Transcript TCS 2411 Software Engineering

Data-Flow Oriented
Design
“From DFD to Structure Chart”
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Lecture Objectives
To describe the process of translating the
DFDs to structure charts
To understand the systematic approach in
producing the design from the analysis
model
To understand the different types of
conversion that can be done in producing
the structure chart
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Expanding the DFD
Initial analysis produces the Context
Diagram and Level 1 DFD
In the design phase, the DFD is expanded
to lower levels, giving more sub-processes
within the original process
Level 1  Level 2  Level 3  …
Consistency is to be maintained from level
to level
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Example of Level 1 DFD
Member
application form
Process
Application
new member record
Members
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Expanded to Level 2
Member
Members
formatted
new member
record
application
form
Get
Form
Check
Form
application
details
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Assign
Code
checked
application
details
Prepare
Record
new
member
details
Format
Record
new
member
record
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Five-Step Process
The type of information flow is
established
Flow boundaries are indicated
The DFD is mapped into program
structure
Control hierarchy is defined by factoring
Resultant structure is refined using design
measures and heuristics
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Types of Information
Flow
Transform Flow
Information enter at a point, transformed,
and exit at another point
Path: incoming flow  central transform 
outgoing flow
Transaction Flow
Single data item, transaction, triggers one of
many paths
Path: incoming flow  transaction centre 
action path
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Transform Mapping
After transform flow is identified, identify
the boundaries of incoming and outgoing
flow
Incoming flow = Afferent flow
Outgoing flow = Efferent flow
In between the boundaries of the two is
the central transform
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Example of Boundaries
Member
Members
formatted
new member
record
application
form
Get
Form
Check
Form
application
details
Afferent flow
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Assign
Code
checked
application
details
Prepare
Record
new
member
details
Central
transform
Format
Record
new
member
record
Efferent flow
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First Level Factoring
Process
Membership
Application
Get
Application
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Process
Application
Generate
New Member
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Second Level Factoring
Process
Membership
Application
Get
Application
Get
Application
Form
Generate
New Member
Process
Application
Check
Application
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Generate
New
Record
Format
New
Record
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Refinement
The hierarchy produced is then refined to
improve the quality of the design
Modules could be exploded or imploded
Apply design concepts of module
independence, good cohesion, minimal
coupling, and ease of maintenance
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Transaction Mapping
If transaction flow is identified, identify
the transaction centre and flow
characteristics of each action path
Similar to transform flow, boundaries are
identified for incoming flow and the
separate action paths
Each action path is a transform flow or
transaction flow as well
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Example of Transaction
Flow
action paths
user commands
read
user
command
command
type
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invoke
command
processing
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Transaction Mapping
transaction
control
a
reception
path
b
d
b
d
a
p
dispatcher
c1
p
q
q
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Factoring and Refining
Each action path has to be refined into a
substructure, either by transform or
transaction mapping
Similar to transaction flow, apply design
concepts to improve quality of design
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Completing the
Structure Chart
For each data flow on the DFD add
exactly one data couple. Use the same
data flow name
For each control module, decide how it
will control subordinates
For modules that select one of several
paths, show selection logic with a
diamond attached to the link
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Design Postprocessing
A processing narrative must be developed
for each module
Interface description is provided for each
module
Define local and global data structures
All design restrictions/limitations noted
Conduct design review
Optimisation is considered
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References
“Software Engineering: A Practitioner’s
Approach” 5th Ed. by Roger S. Pressman,
Mc-Graw-Hill, 2001
“Software Engineering” by Ian Sommerville,
Addison-Wesley, 2001
“The Practical Guide to Structured Systems
Design” by Meilir Page-Jones,
Benjamin/Cummings, 1988
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