This is achieved either by using distinct

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This is achieved either by using distinct documentation forms, such as mini-specifications in structured analysis, or by means of implicit language extensions of data flow models. Language extensions offer the ability to model additional aspects, e.g., the control flow between functions, as done in SA/RT [ Ward and Mellor 1985 , Hatley and Pirbhai 1988 ]. 6.6.3 UML Activity Diagrams UML activity diagrams are well suited to model action sequences [OMG 2007] . Along with activity diagrams in UML, event-driven process chains (EPC) can be used to model sequences of activities [Keller et al. 1992] , especially in information system development. UML activity diagrams depict the control flow between activities or actions. In case of a sequential progression of actions, a subsequent action is executed once every prece- dent action terminates . Figure 6-12 shows important modeling elements of activity diagrams in UML [OMG 2007]. Figure 6-12 Modeling elements of activity diagrams of the UML Action nodes Activity diagrams are control flow graphs that consist of action nodes and the control flow between these action nodes (i.e., the arrows in the control flow graph depicting transitions). Action nodes execute an action. The start and end nodes in activity diagrams have defined semantics. The start node represents an event that initiates the execution of the activity dia- gram. End nodes are special nodes that represent the termination of the activity diagram. Action/Activity Start node End node Synchronization bar Fork Join Control flow Decision Decision node Merge of alternative control flows [Name] [condition] Condition [Name] Object flow [Name] Control flow termination Hierarchization
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80 6 Model-Based Requirements Documentation Control flows, object flows, responsibilities Depicting alternative control flows in activity diagrams can be achieved through the use of decision nodes. At decision nodes, conditions that trigger alternative control flows are annotated. In addition, synchro- nization bars allow for concurrent execution of control flows. A special type of control flows are object flows. By making use of activity partitions (swimlanes), different activities can be documented as the responsibility of specific actors. Sequence Modeling using UML Activity Diagrams The activity diagram in figure 6-13 documents the process “navigate to destination”. Input and output data can be documented by modeling addi- tional object flows along the edges. The data and object flows are special types of control flows of the activity diagram. Every action is executed if and only if previous actions have been carried out and all incoming object flows are available. The action diagram in figure 6-13 also shows object flows that are documented in addition to the actions and control flows.
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