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In this chapter we will: Balance high-level and low-level DFDs; explain differences between current physical, current logical, new physical, and new logical DFDs; use DFDs for analyzing information systems; explain use cases and use case diagrams. | Business System Development CSC581 Lecture 11 Determining System Requirements 2 Summary of the previous lecture In previous lecture, we : Understood logical process modeling via data flow diagrams (DFDs). Drew DFDs of well-structured process models. Decomposed DFDs into lower-level diagrams. Outlines We today’s lecture we will: Balance high-level and low-level DFDs. Explain differences between current physical, current logical, new physical, and new logical DFDs. Use DFDs for analyzing information systems. Explain use cases and use case diagrams DFD Balancing The conservation of inputs and outputs to a data flow process when that process is decomposed to a lower level Balanced means: Number of inputs to lower level DFD equals number of inputs to associated process of higher-level DFD Number of outputs to lower level DFD equals number of outputs to associated process of higher-level DFD Unbalanced DFD This is unbalanced because the process of the context diagram has only one input . | Business System Development CSC581 Lecture 11 Determining System Requirements 2 Summary of the previous lecture In previous lecture, we : Understood logical process modeling via data flow diagrams (DFDs). Drew DFDs of well-structured process models. Decomposed DFDs into lower-level diagrams. Outlines We today’s lecture we will: Balance high-level and low-level DFDs. Explain differences between current physical, current logical, new physical, and new logical DFDs. Use DFDs for analyzing information systems. Explain use cases and use case diagrams DFD Balancing The conservation of inputs and outputs to a data flow process when that process is decomposed to a lower level Balanced means: Number of inputs to lower level DFD equals number of inputs to associated process of higher-level DFD Number of outputs to lower level DFD equals number of outputs to associated process of higher-level DFD Unbalanced DFD This is unbalanced because the process of the context diagram has only one input but the Level-0 diagram has two inputs. 1 input 1 output 2 inputs 1 output Balanced DFD These are balanced because the numbers of inputs and outputs of context diagram process equal the number of inputs and outputs of Level-0 diagram. 1 input 2 outputs Balanced DFD (cont.) These are balanced because the numbers of inputs and outputs to Process 1.0 of the Level-0 diagram equals the number of inputs and outputs to the Level-1 diagram. 1 input 4 outputs Data Flow Splitting A composite data flow at a higher level may be split if different parts go to different processes in the lower level DFD. This remains balanced because the same data is involved, but split into two parts. More DFD Rules Four Different Types of DFD Current Physical Process labels identify technology (people or systems) used to process the data. Data flows and data stores identify actual name of the physical media. Current Logical Physical aspects of system are removed as much as possible. Current system is reduced to data
