A use case realisation is an expression of a use
case??™s flow of events in terms of objects in the system. In UML, this event flow is expressed
with sequence diagrams.
Definition 15: A sequence diagram is a diagram that shows object interactions arranged
in time sequence. In particular, it shows the objects participating in an interaction and the
sequences of messages exchanged. (Rumbaugh, Jacobson & Booch, 2005, p. 585)
A sequence diagram can realise a use case, describing the context in which the invocation
of the use case executes. The sequence diagram describes the objects that are created, ex-
Figure 5. Structural elements
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ecuted, and destroyed in order to execute the functionality described in the use case. It does
not include object relationships. Sequence diagrams provide a critical link of traceability
between the scenarios of the use cases and the structure of the classes. These diagrams can
express the flow in a use case scenario in terms of the classes that will implement them.
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