Transcript The data flow diagram (DFD)
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section II Analysis
Chapter Objectives
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When you complete this chapter you will be able to: Describe why the data flow diagram is called a process model Recognize and describe an abstraction Identify the four elements of a data flow diagram Construct a data flow diagram Decompose a data flow diagram
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Modeling the System
Process model: diagrams how data flows through the system Data model: diagrams the relationships between data files Object model: diagrams the relationships between enterprise objects System models: diagram system functions, hardware, and software
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Figure 3-1: DFD and the Six CIS Components
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TECHNOTE 3-1 Abstractions and Computer Information Systems An abstraction is a simplified description, view, or model of an object or system of objects.
The process, data, object, and system models are abstractions of a computer information system.
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The Data Flow Diagram
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The data flow diagram (DFD) presents a picture of what people and procedures do to transform data into information
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The DFD is composed of four elements:
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External entity … the originator of data (source) or receiver of information (sink)
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Process …a series of steps that manipulate data Data store … a place to keep data for later reference Data flow … describe data and information elements passing between external entities, processes, and data stores
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Figure 3-3: Illustrated DFD Symbols
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The Data Flow Diagram
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Standard rules for construction:
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Data flows must originate or terminate at a process
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Data stores must have at least one entry and one exit data flow
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Processes must have at least one entry and one exit data flow
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Data Flow Diagrams
Bottom-Up DFD creation 1. Develop a narrative of the system 2. Underline the action words 3. Develop a sequential list of the action words 4. Eliminate tasks that do not transform data 5. Identify cohesive tasks 6. Fit all remaining tasks to a cohesive task 7. Develop an IPO chart for each cohesive task
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Reference Figure 3-4: Bottom-Up DFD Creation
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Data Flow Diagrams
A portion of Silhouette Sea Charter’s narrative: Sometimes the office manager cannot determine whether the request is reasonable. When this happens, the request is forwarded to the owner, who makes this determination, contacts the customer if necessary, and either adjusts the request or rejects it outright. All requests are returned to the office manager for scheduling and filing.
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Figure 3 5: Silhouette’s Bottom-Up DFD Worksheet
Action words identified in the narrative
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Figure 3 6: Silhouette’s Task #1 Evaluate Special Requests IPO Chart
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Figure 3 7: Silhouette Sea Charter’s Partial DFD Sketch
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Data Flow Diagrams
Top-Down DFD creation 1. Solicit oral answers to the question “What is the first task of this system?” 2.
Continue with “What happens next?” 3.
Repeat Step 2 until the response is “We do it all over again,” or We are finished.”
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Reference Figure 3-8: Top-Down DFD Creation
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The Context Diagram
The context diagram
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Shows a single process Connected to the external entities Establishes the system boundaries
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Figure 3-9: TKSystem Context Diagram
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Figure 3-10: Revised TKSystem Context Diagram
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Level Decomposition
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The single process in the context diagram is composed of a collection of well-defined activities called cohesive tasks
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Task definition begins by locating the events within a process
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Events identify the end of one task and the beginning of another
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Once a task is identified, the analyst can identify the inputs and outputs
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Level Decomposition
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Issue Access Cards INPUTS:
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class rosters (Source: Data Processing) access cards for registered students (Source: Data Processing) student registration (Source: Data Processing) signed non-piracy agreement (Source: Student) name on class rosters (Data Store: Roster) Issue Access Cards OUTPUTS:
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access cards (Sink: Student) lab handbook (Sink: Student) card numbers already issued (Sink: Data Processing) checkoff on class rosters (Data Store: Roster) new lab account (Data Store: Time) nonpiracy agreement (Data Store: Agreements)
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Reference Figure 3-11: TKSystem Task IPO
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Figure 3-12: TKSystem Issue Access Cards Task DFD
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TECHNOTE 3-2 Logical and Physical DFDs
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Logical DFDs remove all reference to the implementation specifics of the system
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Physical DFDs specify the real world objects that are used to make the system work
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During the analysis phase:
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Develop the physical DFD
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Abstract the logical DFD
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During the design phase:
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Develop the logical DFD
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Develop the physical DFD
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Figure 3-13: TKSystem First-Level DFD
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Figure 3-14: Data Flow Walk-Through
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Figure 3-15: TKSystem DFD Levels
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Thinking
Critically
Model Building for Pay How would you respond to a client who asks why he or she should pay for model building?
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The analysis phase is well underway. The analysts must first understand the existing system before the new system can be designed. The process model is the first abstraction to develop.
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Figure 3-16: Cornucopia Existing System Context Diagram
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Figure 3-17: Cornucopia Existing System First-Level DFD
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Detail Detail Detail Detail
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Figure 3-18: Cornucopia Process Modeling with Visible Analyst
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Figure 3-19: Cornucopia Existing System Context Diagram in Visible Analyst
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Portfolio
project
Portfolio Project
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Team Assignment 3: Process Modeling
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Prepare the existing system context diagram and first-level DFD
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Review the project management sections of Appendices A and B
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Develop the first draft of your week-by-week project budget
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Develop the first draft of your week-by-week project status
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Prepare a cover letter to transmit the budget and status reports to your client
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Chapter Summary (1/2)
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The data flow diagram (DFD) models the processes of an information system
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DFDs consist of processes, external entities, data stores, and data flows
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DFD modeling begins with the context diagram, which depicts the entire system in a single process symbol surrounded by external entities
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Chapter Summary (2/2)
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The first-level DFD shows the major functional components of the system as processes, connected by data flows to external entities, data stores, and other processes
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As needed, successively more detailed levels are developed to promote complete understanding of the system
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