Derived Properties

Derived properties can be calculated based on other values. When we think about a date range (Figure 5.5), we can think of three properties: the start date, the end date, and the number of days in the period. These values are linked, so we can think of the length as being derived from the other two values.

Derivation in software perspectives can be interpreted in a couple of different ways. You can use derivation to indicate the difference between a calculated value and a stored value. In this case, we would interpret Figure 5.5 as indicating that the start and end are stored but that the length is computed. Although this is a common use, I'm not so keen, because it reveals too much of the internals of DateRange.

My preferred thinking is that it indicates a constraint between values. In this case, we are saying that the constraint among the three values holds, but it isn't important which of the three values is computed. In this case, the choice of which attribute to mark as derived is arbitrary and strictly unnecessary, but it's useful to help remind people of the constraint. This usage also makes sense with conceptual diagrams.

Derivation can also be applied to properties using association notation. In this case, you simply mark the name with a /.