Getting the categories and classes distinction right is one of the key drivers of the cost of traditional systems.
We’ve been working with two clients lately, both of whom are using an ontology as a basis for their SOA messages as well as the design of their future systems. As we’ve been building an ontology for this purpose we became aware of a distinction that we think is quite important, we wanted to formalize it and share it here. In an ontology there is no real distinction that I know of between a class and a category. That is: classes are used for categorizing, and you categorize things into classes. If you wanted to make a distinction, it might be that category is used more in the verb form as something you do, and the class is the noun form.
Categories and Classes in Traditional Apps
But back in the world of traditional applications there is a quite significant difference (although again I don’t believe this difference has ever been elaborated). In a traditional (relational or object oriented) application if you just wanted to categorize something, (say by gender: male and female) you would create a gender attribute and depending on how much control you wanted to put on its content you
would either create an enum, a lookup table or just allow anything. On the other hand if you wanted behavioral or structural differences between the categories (let’s say you wanted to distinguish sub
contractors from employees) you would set up separate classes or tables for them, potentially with different attributes and relationships. We’ve been studying lately what drives the cost of traditional systems,
and getting this category/class distinction right is one of the key drivers. Here’s why: in a traditional system, every time you add a new class you have increased the cost and complexity of the system. If you
reverse engineer the function point methodology, you’ll see that the introduction of a new “entity” (class) is the single biggest cost driver for an estimate. So every distinction that might have been a class, that
gets converted to a category, provides a big economic payoff. It’s possible to overdo this. If you make something a category that should have been a class, you end up pushing behavior into the application code, which generally is even less tractable than the schema. So we were interested in coming up with some guidelines for when to make a distinction a category and when to make it a class.
Categories and Classes in gist
As it turns out, we had foreshadowed this distinction, although not for this reason, in gist, our upper ontology. Gist has a class called “category” whose intent is to carry categorical distinctions (from one lower level ontology to another) without necessarily carrying their definitions. For instance when we worked with a State Department of Transportation, we had a class in their enterprise ontology called “Roadside Feature.” A Roadside Feature has properties such as location and when by what process it was recorded. Several of their applications had specific roadside features, for instance “fire hydrants.” In the application fire hydrant is a class, and therefore is one in the application ontology. But in the enterprise ontology “fire hydrant” is an instance of the category class. Instances of fire hydrant are members of the roadside feature class at the enterprise ontology level, and associated with the category “fire hydrant” via a property “categorizedBy.” A fire hydrant can therefore be created in an application and communicated to another application that doesn’t know the definition of fire hydrant, with almost no loss of information. The only thing that is lost on the receiving end is the definition of fire hydrant, not any of the properties that had been acquired by this fire hydrant.
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