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Chapter 3. Black-box thinking: Defining the system context 55
Identifying operations
Here we discuss operation identification by using sequence diagrams.
MDSD Step 7: Operation identification
Operation identification involves the use of a sequence diagram. Sequence
diagrams show the same flow of events described in a use case, but use a very
specific format and method to show them. The flow of events of each use case is
shown as a series of interactions, more specifically requests from one entity to
another. The use case is carried out as entities makes requests of one another.
We create two kinds of sequence diagrams in MDSD—black-box and white-box.
In a black-box sequence diagram, only the entity and its actors are placed on the
diagram while in a white-box sequence diagram, multiple elements within the
entity are used in addition to the actors. For operation identification, we need only
a black-box sequence diagram. We will use white-box sequence diagrams later.
For each use case, draw a black-box sequence diagram with lifelines for the
entity and each of the actors involved in the main flow of that use case, or any of
its alternate flows. Then, following the flow of events in the use case, write a
sequence of requests that fulfill the use case. For example, consider the use
case commute to work mentioned before. The entity is the car. The flow of events
might initially be written as follows:
1. This use case begins when the driver approaches and unlocks the car.
2. The driver starts the car and allows it to warm up.
3. The driver drives the car to the work location.
4. And so forth...
We must
transform this plain language flow of events into a series of requests.
We do this by asking, for each step or set of steps in the use case, what request
is being made of the system do to something. Sometimes this takes a
combination of imagination and reading ahead in the use case to determine the
actual purpose of things.
In the example here, we might ask what request is being made in the first step.
By approaching the car, is the driver making some request of the car? It might be
tempting to draw this on a sequence diagram as an arrow from the driver to the
system, and label it as approach car but this is not correct (Figure 3-4).