Designing Vs Modeling

These days (1999) I distinguish between Modeling and Designing. I say

• "Modeling" is describing something you know. A good model makes correct assertions.
• "Designing" is manipulating space for things you don't know. A good design e.g., lets you change things easily (there are many dimensions of 'goodness').

You take a drawing of the Model into the room full of business and database and other experts. They critique it to see whether it contains any FalseAssertionsAndUnattractiveRestrictions regarding their business.

--AlistairCockburn

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"Designing" for me is manipulating space for things you know for sure (i.e. you have running automated tests for). --KentBeck

I love you - you do all designy things the reverse from me! -- Alistair (in a cheery mood on a sunny day.)

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Just as a good design lets you change things easily, so does a good modeling language. From InformationModelingAndRelationalDatabases by TerryHalpin, the following criteria provide a basis for evaluating conceptual modeling languages:

• Expressibility
• Clarity
• Simplicity and orthogonality
• Semantic stability
• Semantic relevance
• Validation mechanisms
• Abstraction mechanisms
• Formal foundation

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A design is a model without any implementation-specific or technology-specific details. A model is a design with those details.

Programmers are prone to seeing a spectrum of detail between designs and models, but this is false. There is a very sharp distinction between the two determined by the fact that users of a system only ever see its design whereas implementors are most interested in the model details. The ExplicitUserModel of a system is its design.

Hmm. An architecture is a description (with visuals, perhaps) of a system containing modeling information, but lacking implementation or (sometimes) technology-specific details. A design is one solution to an architecture that fits technology to the system. The code is one particular implementation of the design that expresses a particular technology with specific details. This is the normal descent of granularity in designing. Eh?

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