Chapter 4 software design

CHAPTER 3: SOFTWARE DESIGN
3.1 Software Design
- A process through which requirements are translated into a representation of software.
From a project management point of view, software design can be conducted in two main steps:
Preliminary Design
Concerned with the transformation of requirements into data and software architecture.
Detail design
Focuses on refining the architectural representation, and lead to detailed data structure and algorithmic
representations of software.
The three main design activities concerned in the Design phase are: Data design, Architectural
design and Procedural design. In addition, many modern applications have a distinct interface design
activity.
Interface design establishes the layout and interaction mechanisms for human-machine interaction.
Data design
Architectural design
Procedural design
Interface design
Detail design
Preliminary designManagement aspect
Technical aspects
Diagram: Relationship between technical and management aspects of design
3.2 Data Design
Data design is the first (and sometimes the most important) of the three design activities that
are conducted in software engineering. The objective of Data Design is to transform the
information domain into data structures.
Data design principles
1) The systematic analysis principles applied to function and behavior should also be applied
to data
• The same good principles that are followed in analysis principles should also be applied
to help us develop data flow and content, identify data objects, and consider alternative
data organization.
2) All data structures and the operations to be performed on each should be identified.
• The design of an efficient data structure should consider the operations that will be
performed on the data structure itself.
3) A data dictionary should be established and used to define both data and program
design.
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• A data dictionary explicitly represents the relationships among data objects and the
limitations on the elements of a data structure.
4) Low-level data design decisions should be deferred until late in the design process.
• In data design, a top-down approach should be used.
5) The representation of data structure should be known only to those modules that must
make direct use of the data contained within the structure.
• Information Hiding must be practiced.
6) A library of useful data structures and the operations that may be applied to them
should be developed.
• Data structures should be designed for reusability.
7) A software design and programming language should support the specification and
realization of abstract data types.
• The programming language for use should support the creation of complex data
structures.
3.3 Architectural Design
The primary objective in Architectural Design is to develop a modular program structure and
represent the control relationships between modules.
In addition, architectural design combines program structure and data structure, thus defining
interfaces that will enable data to flow throughout the program.
3.4 Procedural Design
The objective in Procedural Design is to transform structural components into a procedural
description of the software.
This step occurs after the data and program structures have been established, i.e. after
architectural design. Procedural details can be represented in different ways:
• Graphical Design Notation
• Tabular Design Notation
• Program Design Language (PDL)
Graphical Design Notation
• The most widely used notation is the flowchart. Some notation used in flowcharts are
(I) Boxes to indicate processing steps; (II) Diamond to indicate logical conditions; (III)
Arrows to indicate flow of control; (IV) Two boxes connected by a line of control will
indicate a Sequence.
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Tabular Design Notation
• Decision tables provide a notation that translates actions and conditions (described in a
processing narrative) into a tabular form.
• The upper left-hand section contains a list of all conditions. The lower left-hand section
lists all actions that are possible based on the conditions. The right-hand sections form a
matrix that indicates condition combinations and the corresponding actions that will
occur for a specific combination.
Conditio
ns
Fixed rate account 1 2 3 4 5
Variable rate account T T F F F
Consumption
<100KWH
T F T F
Consumption
≥100KWH
F T F T
Actions
Minimum monthly
charge
X
Schedule A billing X X
Schedule B billing X
Other treatment X
Program Design Language
• Program Design Language (PDL) is also called structured English, or Pseudocode.
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• The main difference between PDL and its nearest neighbor, 4th Generation Languages,
is that in PDL, the use of narrative text (e.g. English) is embedded directly within PDL
statements.
PDL have the following characteristics:
A fixed syntax of keywords that provide for all structured constructs, data declaration, and
modularity characteristics
A free syntax of natural language that describes processing features
Data declaration facilities that should include both simple (scalar, array) and complex (linked
list or tree) data structures.
Subprogram definition and calling techniques that support various methods of interface
description.
3.5 Software Design Fundamentals
In this part of the chapter, the following Software Design Fundamentals will be discussed.
• Modularity
• Abstraction
• Software Architecture
• Control Hierarchy
• Data Structure
• Software Procedure
• Information Hiding
• Functional Independence
3.5.1 Modularity
Definition of Modularity:
• Software is divided into separately named an addressable components, called
modules, that are integrated to satisfy problem requirements.
• The graph shown refers to the question of Modularity and Software Cost, and is a
useful tool to consider when modularity is to be implemented. In this graph, the
cost or effort drops as more modules are considered, but the cost to interface
modules increases as we have more modules. Thus, we must take care to stay in
the region of minimum cost, i.e. a balance between the number of modules and
the cost or effort.
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3.5.2 Effective Modular Design
 Modularity is an accepted approach in all engineering disciplines
 Reduces complexity and facilitates changes in modules
 Easier implementation
3.5.3 Desirable Characteristics of Module
The attributes of a good module are as follows:
• A small program that can be invoked by the operating system, or it could be a sub-
program invoked by another module(shareable)
• The statement are collectively referred to by a descriptive name called the module name
• A module must return to its caller i.e. have a single entry and exit; (single entry and exit
module to ensure that modules are closed and simplify program maintenance)
• The module should be relatively small in size (small modules allow for more easily
amended programs, estimating and project control more accurate and exhaustive
testing are easier)
• It should be easy to read, modify and use
• A module should preferably have a single function
3.5.4 Advantages and Disadvantages of Modularity
Rationale for Modularity
• Allow large program to be written by several or different people
• Encourage creation of commonly used routines to be placed in library and/or be
used by other programs
• Simplify overlay procedure of loading large program into main storage
• Provide more check point to measure progress
• Simplify design, making program easy to modify and reduce maintenance costs
• Provide a framework for more complete testing, easier to test
• Produces well-designed and more readable program
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Rational Against Modularity
• Execution time may be, but not necessarily, longer
• Storage size may be, but is not necessarily, increased
• Compilation and loading time may be longer
• Intermodule communication problems may be increased
• Demands more initial design time
• More linkage required, run-time may be longer, more source lines must be written
and more documentation has to be done
3.5.5 Abstraction
Abstraction permits one to concentrate on a problem at some level of generalization
without regard to irrelevant low level details.
Use of abstraction also permits one to work with concepts and terms that are familiar, in
the problem environment without having to transform them to an unfamiliar structure.
3.5.6 Software Architecture
Software Architecture can be said to refer indirectly to two important characteristics:
• The hierarchical structure of procedural components (modules) and
• the structure of data
Software architecture is derived through a partitioning process that relates elements of a
software solution to parts of a real-world problem implicitly defined during
requirements analysis.
Thus, in the partitioning process, a big problem is broken up into different software
solutions, and the problem may thus be satisfied by many different candidate
structures.
3.5.7 Control Hierarchy
Control Hierarchy, also called program structure, represents the organization which is
often hierarchical, of program components (modules) and implies a hierarchy of
control.
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There are many different notations used to represent control hierarchy. Some terminology
used in the discussion of control hierarchy are:
• Fan-Out, which is a measure of the number of modules that are directly controlled
by another module;
• Fan-in, which indicates how many modules directly control a given module.
• Depth: Number of levels of control
• Width: Overall span of control
Some other terms used in Control Hierarchy are:
• Superordinate: i.e. a module that controls another module;
• Subordinate: i.e. a module that is controlled by another module.
4.5.8 Data Structure
Data structure is a representation of the logical relationship among individual elements of
data. Data structure dictates the organization, methods of access, degree of
associativity, and processing alternatives for information.
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Examples of classic data structures:
An n - dimensional Space
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• N-dimensional array: result when the sequential vector is extended into an n-
dimensional space.
• Linked list: a data structure that organizes noncontiguous scalar items, vectors, or
spaces in a manner (called nodes) that enables them to be processed as a list.
• Hierarchical data structure: implemented using multilinked lists that contain scalar
items, vectors, and possibly, n-dimensional spaces.
4.5.9 Software Procedure
Software procedure focuses on the processing details of each module individually.
Software procedure must provide a precise specification of processing, including
sequence of events, exact decision points, repetitive operations, and even data
organization/structure.
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Coincindental
Conhesion
Logical
High (most desirable)
Low (least desirable)
Procedural
Functional
Communication
Temporal
4.6 Information Hiding
The principle of information hiding suggests that modules be characterized by design decisions
that (each) hides from the others, i.e. In other words, modules should be specified that
information (procedure and data) contained within a module are inaccessible to other modules
that have no need for such information.
The use of information hiding as a design criterion for modular systems provides the greatest
benefits when modifications are required during testing and later during software maintenance.
This is because as most data and procedures are hidden from other parts of the software,
inadvertent errors introduced during modifications are less likely to propagate to other locations
within the software.
4.7 Functional Independence
The concept of functional independence comes from a direct outgrowth of modularity and the
concepts of abstraction and information hiding.
Functional independence is achieved by developing modules with single-minded function and an
aversion to excessive interaction with other modules. In other words, design software so that
each module addresses a specific subfunction of requirements and has a simple interface when
viewed from other parts of the program structure.
• Independence is measured using two qualitative criteria: cohesion and coupling.
- Cohesion: measures the relative functional strength of a module
- Coupling: measures the relative interdependence among modules.
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Data
Coupling
Stamp
High (least desirable)
Low (most desirable)
Common
Content
Control
4.7.1 Cohesion
A module with high cohesion is able to perform a single task within a software
procedure, requiring little interaction with procedures being performed in other parts
of the program.
In other words, a cohesive module does just one thing and sticks with it.
Low Levels
• Coincidental Cohesion: a module that performs a set of tasks that relate to each
other loosely, if at all.
• Logical Cohesion: a module that performs tasks that are related logically (e.g., a
module that produces all output regardless of type)
• Temporal Cohesion: a module that performs tasks that are related by the fact that all
must be executed with the same span of time.
Moderate Levels
• Procedural Cohesion: this happens when the processing elements of a module are
related and must be executed in a specific order.
• Communication Cohesion: when all processing elements concentrate on one area of
a data structure.
High Levels
• High cohesion is characterized by a module that performs one distinct procedural
task.
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4.7.2 Coupling
Coupling is a measure of interconnection among modules in a software structure.
• Low Coupling Levels (desirable)
• Data coupling: for example passing of simple data like an argument list from one
module to another module.
• Stamp coupling: a portion of data structure, rather than a argument list, is passed via
a module interface.
Moderate Coupling Levels
• Control Coupling: most common in software. Where control is passed via a flag on
which decisions are made in a subordinate or superordinate module.
High Coupling Levels
• Common Coupling: occurs when a number of modules reference a global data area.
• Content Coupling: occurs when one module makes use of data or control
information maintained within the boundary of another module.
4.8 Criteria for Good Design
A design should:
• Exhibit a hierarchical organization that makes intelligent use of control among
components of software
• Be modular; that is, the software should be logically partitioned into components
that perform specific functions and subfunctions
• Contain distinct and separable representation of data and procedure
• Lead to modules that exhibit independent functional characteristics
• Lead to interfaces that reduce the complexity of connections between modules and
with the external environmental
• Be derived using a repeatable method that is driven by information obtained during
software requirements analysis
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Chapter 4 software design

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