for the Small Enterprise
Systems Analysis and Design
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
2
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
4
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
9
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.
Figure 3-5: Silhouette’s Bottom-Up DFD Worksheet
Action words
identified
in the
narrative
11
Silhouette’s Task #1 Evaluate Special Requests IPO Chart
12
Silhouette Sea Charter’s Partial DFD Sketch
13
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
15
Figure 3-9: TKSystem Context Diagram
16
Figure 3-10: Revised TKSystem Context
Diagram
17
Level Decomposition
•
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 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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Cornucopia Existing System Context Diagram
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Cornucopia Existing System First-Level DFD
Detail
Detail
Detail
Detail
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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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