Transcript Chapter 6
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Chapter 6
The Traditional
Approach to
Requirements
Systems Analysis and Design in a Changing World, 2nd Edition, Satzinger, Jackson, & Burd
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Objectives
Explain
how the traditional approach and
the object-oriented approach differ when
an event occurs
List
the components of a traditional
system and the symbols representing
them on a data flow diagram
Describe
how data flow diagrams can
show the system at various levels of
abstraction
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Objectives
Develop
data flow diagrams, data element
definitions, data store definitions, and
process descriptions
Develop
tables to show the distribution of
processing and data access across
system locations
Read
and interpret information
engineering models that can be
incorporated within traditional structured
analysis
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Traditional versus OO Approaches
Figure 6-1
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Data Flow Diagrams
Graphical
system model that shows all main
requirements for an IS
Inputs / outputs
Processes
Data storage
Easy
to read and understand
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Data Flow Diagram Symbols
Figure 6-2
Process
Step-by-step instructions
Data flow
External agent
Data store
Data at rest
Real-time link
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DFD Fragment from the RMO Case
Figure 6-3
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DFD Integrates Event Table and ERD
Figure 6-4
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DFD and Levels of Abstraction
DFDs
are decomposed into additional
diagrams to provide multiple levels of detail
Higher
levels are more general
Lower
levels are more detailed
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Layers of DFD Abstraction
Figure 6-5
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Context Diagram
Academic
Department
Schedule Data
Enrollment
Request
Class List
Faculty
Member
Course
Registration
System
Student
Schedule
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DFD Fragments
Academic
Department
Schedule
data
Schedule
Course
Student List
Enrollment
Request
Student
Schedule
Faculty
Member
Class
List
Offered Course
Enroll
Student
Course enrollment
Offered Course
Student List
Produce
Class
List
Course enrollment
Offered Course
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Diagram 0
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Enroll
Student
Academic
Department
Enrollment
Request
Student
Schedule
Student
List
Schedule
data
Course
enrollment
1
Schedule
Course
3
Offered
Course
Produce
Class
List
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List
Faculty
Member
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Context Diagrams
DFD
that summarizes all processing activity
Highest
Shows
level view of system
system boundaries
Scope
is represented by a single process
and outside agents
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DFD Fragments
Represents
system response to one event
within a single process symbol
Self
contained model
Focuses
attention on single part of system
Shows
only data stores required to respond
to events
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Context Diagram for RMO
Order-Entry Subsystem
Figure 6-8
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Two DFD Fragments for RMO
Order-Entry System
Figure 6-10
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Event Partitioned System Model
DFD
that models system requirements using
a single process for each event in a system
or subsystem
Sometimes
called diagram 0
Decomposition
of the context level diagram
Is
decomposed into more detailed DFD
fragments
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Combining DFD Fragments Figure 6-11
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Decomposing DFD Fragments
Sometimes
DFD fragments need to be
explored in more detail
Broken
into subprocesses with additional
detail
Numbering
scheme doesn’t equate to
execution sequence
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Detailed Diagram for Create New Order
Figure 6-13
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Physical and Logical DFDs
Logical
model
Assumes implementation in perfect technology
Does not tell how system is implemented
Physical
model
Describes assumptions about implementation
technology
Developed in last stages of analysis or in early
design
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Evaluating DFD Quality
Readable
Internally consistent
Accurately represents system
Reduces
information overload
Rule: 7 +/- 2
Minimizes required number of interfaces
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Data Flow Consistency Problems
Differences
in data flow content between a
process and its process decomposition
Data
outflows without corresponding
inflows
Data
inflows without corresponding
outflows
Results
in unbalanced DFDs
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Consistency Rules
All
data that flows into a process must flow
out or be used to generate data that flows
out
All
data that flows out of a process must
have flowed in or been generated from data
that flowed in
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Unnecessary Data Input: Black Hole
Figure 6-15
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Process with Impossible Data Output:
Miracle
Figure 6-16
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Process with Unnecessary Data Input
Figure 6-17
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Process with Impossible Data Output
Figure 6-18
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Documentation of DFD Components
Lowest
level processes need to be
described in detail
Data flow contents need to be
described
Data stores need to be described in
terms of data elements
Each data element needs to be
described
Various options for process definition
exist
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Structured English
Method
of writing process specifications
that combines structured programming
techniques with narrative English
Well suited to lengthy sequential processes or
simple control logic
Ill-suited for complex decision logic or few
sequential processing steps
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Process 2.1 and Structured
English Process Description Figure 6-20
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Decision Tables and Decision Trees
Can
summarize complex decision logic
better than structured English
Incorporates
logic into the table or tree
structure to make descriptions more
readable
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Data Flow Definitions
Textual
description of data flow’s content
and internal structure
Often
coincide with attributes of data entities
included in ERD
Data Flow Definitions
Simply Listing Elements
Figure 6-25
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Data Element Definitions
Data
type description
e.g. string, integer, floating point, Boolean
Very specific
Length
of element
Maximum
and minimum values
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Components of a Traditional Analysis Mode
Figure 6-30
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Information Engineering Models
Focuses
on strategic planning and data
requirements of new system
Shares
features with structured system
development methodology
Developed
by James Martin in early 1980’s
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Information Engineering System
Development Life Cycle Phases
Figure 6-31
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Process Decomposition and
Dependency Models
IE
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process model information types
Decomposition of processes into other
processes
Dependency relationships among processes
Internal processing logic
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Process Dependency Diagram
Figure 6-34
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Process Dependency Diagram
with Data Flows
Figure 6-35
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Locations and Communication
Through Networks
Logical
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information needed during analysis
Number of user locations
Processing and data access requirements at
various locations
Volume and timing of processing and data access
requests
First,
identify locations where work is to be
performed
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RMO Location Diagram
Figure 6-36
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Location Considered
List
functions performed by users at each
location
Place in matrix
Rows are system activities
Columns are locations
Other
matrices
Activities versus data
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