for the Small Enterprise
Systems Analysis and Design
chapter3
section II
Analysis
modeling the system
the data flow diagram
the context diagram
level decomposition
the cornucopia case
portfolio project
Chapter Objectives
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
• The data flow diagram (DFD) presents a picture of
what people and procedures do to transform data
into information
• The DFD is composed of four elements:
– External entity … the originator of data (source)
or receiver of information (sink)
– 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
• Standard rules for construction:
– Data flows must originate or terminate at
a process
– Data stores must have at least one entry
and one exit data flow
– 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
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.”
Reference Figure 3-8: Top-Down DFD Creation
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The Context Diagram
The context diagram
• 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
Chapter 3
•
The single process in the context diagram is
composed of a collection of well-defined
activities called cohesive tasks
•
Task definition begins by locating the events
within a process
•
Events identify the end of one task and the
beginning of another
•
Once a task is identified, the analyst can identify
the inputs and outputs
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Level Decomposition
•
Issue Access Cards INPUTS:
–
–
–
–
–
•
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:
–
–
–
–
–
–
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)
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
•Logical DFDs remove all reference to the
implementation specifics of the system
•Physical DFDs specify the real world objects that
are used to make the system work
•During the analysis phase:
–Develop the physical DFD
–Abstract the logical DFD
•During the design phase:
–Develop the logical DFD
–Develop the physical DFD
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Figure 3-13: TKSystem First-Level DFD
Enlarge
Enlarge
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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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ThinkingCritically
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 Cornucopia Case
THE CORNUCOPIA
CASE
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
Detail
Detail
Detail
Detail
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Figure 3-18: Cornucopia Process Modeling with
Visible Analyst
Enlarge
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Figure 3-19: Cornucopia Existing System
Context Diagram in Visible Analyst
Enlarge
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PortfolioprojectPortfolio Project
Team Assignment 3: Process Modeling
•Prepare the existing system context diagram
and first-level DFD
•Review the project management sections of
Appendices A and B
•Develop the first draft of your week-by-week
project budget
•Develop the first draft of your week-by-week
project status
•Prepare a cover letter to transmit the budget
and status reports to your client
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Chapter Summary (1/2)
• The data flow diagram (DFD) models the
processes of an information system
• DFDs consist of processes, external entities,
data stores, and data flows
• 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)
•
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
•
As needed, successively more detailed levels
are developed to promote complete
understanding of the system
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