvy" <kav@kav.kav> wrote:
> I've recently completed the UK Open University course M206 (Computing and
> Object Oriented Approach) which uses Smalltalk and a take on UML to teach OO
> design and programming.
>
> Upon putting my new skills to practice I've hit upon a problem that the
> course never got as far as explaining. How do you actually manage the
> storage and retreval of objects to make them persist. I'm using SQL and C#,
> and have got as far as guessing that the way forward is some kind of class
> that manages the actual data access to SQL.
>
> However, I'm not sure if I'm supposed to have one class that handles it all
> and then each class uses that. Or do you implement data access into each
> class that requires it?
In almost *any* paradigm you want a *set* of database objects in layer that sites between your OO
domain model and relational database.
Often you want at least 1 class per table. However you may want to create objects that combine data
from multiple objects or that does executes a stored procedure hidden from your OO domain model.
Your OO domain model might send the object data via method arguments, the object runs a stored
procedure, and then your OO domain model uses returned data.
This strategy means for nearly *any* technology - JEEE, CORBA, .Net - that your app will have at
least 3 layers:
Interface
----------
Domain OO model
--------
Relational database
For 3 or n-tier using M$ technology check this out at:
http://www.microsoft.com/downloads/details.aspx?familyid=6eb67f59-d075-4b28-b3c4-e9294ff3cf4a&displaylang=en
*****************************
Visual Basic .NET Code Sample: N-Tier Data Form and Data Layer
This sample shows you how to build an N-Tier data application. This demonstrates the use of a 3-Tier
system, using SQL Server or MSDE as a back end (accessing the Northwind database), a data access
layer middle tier, and a data-bound form as the front end.
Quick Info
File Name:
n-tierdataformanddatalayer.exe
Download Size:
103 KB
Date Published:
7/29/2002
Version:
1.0
************************************************
> Also :)
>
> Considering that the way I've been taught suggests that a system contains
> all the instances of objects that are currently created, this implies that
> you load all the previously created objects from your database upon
> execution of the application, and then save whatever objects are left at the
> end? If this is the case, then what is the best method to store an objects
> relationship with another?
Why load *all* instances at the start for *every* app.?
If you can speed things up by only loading 1 or a few then only load 1 or a few. No one's gonna
shoot you and I've never heard all instances should be created and loaded at run-time start. Some
do it for speed *IF* they have the RAM to hold it, or the disk space to hold a number of already
created, "rehydrated" object instances in a disk cache and thus avoid facing database processing and
object "rehydration" at later time.
Elliott
--
Theory Leads, Practice Verifies
Global Plans + Iterative/Incremental Development
Profiteer US Out of Iraq Now!
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