Lifelines represent parts (or references) of the block that owns the interaction. The following list highlights key aspects of interactions: The most significant source of occurrences is the exchange of messages between lifelines, which may trigger executions. When an interaction executes, it evaluates the set of event occurrences generated by instances of its lifelines and determines whether they are valid. An interaction is specified using occurrence specifications, which are organized into a hierarchy, and ordered by interaction operators. Sequence diagrams describe interactions, which are used to capture system scenarios as a set of specified occurrences across several parts of the system, represented by lifelines. At the same time, such a transformation may be used for a formalization of Sequence Diagrams in UML since MSC has a formal semantics based on process algebra. Such a transformation demonstrates the big advantage of MSC concerning composition mechanisms, particularly, in comparison with the rather obscuring branching constructs in Sequence Diagrams. It is argued that Sequence Diagrams can be transformed into MSC diagrams if some enhancements of MSC are introduced. In this context, it has to be noted that the acceptance of a language in the OO community essentially depends on a clear visualization of constructs typical for OO modelling. On the other hand, the application area of MSC might be considerably enlarged. A more formal and powerful notation for Sequence Diagrams may arise, on the one hand. Both notations would benefit from a harmonization. They can be seen as OO variants of the ITU-T standard language Message Sequence Chart (MSC) which is very popular mainly in the telecommunication area. Sequence Diagrams as part of UML play an important role within use case driven object oriented (OO) software engineering. Towards a Harmonization of UML-Sequence Diagrams and MSCĮkkart Rudolph.
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