Software Designing - Software Engineering

INTRO
 Goal is to produce a model or repesentation
 Designer should
 Stock all material of design like components, component solutions &
knowledge
 Choose elements from stock that meet requirements
 Alternatives are also considered & accepted/rejected for final
confirmation

Design Process
 To achieve intended goal of software – successful
implementation
 Always refers to finding solution or problem solving
 Includes algorithms, diagrams, formulae designs, low level
design etc.
 Follow semi standard methods like UML for modelling the
software
 Use different diagrams like activity, use case and sequence
 Use of design patterns and architectural guides (if required)

Design Concepts
 Abstraction
 For modular solution many level of abstraction is possible
 High level solution – solution stated in the language of the problem
environment
 lower levels of abstraction - used for detailed description
 data abstraction is actually a collection of data that describes a data
object

 Architecture
 Software architecture is the complete structure or
arrangement of the program components in such a way that
they interact with each other
 data structures may be used by these components
 these components are the building blocks of the overall
structure of the application being developed
 architectural design can be represented using one or more of a
number of different models
 Patterns
 a design pattern describes a design structure that solves a
particular design problem within a specific context

 Modularity
 consists of software architecture and design patterns
 defined as the modularization of a single attribute (main
module) of software into number of small parts (smaller
modules) such that these parts can be easily managed
 Consider
 C(x) – function of a problem x that define its complexity
 E(X) - function of a problem x that define efforts required to solve
that problem
 Let there are two problems p1 and p2 for which,
If C(p1) > C(p2) then E(p1) > E(p2)

 interesting characteristic is uncovered through observations
and experimentation
C(p1 + p2) > C(p1) + C(p2)
 complexity of two combined problems p1 and p2 is greater than
the complexity when the problems are considered separately
 From previous equations it follows,
E(p1 + p2) > E(p1) + E(p2)
 That leads to a strategy called divide and conquer
 always easy to solve a complex problem when it is divided in
small sub-problems that are easily manageable

 the effort (cost) to develop an
individual software module does
decrease as the total number of
modules increases
 For large size problem, the
number of modules will be
increased and the effort (cost)
associated with integrating the
modules also increases
• M is number of modules that would result in minimum development
cost, but there is no any proper and sophisticated method to predict M
• divide the problem into modules, but utmost care must be taken to stay in
the region of M

 five criteria defined for an effective modular system
 Modular decomposability : complex problem may be solved very
easily by decomposing into smaller sub problems
 Modular composability : Different modules and components are
integrated to produce a new system
 Modular understandability : While integrating a system, if each of the
individual modules or components are understood well, then it become
very easy to build the system once again
 Modular continuity : to minimize side effect of changes it’s always
better to make changes module-wise instead of making changes system-
wise
 Modular protection : for any abnormal condition within a module,
the effects are within that module only. Thus the side effects are
minimized and problems will not spread outside that module

 Information Hiding
 modules of the larger problem must be specified and designed
properly so that information (i.e. algorithms and data) present within
a module is not available to other modules that do not require such
information
 To achieve effective modularity that have proper necessary
communication among modules
 Refinement
 uses a top-down strategy
 the procedural details of a program are refined in various levels
 hierarchy is generated by modularizing the statements of a program

 Functional Independence
 Its a type of modularity and related to the concepts of
abstraction and information hiding
 In this, each module addresses a specific sub function of
requirements and has a simple interface
 Independence is assessed by two criteria
 Cohesion : a relative functional strength of a module
 Coupling : relative interdependence among modules

 Cohesion
 extension of the concept of information hiding
 a module performs only one task within a software procedure (or
required only little interaction with other procedures of the software)
 module that performs different tasks that are loosely related to each
other are termed as coincidentally cohesive
 module that performs tasks that are related logically is logically
cohesive
 When a module contains tasks that are related by the fact that all must
be executed with the same span of time, the module exhibits temporal
cohesion
 Procedural cohesion exists when processing elements of a module are
related and are executed in a specific order

 When all the processing elements do concentrate on only one area of a
data structure, then communicational cohesion is available
 High-end cohesion is characterized by a module that performs one
single and unique procedural task
 Coupling
 is a measure of inter-relationship among various modules in a software
structure
 always depends on the interface complexity among modules
 to make bit easy to understand, connectivity between different modules
of software is made simple by minimum possible coupling
 Simple connectivity  avoid “Ripple Effect” - (errors occurring at one
particular location in the system and propagating through a system)

 Types of Coupling
 Low data coupling (a – c)
 Data passed
 Stamp coupling (a - b)
 Data Structure passed
 Control Coupling (d - e)
 Control flag passed
 High level of coupling
(external coupling)
 Modules tied to external environments
 High (common) Coupling (c – g – k )
 Modules refer a global data area

 Refactoring
 reorganization technique that simplifies the design (or code) of a
component without changing its function or behavior
 it does not alter the external behavior of the code [design] yet improves
its internal structure
 Importance
 To improve quality of software
 Improving the quality of the code, working becomes easier  addition of new
code and maintenance of the code becomes easy
 Improves the code over the period of time
 Play important role in splitting long functions into reasonably small sub
functions
 Helps in creating reusable codes
 Error handling is much easier and within control

 Design Classes
 Each of the software team should define a set of design classes
 Classes should describe the elements of problem domain and that should focus
the customer requirement
 It should,
 Refine the analysis classes that are implemented
 Create a new set of design classes that must support the business solutions

Design Principles
 s/w design must be in such a way that the system is easily
traceable to the analysis model
 software design must not reinvent the wheel (i.e. repetition of the
same tasks), since the time is limited and the resources are also
limited, the design should use the existing ideas and patterns
 design must be in such a way that it minimizes the intellectual
distance between the application or the system and the related
problems
 design must demonstrate uniformity and integration of different
components to their interfaces

 design structure must be flexible to accommodate
modification
 design must withstand against abnormal data, abnormal
events and abnormal operating conditions
 design should not be the coding, and coding should not be the
design
 design should be able to evaluate the quality
 design must be reviewed to minimize the conceptual errors

Architectural Design
 architecture of a program or computing system is actually the
structures of the system that consists of following attributes
 Software Components
 Externally visible property of s/w component
 Relationships among this components
 It’s a representation that enable s/w engineer to analyse,
 Analyse the effectivness of the design
 Consider architectural alternatives
 Reduce the associated risks

 Importance of architectural design
 enables communication between all stakeholders involved in the
development of the system
 emphasizes early design decisions that will have a deep influence on all
software engineering tasks (success mantra for the system)
 builds a small, intellectually model that will help the system to integrate
its component

Architectural Styles
 It explains the system category that consist of,
 set of components (i.e. a database or the computational modules) that
perform a function required by the system
 set of connectors used for communication, coordination, and
cooperation between the components
 constraints used to define the integration of various components into
system
 semantic models used to understand the overall properties of a system
with the help of analysis of its elements

Data Centered Architecture
 Data stored – accessed by various components of the system
 Access a central repository
 Independent of other component changes and update

Data Flow architectures
 When i/p data are to be transformed through series of computational components
to get o/p data
 Pipe and filter pattern  transmit data from one component to next
 Each filter designed to expect data i/p of certain form and produce o/p in specified
form
 Doesn’t require working knowledge of neighbouring filter

 Call & Return architecture
 Enables program structure that is easy to modify and scales
 No of sub style
 Main Program/Sub Program Arch. : decomposes function into control
hierarchy where main program invokes no of other components
 Remote Procedure Call Archi. : component of main program / subprogram
are distributed across multiple computers on n/w
 Object Oriented Architecture
 Component encapsulate data and operations
 Component communication happen via message passing

 Layered Archi.
 No of different layers are
defined having operations that
progressively become closer to
solution

 Other Architectures
 Publisher Subscriber Architecture
 Client Server Architecture
 Peer to Peer Architecture
 Event Based s/w Architecture

Component Level Design
 comes after architectural design is completed
 It’s an alternative approach to the architectural design approach
 Defines data structures, algorithms, interface characteristics & comm
mechanism
 Three different view of a component design
 Objet Oriented view (defines attributes and operations)
 ConventionalView (component with – processing logic + data Structures + interface)
 Process RelatedView (focus on the reusability of the existing software components)

Object OrientedView ConventionalView

User Interface Design
 Creates an effective communication between user and system
 Many factors are considered before designing UI
 Type of the user, age, education level, purpose of the s/w
 Provided at h/w as well as s/w level
 Need good understanding of user needs
 Types of UI - Command Interpreter & GUI
 Benefits of good UI
 Higher revenue
 Increased user efficiency and satisfaction
 Reduced development costs
 Reduced support costs

Design rules for UI
 Place the user in Control
 Define the interaction modes in such a way that the user is not forced
to perform unnecessary or undesired actions
 The interaction should be flexible
 User interaction should be interruptible and undoable
 The different levels of difficulty in interaction for different classes of
users must be customized
 Hide complexities from the casual user
 direct interaction with objects that appear on the screen

 Reduce the User’s Memory Load
 Reduce the demand for short-term memory
 Define shortcuts that can easily remembered
 Established meaningful defaults
 information should be disclosed in a progressive fashion
 Make the Interface Consistent

UI Design Models
 User Profile Model
 Novices (no syntactic but little semantic knowledge)
 Knowledgeable, intermittent users (low syntactic & reasonable semantic
knowledge)
 Knowledgeable, frequent users (Good Syntactic and semantic knowledge)
 Design Model
 Implementation Model
 User’s System Perception

Web Application Design
 WebApp Design involves technical and non technical acts like,
 Establishing look and feel of web app
 Great UI
 Defining overall architectural structure
 Developing content and functionality that resides within his architecture
 Planning the navigation that occurs within the WebApp

WebApp Design Quality Requirement

WebApp Interface Design
 Main objectives
 Establish a consistent
window into the content
an functionality that
provided by the interface
 Guide the user with series
of interactions with the
WebApp
 Organize the navigation
option and content
available to the user

 Aesthetic Design (graphic design)
 Is an artistic endeavour that complements technical aspects of
WebApp Design
 Content Design
 Focus on two different task (addressed by individuals)
 Design representation for content objects and relationship between
them developed
 Conducted by copywriters, graphic designers & others who generate
the content to be used within WebApp

 Architectural Design (in parallel with previous phase)
 Tied to the goals
 Content to be presented
 Users who visit the WebApp
 Navigation philosophy established within the WebApp
 Decisions made during this phase has strong influence on
Navigation Designing
 Navigation Design
 Define the navigation pathways that enable user to access
components and functions of the WebApp (pages, functions,
scripts and other pages)

 Component Level Design
 ModernWebApps deliver sophisticated processing functions
that perform
 localized processing (to generate content and navigation dynamically)
 Computation or data processing capability
 Provide sophisticated database query and access
 Establish data interface with external components

Software Designing - Software Engineering

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