On 26 Nov 2003 01:55, loverde wrote:

>As strange as it sounds, I think that what is really needed in this type
>of layered architecture is yet another mapping framework.  (And by
>"needed", I really mean what would be nice.)

CarrierWave (http://carrierwave.sourceforge.net/) is a framework for
that purpose. It is a mix of the DTO, DTO Assembler and various other
patterns. Basically, it decouples the client view of data from the
server implementation of that view. What you get is the ability to
transfer arbitrary object graphs from the server to the client and, much
more exciting, send that graph back to the server after you modify the
objects client-side. Optimistic concurrency checking (versions) is done
on the server side to ensure transaction isolation.

CarrierWave also has nice features that help separating the client
developers from the server developers: On the server side, you just
create "Finders", "Actions" and mark the parts of your domain model
"Imageable" if you want to allow client side copies/retrieval.

The client developers uses this clear interface to program his data
binding logic, that is, what Finders he calls and what objects (copy of
objects, called "Images") he can retrieve and work with.

CarrierWave generates all this for you, the DTO ("Image") classes and
everything, you just mark up your domain model with XDoclet like tags.

The problem is: CarrierWave is not mature. It has been used as an
in-house framework and was made open source later, so it still has a
certain complexity and warts that you wouldn't find in a clean room
implementation of the patterns.

In my opinion, it is currently the best way to have distributed,
multi-layered systems with a rich domain model. Have a look.

(BTW, the new SDO stuff is basically the same, just not as powerful.)