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Jamal Munshi, Sonoma State Univesity, 1992 | ||
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systems analysis tools produce requirements specifications functional models (dataflow diagrams dfd) identify functions in the user environment data models identify entities in the user environment yourdon-demarco methodology functional model (dataflow diagram) process - circle - verb phrase - a logical function in the user environment terminal - rectangle - noun phrase - external entity, outside the system interacts w/system datastore - open rectangle - noun phrase - holds data for later use dataflows - lines with arrows - identifies data that must be passed not a flowchart - no time sequence or logic described careful of 'then' or other references to time stepwise decomposition root diagram is 'context diagram' - define system boundaries separates system from not-the-system define all external entities and all dataflows between system and the outside must be called 'context diagram' consists of one process and all external entities and external dataflows no datastores the single circle represents the system itself. name is noun phrase top level dataflow diagram use up to 9 processes to describe process in context diagram identify all dataflows between them and all datastores needed dataflows to and from datastores must be from processes further decomposition use dfd to describe function of each process continue until processes are simple enough to state in structured english these are primitive processes consistency checks all dataflows that appear in the context diagram must appear in the top level dfd all external entities that appear in the model must appear in the context diagram all dataflows that flow to/from a process must appear in its decomposed dfd direct dataflows between datastores or between external entities are not possible do not attempt to use dfd to model logic or sequence. use other tools a dataflow arrow only indicates that that data has to flow for the system to work it says nothing about when that dataflow occurs or under what condition std (state transition diagrams) are used to model sequence in real time systems structured english is used to model logic and sequence |