Page 92 - Software and Systems Requirements Engineering in Practice

62 S o f t w a r e & S y s t e m s R e q u i r e m e n t s E n g i n e e r i n g : I n P r a c t i c e

A The Bank A Cash
Customer A Teller A Check Computer System Drawer
Please cash this check
Please show ID
Here is my ID
Verify ID
Here is your
ID back
Get check info
Check info
Cash a check
Okay
Get money (amount)
Money
Money
FIGURE 3.12 Detailing a use case with a scenario diagram

3.4 Customer-Specific Business Rules
Business rules are a special category of customer requirements. They
are different in that rather than defining a fixed customer need, they
describe the implementation of a customer policy that may be changed
by the customer after delivery of a product or system. Hence they
describe a special category of user-implemented extensibility.
A business can enact, revise, and discontinue the business rules
that govern and guide it. A business policy is an element of governance
that is not directly enforceable, whose purpose is to guide an enterprise.
Compared to a business rule, a business policy tends to be less
structured; i.e., less carefully expressed in terms of a standard
vocabulary and not directly enforceable. For example, a banking
business policy might be: “Bank customers should not be able to
make too many bank withdrawals in a single day or withdraw more
than a certain amount of money in a fixed period of time; the
maximum amount being based on their total account value and
history.”

Why Are Customer-Specific Business Rules Important?
Customer-specific business rules must be kept separate from regular
requirements (at least logically, using database tags or attributes),
since they are not requirements. However, customer requirements
can be derived from the business rules; the requirements may look
different than the rules that they derive from.

What Are Their Characteristics?
Customer-specific business rules are implementations of the customer’s
company policies, where the business rules may change after system