Rockford Lhotka

 Friday, August 5, 2005
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Ted Neward believes that “distributed objects” are, and always have been, a bad idea, and John Cavnar-Johnson tends to agree with him.

 

I also agree. "Distributed objects" are bad.

 

Shocked? You shouldn’t be. The term “distributed objects” is most commonly used to refer to one particular type of n-tier implementation: the thin client model.

 

I discussed this model in a previous post, and you’ll note that I didn’t paint it in an overly favorable light. That’s because the model is a very poor one.

 

The idea of building a true object-oriented model on a server, where the objects never leave that server is absurd. The Presentation layer still needs all the data so it can be shown to the user and so the user can interact with it in some manner. This means that the “objects” in the middle must convert themselves into raw data for use by the Presentation layer.

 

And of course the Presentation layer needs to do something with the data. The ideal is that the Presentation layer has no logic at all, that it is just a pass-through between the user and the business objects. But the reality is that the Presentation layer ends up with some logic as well – if only to give the user a half-way decent experience. So the Presentation layer often needs to convert the raw data into some useful data structures or objects.

 

The end result with “distributed objects” is that there’s typically duplicated business logic (at least validation) between the Presentation and Business layers. The Presentation layer is also unnecessarily complicated by the need to put the data into some useful structure.

 

And the Business layer is complicated as well. Think about it. Your typical OO model includes a set of objects designed using OOD sitting on top of an ORM (object-relational mapping) layer. I typically call this the Data Access layer. That Data Access layer then interacts with the real Data layer.

 

But in a “distributed object” model, there’s the need to convert the objects’ data back into raw data – often quasi-relational or hierarchical – so it can be transferred efficiently to the Presentation layer. This is really a whole new logical layer very akin to the ORM layer, except that it maps between the Presentation layer’s data structures and the objects rather than between the Data layer’s structures and the objects.

 

What a mess!

 

Ted is absolutely right when he suggests that “distributed objects” should be discarded. If you are really stuck on having your business logic “centralized” on a server then service-orientation is a better approach. Using formalized message-based communication between the client application and your service-oriented (hence procedural, not object-oriented) server application is a better answer.

 

Note that the terminology changed radically! Now you are no longer building one application, but rather you are building at least two applications that happen to interact via messages. Your server doesn't pretend to be object-oriented, but rather is service-oriented - which is a code phrase for procedural programming. This is a totally different mindset from “distributed objects”, but it is far better.

 

Of course another model is to use mobile objects or mobile agents. This is the model promoted in my Business Objects books and enabled by CSLA .NET. In the mobile object model your Business layer exists on both the client machine (or web server) and application server. The objects physically move between the two machines – running on the client when user interaction is required and running on the application server to interact with the database.

 

The mobile object model allows you to continue to build a single application (rather than 2+ applications with SO), but overcomes the nasty limitations of the “distributed object” model.