This page contains Chapter 6 of the paper "Defining Business Rules ~ What Are They Really?", produced by the Business Rules Group. Other formats in which this paper is available are described in Defining Business Rules -- What Are They Really? (Abstract & Table of Contents)

The subsections in this chapter are:

Defining Business Rules ~ What Are They Really?

Chapter 6 - Derivations

Figure 13 shows the business rules model of derivations. In this view, a derived fact is shown as a kind of fact, along with base fact.

Each derived fact must be derived using one derivation. The derivation, in turn, must be based on one or more business rules. In other words, a derivation also must be used to derive at least one and possible more derived facts. However, a derived fact may be treated just like any other fact. Its derivation is not visible when the fact is referred to.

Derived Facts and Derivations

Figure 13: Derived Facts and Derivations

Kinds of Derivation

A derivation is itself a kind of business rule, and it may be either a mathematical calculation (such as 'charge rate multiplied by hours') or an inference (such as the fact that the time sheet entry's 'charge rate' is in fact the corresponding person's 'charge rate.'

Note, by the way, that implicit in the determination of which is a base fact and which is a derived fact are assumptions about the order in which information comes available. Here we assumed that the 'charge rate' and 'hours' were known before the 'total cost.' In a different environment, the reverse could be true. It is the job of the analyst to consider these factors and make this judgment.

An example of how derived facts can appear on a conceptual model is shown in Figure 14. Base facts shown include the presence of 'insurance rate' as an attribute of 'Insurance Coverage,' 'rental rate' as an attribute of 'Car Group,' and so forth.

Derived Facts

Figure 14: Derived Facts

For the car rental business, the derivations of derived facts can be specified as follows:

  • Derivation / Inference
    • The 'insurance rate' in Rental is inferred from the 'insurance rate' of the Insurance Coverage for the Rental that is the user of that Coverage, through a many-to-one relationship.

    • The 'rental rate' in Rental is inferred from the 'rental rate' of the Car of that the Rental is of, through a many-to-one relationship. This in turn is inferred from the 'rental rate' of the Car Group that the Car is an example of.

    • The 'late rate' in Rental is inferred from the 'late rate' of the Car that the Rental is of, through a many-to-one relationship. This in turn is inferred from the 'late rate' of the Car Group that the Car is an example of.

  • Derivation / Mathematical Calculation
    • The 'insurance amount' in Rental is calculated from the Rental 'insurance rate' multiplied by its 'number of days.'

    • The 'rental amount' in Rental is calculated from the Rental 'rental rate' multiplied by its 'number of days.'

    • The 'late charge' in Rental is calculated as the difference between the 'due date, time' and the 'actual date, time' multiplied by the 'late rate.'

    • The 'total cost' of the Rental is calculated from the sum of its 'insurance amount,' 'rental amount,' and 'late charge.'

In each case, the formula involved is an occurrence of derivation that is based on one or more business rules, and used to calculate one derived fact.

Definitions

The following summarizes the definitions of the concepts relating to Derivations .

Base Fact
a Fact that is given in the world and stored.
Derivation
an algorithm used to compute or infer a Derived Fact.
Derived Fact
a Fact whose value is created by an Inference or a Mathematical Calculation from Terms, Facts, other Derivations, or Action Assertions.
Inference
a Derivation that produces a Derived Fact using logical induction (from particulars) or deduction (from general principles).
Mathematical Calculation
a Derivation that produces a Derived Fact according to a specified mathematical algorithm.

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