Databases Reference
In-Depth Information
STD can help the analyst dig out hidden business policies. Three ele-
mentary questions can be posed to discover those policies (see Table 3.2):
Can any of the events that appear in the STD be caused by other
events? For instance, the analyst may be intrigued to know what
causes a course to start. Distinct alternatives are possible, depending
on the organizations policy: The course can start at an established
date or once the number of students surpasses a given threshold. In
the latter case, the enrollment of a new student causes the beginning
of the course.
·
Can an event cause other events to happen?
·
What if an event does not happen? Does it matter? For instance,
what if not enough students (i.e., the threshold number of students
set for the course to take place) are registered in a reasonable period
of time? Is the course cancelled or postponed? What happens with
the already enrolled students? The analyst should address all those
questions and find the answers from the stakeholders.
·
This situation leads to the so-called nonevents [3]. The effect of one
event, such as enrollment, may prepare the environment for the next expected
event. Hence, when a course is approved, the system sets up a window of
expectation (e.g., a month) for the students to register, and then the course
begins. However, there is no guarantee that that will happen. That is, the
required number of enrollment events may not occur. Nonevents are often
overlooked and omitted, but they represent the clue to ascertaining impor-
tant business policies.
Table 3.2
Causal Business Policies
Effect
Cause
Kind of Cause
A course starts when
¼
the number of students is above 20.
State condition
A student is reassigned when
¼
the students course is cancelled.
Operational event
A course is cancelled when
¼
fewer than 20 enrollments are produced
in a month since the course was
approved.
Composite event
