Systems Analysis and Design in a Changing World, Fourth Edition

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Systems Analysis and Design in a
Changing World, Fourth Edition

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Systems Analysis and Design in a Changing World, 4th Edition 2
Learning Objectives
Explain how the traditional approach and the
object-oriented approach differ when modeling
the details of a use case
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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Learning Objectives (continued)
Develop data flow diagrams, data element
definitions, data store definitions, and process
descriptions
Read and interpret Information Engineering
models that can be incorporated within traditional
structured analysis
Develop tables to show the distribution of
processing and data access across system
locations

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Overview
What the system does and what event occurs –
activities and interactions (use case)
Traditional structured approach to representing
activities and interactions
Diagrams and other models of the traditional
approach
RMO customer support system example shows
how each model is related
How traditional and IE approaches and models
can be used together to describe system

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Traditional versus Object-Oriented
Approaches

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Traditional Approach in this Chapter

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Data Flow Diagrams (DFDs)
Graphical system model that shows all main
requirements for an IS in one diagram
Inputs/outputs
Processes
Data storage
Easy to read and understand with minimal
training

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Data Flow
Diagram
Symbols
(Figure 6-3)

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DFD Fragment Showing Use Case Look
up item availability from the RMO (Figure 6-4)

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DFD Integrates Event Table and ERD (Figure 6-5)

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DFD and Levels of Abstraction
Data flow diagrams (DFDs) are decomposed into
additional diagrams to provide multiple levels of
detail
Higher-level diagrams provide general views of
system
Lower-level diagrams provide detailed views of
system
Differing views are called levels of abstraction

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Layers of
DFD
Abstraction
for Course
Registration
System
(Figure 6-6)

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Context Diagrams
DFD that summarizes all processing activity for
the system or subsystem
Highest level (most abstract) view of system
Shows system boundaries
System scope is represented by a single process,
external agents, and all data flows into and out of
the system

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DFD Fragments
Created for each use case in the event table
Represent system response to one event within a
single process symbol
Self-contained models
Focus attention on single part of system
Show only data stores required in the use case

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Three Separate DFD Fragments for Course
Registration System

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Event-Partitioned System Model
DFD to model system requirements using single process
for each use case/activity in system or subsystem
Combines all DFD fragments together to show
decomposition of the context-level diagram
Sometimes called “diagram 0”
Used primarily as a presentation tool
Decomposed into more detailed DFD fragments

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Combining
DFD
Fragments to
Create
Event-
Partitioned
System
Model
(Figure 6-8)

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Context
Diagram for
RMO
Customer
Support
System
(Figure 6-9)

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RMO Subsystems and Use Cases/Activities from
Event Table (Figure 6-10)

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Context Diagram for RMO
Order-Entry Subsystem (Figure 6-11)

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Five Separate DFD Fragments for RMO
Order-Entry Subsystem (Figure 6-12)

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Decomposing DFD Fragments
Most DFD fragments can be further described using
structured English
Sometimes DFD fragments need to be diagrammed in
more detail
Decomposed into subprocesses in a detailed DFD
DFD numbering scheme
Hierarchical decomposition
DFD Fragment 2 is decomposed into Diagram 2
Diagram 2 has processes 2.1, 2.2, 2.3, 2.4

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Detailed
DFD for
Create
new order
DFD
Fragment
(Figure 6-14)

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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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Physical
DFD for
Scheduling
Courses
(Figure 6-15)

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Evaluating DFD Quality
Readable
Internally consistent and balanced
Accurately represents system requirements
Reduces information overload – rule of 7 +/- 2
Single DFD should not have more than 7 +/-2
processes
No more than 7 +/- 2 data flows should enter or
leave a process or data store in a single DFD
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 of the process, or
Be used to generate data that flows out of the
process
All data that flows out of a process must
Have flowed into the process, or
Have been generated from data that flowed into
the process

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Unnecessary Data Input: Black Hole

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Process with Impossible Data Output:
A Miracle (Figure 6-17)

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Process with Unnecessary Data Input
(Figure 6-18)

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Process with Impossible Data Output
(Figure 6-19)

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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
Combines structured programming techniques
with narrative English
Well-suited for lengthy sequential processes or
simple control logic (single loop or if-then-else)
Ill-suited for complex decision logic or few (or no)
sequential processing steps

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Structured English Example (Figure 6-20)

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Process 2.1 and Structured
English Process Description (Figure 6-21)

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Decision Tables and Decision Trees
Can summarize complex decision logic better than structured
English
Incorporate logic into the table or tree structure to make
descriptions more readable

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Decision Tree for Calculating
Shipping Charges (Figure 6-24)

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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 plus computed values
Algebraic notion describes data elements on data flow
plus data structure

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Data Flow Definition for RMO Products
and Items Control Break Report (Figure 6-29)

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Data Element Definitions
Data type description
String, integer, floating point, Boolean
Sometimes very specific written description
Length of element
Maximum and minimum values
Data dictionary – repository for definitions of data
flows, data stores, and data elements

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Data Element Definition Examples
(Figure 6-30)

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Components of a Traditional Analysis Model
(Figure 6-31)

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Information Engineering Models
Focus on strategic planning, enterprise
applications, and data requirements of new
system
Share features with structured system
development methodology
Developed by James Martin in early 1980’s
Thought to be more rigorous and complete than
the structured approach

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Information Engineering System
Development Life Cycle Phases (Figure 6-32)

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Process Decomposition and Dependency
Models
IE process models show three information types
Decomposition of processes into other processes
Dependency relationships among processes
Internal processing logic
Process decomposition diagram – represents
hierarchical relationship among processes at
different levels of abstraction
Process dependency model – describes ordering
of processes and interaction with stored entities

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Process
Decomposition
Diagram for RMO
(Figure 6-34)

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Process Dependency Diagram (Figure 6-35)

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Locations and Communication
Through Networks
Logical 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
Needed to make initial design decisions such as
Distribution of computer systems, application
software, database components, network capacity

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Gathering Location Information
Identify locations where work is to be performed
Draw location diagram
List functions performed by users at each location
Build activity-location matrix
Rows are system activities from event table
Columns are physical locations
Build activity-data (CRUD) matrix
CRUD – create, read, update, and delete

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RMO Activity-Location Matrix (Figure 6-38)

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RMO Activity-Data Matrix (CRUD)
(Figure 6-39)

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Summary
Data flow diagrams (DFDs) are used in combination with
event table and entity-relationship diagram (ERD) to
model system requirements
DFDs model system as set of processes, data flows,
external agents, and data stores
DFDs easy to read – graphically represent key features of
system using small set of symbols
Many types of DFDs – context diagrams, DFD fragments,
subsystem DFDs, event-partitioned DFDs, and detailed
process DFDs

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Summary (continued)
Each process, data flow, and data store requires
detailed definition
Analyst may define processes as structured
English process specifications, decision tables,
decision trees, or detail process DFDs
Detailed process decomposition DFDs used
when internal process complexity is great
Data flows are defined by component data
elements and their internal structure (algebraic
notation)

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Summary (continued)
Models from IE may supplement DFDs
Process decomposition diagram (how processes
on multiple DFD levels are related)
Process dependency diagram (emphasizes
interaction with stored entities)
Location diagram (where system is used)
Activity-location matrix (which processes are
implemented at which locations)
Activity-data (or CRUD) matrix (where data is
used)

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