I am looking for example of source code (preferably open source)
which
exhibits a good design pattern to break up my functionality into 3
layers, a presentation layer, a business layer, and a data layer.
Actually, I have not encountered any distributed application accessing
a database that does /not/ follow this design pattern (3-tier).
However, this is a *pattern*, not a *framework* and example code will
give you limited help only --- like technical things on how to realize
database connectivity. There will be oh-so-many design decisions that
depend on your concrete task at hand and you do not see that easily
from other people's code. For example you will not see alternative
approaches considered for any detail and the reasons why the
implemented ones were favored over the alternatives.
I would recommend studying a book, my over the top favorite on this
topic is Martin Fowler, "Patterns of Enterprise Application
Architecture", Addison-Wesley (2003). A major part of it covers your
problem in-depth, in very understandable language. The example codes
given in the book are C# and Java but it should be very easy to adapt
to C++ --- it is dealing with patterns, not implementations.
Mostly, I am looking for a data layer API which is able to deal with
data stored either in a file system like that used by dBase (v5) or
[MS] SQL layout.
I am writing my code in C++!
C++ does not offer any database access as part of the standard. You
most likely will have to resort to platform specific libraries. ODBC
is reasonably widely available, but feels too low level --- you'd
still need to embed SQL statements into your code and given the
miserable state of interoperability of current databases at SQL level,
I cannot really recommend it.
Currently I am working with Microsoft's Entity Framework giving me an
abstraction layer for any data tier with an ADO.NET adapter (eg. SQL
Server, Oracle, XML, and some more). This would limit you to .NET
platform, and C++.NET is not standard C++.
Maybe some other friendly person in this news group can hint to a C++
equivalent of Java's hibernate or Entity Beans frameworks.
Right now, I am trying to build the functionality to hide the data
representation
I am getting into lots of problems deciding which data should go into
which layer.
I usually start with a UML domain model, analyzing the data my
application needs to deal with at real world level. A car is a car in
this view and not a join on many tables where one is representing
wheels and others welding connections. The domain model purposely
represents my data denormalized. I next derive two completely
different refinements from this, first the database model (normalizing
the data, selecting database specific datataypes) and the data object
model, i.e. how I like the data be represented as class instances in
my application (using pointers, native and STL datatypes).
How these two interact is a complex design decision, read Martin
Fowler's book mentioned above.
Most of the time I want to have a database layer that deals with data
strictly in database format and offers a generic interface to the
upper layer. This will be an implementation specific to the DB
management system and you'd need to create one for SQL Server, one for
dBase and one for each DB you might want to add on top. On the other
side I do a business layer that does not care about how data is stored
in any DB and acts only on the data object model.
So actually, I put most of the involved data into my designs *twice*
(actually, three times, I do the same approach for GUIs, defining a
user visible representation of my data, independent of the business
tier, see Model-View-Controller [MVC] pattern).
So its not really a question of what data to put where but rather how
to communicate the different representations of the same data between
layers.
When querying Google, I see lots of articles long on top level design
details, but very little source code.
Looks like most other people feel the same about this topic as me
[..]
Can anyone point me to a URL meeting these criteria.
This newsgroup has some smart people, so if you know of another group
more appropriate to my question, please redirect me.
I am sorry I cannot be more specific, but I really feel specifics
would do you no good anyway.
best regards,
MiB
