Software Design
(Lecture 4)
Prof. R. Mall
Dept. of CSE, IIT, Kharagpur
Organization of This Lecture
Introduction to software design Goodness of a design Functional Independence
Cohesion and Coupling
Function-oriented design vs. Objectoriented design Summary
Introduction
Design phase transforms SRS document:
To a form easily implementable in some programming language.
SRS Document
Design Activities
Design Documents
Items Designed During Design Phase
Module structure, Control relationship among the modules
call relationship or invocation relationship Data items exchanged among different modules,
Interface among different modules,
Data structures of individual modules, Algorithms for individual modules.
Module Structure
Introduction
A module consists of:
Several
functions Associated data structures.
D1 .. D2 .. D3 .. F1 .. F2 .. F3 .. F4 .. F5 ..
Data
Functions
Introduction
Good software designs: Seldom arrived through a single step procedure:
But
through a series of steps and iterations.
Introduction
Design activities are usually classified into two stages:
Preliminary (or high-level) design.
Detailed design.
Meaning and scope of the two stages:
Vary considerably from one methodology to another.
High-Level Design
Identify:
Modules Control relationships among modules Interfaces among modules.
d1 d2
d3
d1
d4
High-Level Design
The outcome of high-level design: Program structure (or software architecture).
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High-Level Design
Several notations are available to represent high-level design: Usually a tree-like diagram called structure chart is used.
Other
notations:
Jackson diagram or WarnierOrr diagram can also be used.
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Detailed Design
For each module, design: Data structure
Algorithms
Outcome of detailed design: Module specification.
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A Classification of Design Methodologies
Procedural (aka Functionoriented) Object-oriented More recent:
Aspect-oriented Component-based
Server)
(Client13
Does a Design Technique Lead to a Unique Solution?
No:
Several
subjective decisions need to be made to trade off among different parameters. Even the same designer can come up with several alternate design solutions.
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Analysis versus Design
An analysis technique helps elaborate the customer requirements through careful thinking:
And at the same time consciously avoids making any decisions regarding implementation.
The design model is obtained from the analysis model through transformations over a series of steps:
Decisions regarding implementation are consciously made.
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A Fundamental Question
How to distinguish between the superior of two alternate design solutions? Unless we know what a good software design is: We can not possibly design one.
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Good and Bad Designs
There is no unique way to design a system. Even using the same design methodology:
Different
designers can arrive at very different design solutions.
We need to distinguish between good and bad designs.
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Which of Two is a Better Design?
Should implement all functionalities of the system correctly.
Should be easily understandable. Should be efficient. Should be easily amenable to change, i.e. easily maintainable.
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Which of Two is a Better Design?
Understandability of a design is a major issue:
Determines A
goodness of design:
design that is easy to understand:
Also easy to maintain and change.
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Which of Two is a Better Design?
Unless a design is easy to understand,
Tremendous effort needed to maintain it
We already know that about 60% effort is spent in maintenance.
If the software is not easy to understand:
Maintenance effort would increase many times.
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Understandability
Use consistent and meaningful names:
For
various design components.
Should make use of abstraction and decomposition principles in ample measure.
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How are Abstraction and Decomposition Principles Used in Design?
Two principal ways:
Modular Layered
Design Design
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Modularity
Modularity is a fundamental attributes of any good design.
Decomposition Modules
of a problem cleanly into modules:
are almost independent of each other Divide and conquer principle.
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Modularity
If modules are independent:
Modules
can be understood separately,
To
Reduces the complexity greatly.
understand why this is so,
Remember that it is very difficult to break a bunch of sticks but very easy to break the sticks individually.
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Layered Design
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Layered Design
Neat arrangement of modules in a hierarchy means:
Low
fan-out
Control
abstraction
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Modularity
In technical terms, modules should display:
High Low
cohesion
coupling. and coupling.
We shall next discuss:
cohesion
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Cohesion and Coupling
Cohesion is a measure of:
functional
strength of a module. A cohesive module performs a single task or function.
A
Coupling between two modules:
measure of the degree of the interdependence or interaction between the two modules.
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Cohesion and Coupling
A module having high cohesion and low coupling:
functionally
independent of other modules:
A functionally independent module has minimal interaction with other modules.
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Advantages of Functional Independence
Better understandability and good design: Complexity of design is reduced, Different modules easily understood in isolation:
Modules
are independent
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Advantages of Functional Independence
Functional independence reduces error propagation.
Degree of interaction between modules is low. An error existing in one module does not directly affect other modules.
Reuse of modules is possible.
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Advantages of Functional Independence
A functionally independent module:
Can
be easily taken out and reused in a different program.
Each module does some well-defined and precise function The interfaces of a module with other modules is simple and minimal.
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Functional Independence
Unfortunately, there are no ways: To quantitatively measure the degree of cohesion and coupling.
Classification
of different kinds of cohesion and coupling:
Can give us some idea regarding the degree of cohesiveness of a module.
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Classification of Cohesiveness
Classification is often subjective:
Yet
gives us some idea about cohesiveness of a module.
By examining the type of cohesion exhibited by a module:
We
can roughly tell whether it displays high cohesion or low cohesion.
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Classification of Cohesiveness
functional sequential communicational procedural temporal logical coincidental
Degree of cohesion
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Coincidental Cohesion
The module performs a set of tasks:
Which
relate to each other very loosely, if at all.
The module contains a random collection of functions. Functions have been put in the module out of pure coincidence without any thought or design.
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Logical Cohesion
All elements of the module perform similar operations:
e.g.
error handling, data input, data output, etc. set of print functions to generate an output report arranged into a single module.
An example of logical cohesion:
A
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Temporal Cohesion
The module contains tasks that are related by the fact:
All the tasks must be executed in the same time span. The set of functions responsible for
Example:
initialization, start-up, shut-down of some process, etc.
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Procedural Cohesion
The set of functions of the module:
All Certain
part of a procedure (algorithm)
sequence of steps have to be carried out in a certain order for achieving an objective,
e.g. the algorithm for decoding a message.
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Communicational Cohesion
All functions of the module:
Reference
structure,
or update the same data
Example:
The
set of functions defined on an array or a stack.
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Sequential Cohesion
Elements of a module form different parts of a sequence,
Output
from one element of the sequence is input to the next.
Example:
sort
search display
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Functional Cohesion
Different elements of a module cooperate:
To
achieve a single function,
e.g.
managing an employee's pay-roll.
When a module displays functional cohesion,
We
can describe the function using a single sentence.
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Determining Cohesiveness
Write down a sentence to describe the function of the module
If
the sentence is compound,
If
it has words like first, next, after, then, etc. it has words like initialize,
It has a sequential or communicational cohesion.
If
It has sequential or temporal cohesion.
It probably has temporal cohesion.
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Coupling
Coupling indicates:
How
closely two modules interact or how interdependent they are. degree of coupling between two modules depends on their interface complexity.
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The
Coupling
There are no ways to precisely determine coupling between two modules:
Classification of different types of coupling will help us to approximately estimate the degree of coupling between two modules.
Five types of coupling can exist between any two modules.
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Classes of coupling
data stamp
control common content
Degree of coupling
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Data coupling
Two modules are data coupled,
If
they communicate via a parameter: an elementary data item, e.g an integer, a float, a character,
etc.
The
data item should be problem related: Not used for control purpose.
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Stamp Coupling
Two modules are stamp coupled,
If
they communicate via a composite data item
such as a record in PASCAL
or a structure in C.
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Control Coupling
Data from one module is used to direct:
Order
of instruction execution in another.
flag set in one module and tested in another module.
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Example of control coupling:
A
Common Coupling
Two modules are common coupled,
If
they share some global data.
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Content Coupling
Content coupling exists between two modules:
If they share code,
e.g, branching from one module into another module.
from data coupling to content coupling.
The degree of coupling increases
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Neat Hierarchy
Control hierarchy represents:
Organization
Control
of modules.
hierarchy is also called program structure.
Most common notation:
A
tree-like diagram called structure chart.
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Layered Design
Essentially means:
Low
fan-out
Control
abstraction
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Characteristics of Module Hierarchy
Depth:
Number Overall A
of levels of control
span of control.
Width:
Fan-out:
measure of the number of modules directly controlled by given module.
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Characteristics of Module Structure
Fan-in:
Indicates
how many modules directly invoke a given module.
High
fan-in represents code reuse and is in general encouraged.
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Module Structure
Fan out=2 Fan in=0 Fan Fan in=1 out=1 Fan in=2
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Layered Design
A design having modules:
With A
high fan-out numbers is not a good design: module having high fan-out lacks cohesion.
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Goodness of Design
A module that invokes a large number of other modules:
Likely Not
to implement several different functions:
likely to perform a single cohesive function.
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Control Relationships
A module that controls another module:
Said
to be superordinate to it.
Conversely, a module controlled by another module:
Said
to be subordinate to it.
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Visibility and Layering
A module A is said to be visible by another module B,
If
A directly or indirectly calls B.
The layering principle requires Modules at a layer can call only the modules immediately below it.
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Bad Design
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Abstraction
A module is unaware (how to invoke etc.) of the higher level modules. Lower-level modules:
Do
input/output and other low-level functions. more managerial functions.
Upper-level modules:
Do
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Abstraction
The principle of abstraction requires:
Lower-level
modules do not invoke functions of higher level modules.
Also
known as layered design.
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High-level Design
High-level design maps functions into modules {fi} {mj} such that:
Each
module has high cohesion
Coupling
among modules is as low as possible are organized in a neat hierarchy
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Modules
High-level Design
f1 f2 f3
fn
d1 d2
d3
d1
d4
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Design Approaches
Two fundamentally different software design approaches:
Function-oriented
Object-oriented
design
design
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Design Approaches
These two design approaches are radically different.
However,
are complementary
Rather than competing techniques.
Each
technique is applicable at
Different stages of the design process.
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Function-Oriented Design
A system is looked upon as something
That performs a set of functions.
Starting at this high-level view of the system:
Each function is successively refined into more detailed functions. Functions are mapped to a module structure.
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Example
The function create-new-librarymember:
Creates
the record for a new member,
Assigns
Prints
a unique membership number
a bill towards the membership
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Example
Create-library-member function consists of the following subfunctions:
Assign-membership-number
Create-member-record Print-bill
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Function-Oriented Design
Each subfunction:
Split
into more detailed subfunctions and so on.
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Function-Oriented Design
The system state is centralized:
Accessible
to different functions, Member-records:
Available for reference and updation to several functions: Create-new-member Delete-member Update-member-record
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Function-Oriented Design
Several function-oriented design approaches have been developed:
Structured design (Constantine and Yourdon, 1979)
Jackson's structured design (Jackson, 1975) Warnier-Orr methodology
Wirth's step-wise refinement
Hatley and Pirbhai's Methodology
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Object-Oriented Design
System is viewed as a collection of objects (i.e. entities). System state is decentralized among the objects:
Each
object manages its own state information.
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Object-Oriented Design Example
Library Automation Software:
Each
library member is a separate object
Functions
With its own data and functions.
defined for one object:
Cannot directly refer to or change data of other objects.
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Object-Oriented Design
Objects have their own internal data:
Defines their state.
Similar objects constitute a class.
Each object is a member of some class.
From a super class.
Classes may inherit features
Conceptually, objects communicate by message passing.
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Object-Oriented versus FunctionOriented Design
Unlike function-oriented design,
In
OOD the basic abstraction is not functions such as sort, display, track, etc., But real-world entities such as employee, picture, machine, radar system, etc.
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Object-Oriented versus FunctionOriented Design
In OOD:
Software
is not developed by designing functions such as:
update-employee-record, get-employee-address, etc. employees, departments, etc.
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But
by designing objects such as:
Object-Oriented versus FunctionOriented Design
Grady Booch sums up this fundamental difference saying:
Identify
verbs if you are after procedural design and nouns if you are after objectoriented design.
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Object-Oriented versus FunctionOriented Design
In OOD:
State But
information is not shared in a centralized data. is distributed among the objects of the system.
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Example:
In an employee pay-roll system, the following can be global data:
employee
Whereas, in object oriented design:
Data
names, code numbers, basic salaries, etc.
is distributed among different employee objects of the system.
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Object-Oriented versus FunctionOriented Design
Objects communicate by message passing.
One
object may discover the state information of another object by interrogating it.
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Object-Oriented versus FunctionOriented Design
Of course, somewhere or other the functions must be implemented:
The
functions are usually associated with specific real-world entities (objects) access only part of the system state information.
Directly
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Object-Oriented versus FunctionOriented Design
Function-oriented techniques group functions together if:
As a group, they constitute a higher level function.
On the other hand, object-oriented techniques group functions together:
On the basis of the data they operate on.
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Object-Oriented versus FunctionOriented Design
To illustrate the differences between object-oriented and function-oriented design approaches,
let An
us consider an example ---
automated fire-alarm system for a large building.
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Fire-Alarm System
We need to develop a computerized fire alarm system for a large multistoried building:
There
are 80 floors and 1000 rooms in the building.
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Fire-Alarm System
Different rooms of the building:
Fitted
with smoke detectors and fire alarms.
The fire alarm system would monitor:
Status
of the smoke detectors.
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Fire-Alarm System
Whenever a fire condition is reported by any smoke detector:
the
fire alarm system should:
Determine the location from which the fire condition was reported
Sound the alarms in the neighboring locations.
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Fire-Alarm System
The fire alarm system should:
Flash
an alarm message on the computer console:
Fire fighting personnel man the console round the clock.
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Fire-Alarm System
After a fire condition has been successfully handled,
The
fire alarm system should let fire fighting personnel reset the alarms.
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Function-Oriented Approach:
/* Global data (system state) accessible by various functions */ BOOL detector_status[1000]; int detector_locs[1000]; BOOL alarm-status[1000]; /* alarm activated when status set */ int alarm_locs[1000]; /* room number where alarm is located */ int neighbor-alarms[1000][10];/*each detector has at most*/ /* 10 neighboring alarm locations */ The functions which operate on the system state: interrogate_detectors(); get_detector_location(); determine_neighbor(); ring_alarm(); reset_alarm(); report_fire_location();
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Object-Oriented Approach:
class detector attributes: status, location, neighbors operations: create, sense-status, getlocation, find-neighbors class alarm attributes: location, status operations: create, ring-alarm, get_location, reset-alarm In the object oriented program,
appropriate number of instances of the class detector and alarm should be created.
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Object-Oriented versus FunctionOriented Design
In the function-oriented program :
The system state is centralized Several functions accessing these data are defined.
In the object oriented program,
The state information is distributed among various sensor and alarm objects.
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Object-Oriented versus FunctionOriented Design
Use OOD to design the classes:
Then
applies top-down function oriented techniques
To design the internal methods of classes.
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Object-Oriented versus FunctionOriented Design
Though outwardly a system may appear to have been developed in an object oriented fashion,
But
inside each class there is a small hierarchy of functions designed in a top-down manner.
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Summary
We started with an overview of:
Activities undertaken during the software design phase.
We identified:
The information need to be produced at the end of the design phase:
So that the design can be easily implemented using a programming language.
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Summary
We characterized the features of a good software design by introducing the concepts of:
fan-in,
fan-out,
cohesion,
coupling,
abstraction,
etc.
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Summary
We classified different types of cohesion and coupling:
Enables
us to approximately determine the cohesion and coupling existing in a design.
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Summary
Two fundamentally different approaches to software design:
Function-oriented
approach Object-oriented approach
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Summary
We looked at the essential philosophy behind these two approaches
These
two approaches are not competing but complementary approaches.
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