Chapter 3
The receipt of the message at passenger services initiates activities, which is indicated by
the gray vertical bar (7). The diagram does not show how passenger services handle the
process, meaning that it does not show which activities are conducted:
Figure 3.23 Sequence diagram "Passenger Check-In"
Only the comment (5) can include a clue. Comments can be inserted at the left margin of
the sequence diagram. An exact description of the processing can be found in the activity
diagram 'passenger checks in' (see Figure 3.21 above).
In a final step, passenger services
issues (8) a boarding pass (9) to the passenger. With
that, the interaction that is illustrated in this sequence diagram is completed for both
parties. This is indicated by the end of the wide gray vertical bar (10).
In the business model we do not utilize all the options of the sequence diagram. UML
provides many more possibilities for this diagram type, but our experience showed that
this is sufficient to communicate the essential aspects.
3.3.8 Constructing Sequence Diagrams
The following checklist shows the necessary steps for the construction of sequence
diagrams. Subsequently, we will further explain the individual steps.
Checklist 3.5 Constructing Sequence Diagrams in the External View :
Designate actors and business system—Who is taking part?
Designate initiators—Who starts interactions?
Describe the message exchange between actors and business system—Which
messages are being exchanged?
Identify the course of interactions—What is the order?
Insert additional information—What else is important?
Verify the view—Is everything correct?
Modeling Business Systems
Designate Actors and Business System—Who is Taking Part?
Sequence diagrams illustrate the interactions between actors and the business system.
Fundamentally we have a pool of interaction partners from the use case diagrams.
Depending on the flow that is being depicted in the sequence diagram, the appropriate
actors and business systems can be selected from this pool.
In our case study (see Figure 3.24), we find the interaction partners passenger and
passenger services for the above sequence diagram (Figure 3.23):
Figure 3.24 Constructing sequence diagrams
Designate Initiators—Who Starts Interactions?
For every sequence of interactions the actor who starts the interaction has to be identified.
This actor is called the
initiator. Since in the external view of the business model each
business use case is initiated by an actor, we can here also select the actor from the pool
of actors in the use case diagrams.
In our sequence diagram passenger check-in, the passenger starts the interaction by
utilizing the service check-in from passenger services.
Chapter 3
Describe the Message Exchange between Actors and the
Business System—Which Messages are being Exchanged?
After the initiator has been defined, the subsequent progression of interactions has to be
identified. For each communication step it has to be determined what information is
exchanged. In this way the message will be defined. Messages are requests to do
something directed toward a particular partner. The business objects that are exchanged
with these messages also have to be defined.
Identify the Course of Interactions—What is the Order?
All messages are exchanged in a chronological order that has to be identified.
Messages are inserted along the
y-axis in increasing chronological order, from top to
bottom (see Figure 3.25):
Figure 3.25 Constructing sequence diagrams
Insert Additional Information—What Else is Important?
Important activities of involved actors and business systems and important conditions can
be inserted into the diagram as comments. Comments are inserted at the level of the
appropriate message. Restrict this to important comments that have significance so that
the diagram is not overcrowded with text (see Figure 3.26):

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