Flowing dataA data flowdescribes data that is transported between processes, datastores, and sources/sinks [Yourdon 1989]. In requirements models, a dataflow can model both the transport of material and immaterial objects, e.g.,information flow or material flow. Typically, only the most important dataflows are modeled in data flow diagrams. Data flows that are not relevantfor the requirements of the system can be neglected.Example of a Data Flow DiagramSystem interfacesFigure 6-11 shows a simplified data flow diagram of a navigation system inthe notation suggested by DeMarco. The interfaces of the system to theData FlowProcessSource/SinkData Store[Name][Name][Name][Name]7.In the structured analysis, the flow of data, information, documents, or material isconsidered a data flow.
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786Model-Based Requirements Documentationcontext are defined by the data flows to the sources “GPS satellite system”and “traffic information server” as well as to the sink “driver”.Process “calculate route”The functionality of the navigation system is separated into three dis-tinct processes. Process one, named “calculate route”, receives up-to-datetraffic information via its interface to the source “traffic informationserver” as well as data about the current location via its interface to thesource “GPS satellite system”. In addition, the process “calculate route” isprovided with the desired destination by the driver of the vehicle. The cal-culated route is stored in the data store “route data”. Process “determine nextwaypoint”Process two—“determine next waypoint”—accesses the data store andretrieves data concerning the current route. The process determines thenext waypoint and outputs this information. Process “recalculate route”Process three—“recalculate route”—plots a new course to the destina-tion. In order to do so, it gathers traffic information from the source “traf-fic information server” and, potentially, information about the currentlocation. The newly calculated course is stored in the data store “routedata”. Figure 6-11 Data flow diagram in the notation suggested by DeMarco6.6.2Models of the Functional Perspective and Control FlowIn data flow diagrams, it cannot be seen which conditions trigger whichprocesses. Data flow diagrams merely depict data dependencies of theprocesses in a system and document necessary input and generated outputdata. Approaches used in structured system analysis, however, often offerRoute dataDriverRecalculaterouteDeviant locationDeterminenextwaypointCalculate routeLocation dataTraffic information serverGPS satellite system