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Interactions are used in a number of different situations. They are used to get a better grip of an interaction situation
for an individual designer or for a group that needs to achieve a common understanding of the situation. Interactions are
also used during the more detailed design phase where the precise inter-process communication must be set up according to
formal protocols. When testing is performed, the traces of the system can be described as interactions and compared with those
of the earlier phases.
The Interaction package describes the concepts needed to express Interactions, depending on their purpose. An interaction
can be displayed in several different types of diagrams: Sequence Diagrams, Interaction Overview Diagrams, and Communication
Diagrams. Optional diagram types such as Timing Diagrams and Interaction Tables come in addition. Each type of diagram provides
slightly different capabilities that makes it more appropriate for certain situations.
Interactions are a common mechanism for describing systems that can be understood and produced, at varying levels of detail,
by both professionals of computer systems design, as well as potential end users and stakeholders of (future) systems.
Typically when interactions are produced by designers or by running systems, the case is that the interactions do not tell
the complete story. There are normally other legal and possible traces that are not contained within the described interactions.
Some projects do, however, request that all possible traces of a system shall be documented through interactions in the form
of (e.g., sequence diagrams or similar notations).
The most visible aspects of an Interaction are the messages between the lifelines. The sequence of the messages is considered
important for the understanding of the situation. The data that the messages convey and the lifelines store may also be very
important, but the Interactions do not focus on the manipulation of data even though data can be used to decorate the diagrams.
In this chapter we use the term trace to mean sequence of event occurrences, which corresponds well with common use in the
area of trace-semantics, which is a preferred way to describe the semantics of Interactions. We may denote this by <eventoccurrence1,
eventoccurrence2, ...,eventoccurrence-n>. We are aware that other parts of the UML language definition of the term trace is
used also for other purposes.
By interleaving we mean the merging of two or more traces such that the events from different traces may come in any order
in the resulting trace, while events within the same trace retain their order. Interleaving semantics is different from a
semantics where it is perceived that two events may occur at exactly the same time. To explain Interactions we apply an Interleaving
Semantics.