Domain requirements

Domain requirements are derived from the application domain of the system rather
than from the specific needs of system users. They Ulsually include specialised domain
terminology or reference to domain concepts. They may be new functional requireThe deceleration of the train shall be computed as:
Otroln- O_+ O _
where Oplientis 9.81 ms2
• compensated gradient/alpha and where the values of
9.81 ms2/alpha are known for different types of train.
ments in their own right, constrain existing functional requirements or set out how
particular computations must be carried out. Because these requirements are specialised,
software engineers often find it difficult to understand how they are related
to other system requirements.
Domain requirements are important because they often reflect fundamentals of
the application domain. If these requirements are not satisfied, it may be impossible
to make the system work satisfactorily. The LIBSYS system includes a number
of domain requirements:
I. There shall be a standard user interface to all databases that shall be based on
the Z39.50 standard.
2. Because of copyright restrictions, some documents must be deleted immediately
on arrival. Depending on the user s requirements, these documents will
either be printed locally on the system server for manual forwarding to the user
or routed to a network printer.
The first requirement is a design constraint. It specifies that the user interface to
the database must be implemented according to a specific library standard. The developers
therefore have to find out about that standard before starting the interface design.
The second requirement has been introduced because of copyright laws that apply
to material used in libraries. It specifies that the system must include an automatic
delete-on-print facility for some classes of document. This means that users of the
library system cannot have their own electronic copy of the document.
To illustrate domain requirements that specify how a computation is carried out,
consider Figure 6.7, taken from the requirements specification for an automated train
protection system. This system automatically stops a train if it goes through a red
signal. This requirement states how the train deceleration is computed by the system.
It uses domain-specific terminology. To understand it, you need some understanding
of the operation of railway systems and train characteristics.
The requirement for the train system illustrates a major problem with domain requirements.
They are written in the language of the application domain (mathematical equations
in this case), and it is often difficult for software engineers to understand them.
Domain experts may leave information out of a requirement simply because it is so
obvious to them. However, it may not be obvious to the developers of the system,
and they may therefore implement the requirement in the wrong way.