The Traditional Approach to
Requirements
(Data Flow Diagrams)
Excerpts from chapter 6:
Systems Analysis & Design in a Changing World
by
Satzinger, J.; Jackson, R.; & Burd, S. (5th edition)
The Traditional Approach to
Requirements

System is a collection of processes

Process is an activity carried out by people or
computers

Diagrams that represent processes are called
PROCESS MODELS

DataFlow Diagram (DFD) is a process model
Traditional Modeling
 System
is a collection of processes
 Processes interact with data entities
 Processes accept inputs
 Processes product outputs
Data Flow Diagram (DFD)

Graphical model that shows all the main
requirements of the system in a diagram

5 components and symbols
 Process
= circle (or rectangle w/ rounded corners)
 Data store = open ended rectangle
 Entity = rectangle
 Inputs = lines w/ arrows going inwards
 Outputs = lines w/ arrows going outwards
Components of DFD

Entity = external agent who is a source or destination
for some data outside the system (could be a person or
a computer)

Process = activity that transforms inputs into outputs

Data store = file that stores data

Data flow


Input = flow of data into a process
Output = flow of data out of a process
Components of DFD
Steps in Developing DFD
 Draw
a Context Diagram
 Draw a Zero level DFD
 Decompose each process into lower level
DFDs (as necessary).
 Develop a process description for the
lowest level process decomposition
Context Diagram

Highest level of abstraction the system

Defines the system boundary/scope

Shows the system in the center as a bubble (or
box in some books)

Shows the external agents/entities which may
be people or computers

Does not show data stores
Evaluating Quality of DFDs

Minimizing complexity – the 7 plus or minus rule
- for the # of processes and interfaces

Data flow consistency - balancing data outflows
and inflows

Content Consistency – content b/w process and
its process decomposition
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
Systems Analysis and Design in a Changing World, 5th Edition
10
Unnecessary Data Input:
Black Hole
Figure 6-16
Systems Analysis and Design in a Changing World, 5th Edition
11
Process with Impossible
Data Output: a Miracle
Figure 6-17
Systems Analysis and Design in a Changing World, 5th Edition
12