Transcript TCS 2411 Software Engineering

Software Design Principles
“Producing the software blueprint”
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Lecture Objectives
To understand the importance of design in
developing quality software
To describe the translation from the
requirements analysis model to the design
model
To understand the principles that guide
proper design of software
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Designing A House
If you are asked to design a house…
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Kitchen
Room 2
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Living
Room
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Room 1
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What Is Design?
Explaining the idea/concept of something
Usually with graphical diagrams
With the intention to build from the
explanation
The design is a representation of a
product or a system with sufficient detail
for implementation
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The Second Task
Analysis
Design
Problem
Models
Development
Testing
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Solution
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Designing Software
From our understanding of the problem,
we start building the software
Translate the analysis model into the
design model
Map the information from the analysis
model to the design representations - data
design, architectural design, interface
design, procedural design
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Translation Model
EntityRelationship
Diagram
Data Flow
Diagram
Procedural
design
Data Dictionary
Interface
design
State-Transition
Diagram
Architectural design
Data design
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Design Principles
Design process should not suffer from
“tunnel vision”
The design should be traceable to the
analysis model
The design should not reinvent the wheel;
Time is short
The design should “minimize intellectual
distance” between the software and the
problem in the real world
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Design Principles (Continued)
The design should exhibit uniformity and
integration
The design should be structured to
accommodate change
The design should be structured to
degrade gently.
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Design Principles (Continued)
Design is not coding, coding is not design
The design should be assessed for quality
as it is being created, not after the fact
The design should be reviewed to
minimize conceptual errors
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Design Concepts
Fundamental concepts which provide
foundation to design correctly:
Abstraction
Structural
Partitioning
Refinement
Data Structure
Modularity
Software Procedure
Software
Architecture
Information Hiding
Control Hierarchy
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Abstraction
Identifying important features for
representation
There are many levels of abstraction
depending on how detailed the
representation is required
Data abstraction - representation of data
objects
Procedural abstraction - representation of
instructions
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Refinement
Stepwise refinement - top-down design
strategy by Niklaus Wirth
Starting at the highest level of abstraction,
every step of refinement ‘decompose’
instructions into more detailed instructions
Complementary to abstraction
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Modularity
Software is divided into separately named
and addressable modules
“Divide and conquer” approach - problem
is broken into manageable pieces
Solutions for the separate pieces then
integrated into the whole system
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Divide And Conquer
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Software Architecture
Modules can be integrated in many ways
to produce the system
Software architecture is the overall
structure of the software
The hierarchy of components and how
they interact, and the structure of data
used by the components
Use of framework models, and possible
reuse of architectural patterns
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Software Architecture Patterns
Recurring pattern help designers reuse
successful designs by basing new designs
on prior experience.
A designer who is familiar with such
patterns can apply them immediately to
design problems without having to
rediscover them.
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Why use Design Patterns?
Reuse successful practices.
Not new – recognised that this is something
that engineers have done for years.
Improve communication
Step towards a software engineer’s
handbook
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Examples of Software
Architecture
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Batch
sequential
pattern
Program structure
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Control Hierarchy
Hierarchy of modules representing the
control relationships
A super-ordinate module controls another
module
A subordinate module is controlled by
another module
Measures relevant to control hierarchy:
depth, width, fan-in, fan-out
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Structure Terminology
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Fan-out
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b
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Depth
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Fan-in
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Width
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Structural Partitioning
Program structure partitioned horizontally
and vertically
Horizontal partitioning defines separate
branches for each major program function
- input, process, output
Vertical partitioning (aka factoring) defines
control (decision-making) at the top and
work at the bottom
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Software Procedure
Processing details of individual modules
Precise specification of processing,
including sequence of events, exact
decision points, repetitive operations, and
data organization/structure
Procedure is layered - subordinate
modules must be referenced in processing
details
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Information Hiding
Information (procedure and data) contained
within a module is inaccessible to other
modules that have no need for such
information
Effective modularity is achieved by
independent modules, that communicate
only necessary information
Ease of maintenance - testing, modification
localized and less likely to propagate
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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
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