Monday, May 17, 2004

 

Models

What is a model? The dictionary defines a model to be a miniature representation of something; also: a pattern of something to be made. It is common engineering practice to use models to facilitate the definition and design of a system being built. Booch points out that; unlike theorems, models are neither right nor wrong. Instead, models are judged to be more or less useful to the engineering process.

Why do we model? Models allow us to focus on some aspects of a structure while ignoring others. For example, a schematic lets us focus on the electronic properties of a circuit and ignore the physical layout of the components. When modeling requirements we want to ignore the implementation – concentrate on the “what” and ignore the “how”. In design we focus on the architecture, ignoring the yet to be implemented code.

By suppressing unwanted details, models make a system easier to understand. Because we are humans our models are often expressed in language or as drawings. This facilitated communication and discussion of proposals, which is another reason that we model. Diagrams, especially, make it possible to visualize a system. Simulations and prototypes allow us to experience a system before we have created a complete construction.

But models can do more than focus our attention or allowing us to visualize a system; in some cases the model provides a framework for thinking and reasoning about the system. A heat flow model of an electronic circuit allows quantitative evaluation of the thermal parameters associated with the component. UML Sequence Diagrams can be used to show precise timing among a set of system events.

Should we model the problem space or the solution space? Up to now Computer Aided Software Engineering (CASE) has focused on modeling the solution space. UML is used primarily to model software elements – classes, objects, interactions, states, etc. Model Driven Architecture (MDA) emphasizes transformations from one model to the next, presumably eventually arriving at a source code model. But it is unclear how (or even if) the MDA approach can cross that all-important chasm between models of the problem and models of the solution.


Comments:
Hello,

I'm a 25 year old up and comming developer, working with a small company while I'm going to school.

One of the things my manager is trying to instill in me is the importance of modeling. He insists on "blueprinting" the application before I write a line of code. We're using the Rational Unified Process to develop our product.

Now understand that I have come from a Microsoft Access/Visual Basic 6 background. The desire to just code first is incredible. Development Lifecycle models like "Agile" or "eXtreme Programming" sound extremely appealing.

I'm learning to like UML and designing before I code.

I was curious how you felt about it? Good/Bad/Indifferent?

Would you be willing to give me a little advise on making the long journey from apprentice coder to software architect?

Joshua
(joshua@hayworth.com)
 
Hello, you've been Scobleized (http://radio.weblogs.com/0001011/2004/06/06.html#a7712), so expect more visitors!  I'm grateful for the connection.  Your essay on models fits perfectly with a conversation about models that I have been having, including a little demonstration of abstraction on the Numbering Peano blog (http://miser-theory/astraendo/pn/).  Thanks.
 
Hi Joshua,

25 as well :D
There is nothing wrong with the desire to code first. This is a misinterpretation of your manager and I’m expecting your manager is steering you in the “design” direction for his own understanding of the application being build. Or to not let you “drive” on your own. You see, it doesn’t matter if you write code or model first. Both a model and its implementation capture knowledge. I would restrict you to write Unit-Test and Mock objects first, just to make sure you don’t get carried away to much and gain solid understanding of what you are about to build. There’s nothing wrong with prototyping small parts of the application you aren’t feeling to confident about. This way you identity the bottlenecks in an early stage and it gives you the opportunity to find a simpler solution or consultancy in that particular area. Remember that the model is your backbone. It lets you and your team communicate using the same language. So at the end of the day you should be able to bring your work back into the model. You use that model the next day to “model out loud” and take any new ideas back down to your model diagrams and code. No big design upfront but small iterations, model incarnations and working software at the end of the day.

Buy this book http://www.domaindrivendesign.org/book/ and subscribe to the mailing list.

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