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1
SWE 205 - Introduction to Software Engineering
Lecture 4
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Lecture Objectives Software processes and their principle
characteristics. Generic software process models
Waterfall model Evolutionary development Component based system development
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What is a software process? A set of ordered tasks to produce indented
output of some kind Involving activates,
constraints; and Resources
The process of building a software product Life cycle - describes the life of the software from
conception through its implementation, delivery, use and maintenance.
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Why we need software process?
Common understanding of the activities, resources and constraints involved in software development.
Creating processes helps Find inconsistencies, Redundancies; and Omissions
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Generic software process models The waterfall model
Separate and distinct phases of specification and development.
Evolutionary development Specification, development and validation are
interleaved. Component-based software engineering
The system is assembled from existing components.
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Waterfall ModelRequirements
definition
System andsoftware design
Implementationand unit testing
Integration andsystem testing
Operation and
maintenance
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Waterfall Model Inflexible partitioning of the project into
distinct stages Makes it difficult to respond to changing
customer requirements. Model is only appropriate when the
requirements are well-understood; and changes will be fairly limited during the design
process.
Few business systems have stable requirements.
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Evolutionary development Exploratory development
Objective is to work with customers and to evolve a final system from an initial outline specification.
Starts with well-understood requirements and add new features as proposed by the customer.
Throw-away prototyping Objective is to understand the system
requirements. Starts with poorly understood requirements to
clarify what is really needed.
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Evolutionary DevelopmentConcurrentactivities
ValidationFinal
version
DevelopmentIntermediate
versions
SpecificationInitial
version
Outlinedescription
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Evolutionary Development Problems
Lack of process visibility; Systems are often poorly structured; Special skills (e.g. in languages for rapid
prototyping) may be required. Applicability
For small or medium-size interactive systems; For parts of large systems (e.g. the user interface); For short-lifetime systems.
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Component Based System Engineering Systematic reuse where systems are
integrated from existing components or COTS (Commercial-off-the-shelf) systems.
This approach is becoming increasingly usedas component standards have emerged.
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Component Based System Engineering
Requirementsspecification
Componentanalysis
Developmentand integration
System designwith reuse
Requirementsmodification
Systemvalidation
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Process Iterations System requirements ALWAYS evolve in the
course of a project Process iteration where earlier stages are
reworked is always part of the process for large systems.
Iteration can be applied to any of the generic process models.
Two (related) approaches Incremental delivery; Spiral development.
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Incremental Delivery Rather than deliver the system as a single
delivery, Development and delivery is broken down into
increments; With each increment delivering part of the
required functionality. User requirements are prioritised and the
highest priority requirements are included in early increments.
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Incremental Delivery
Validateincrement
Develop systemincrement
Design systemarchitecture
Integrateincrement
Validatesystem
Define outline requirements
Assign requirements to increments
System incomplete
Finalsystem
Requirements are frozen though requirements for later increments can continue to evolve.
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Incremental Delivery - Advantages Customer value can be delivered with each
increment so system functionality is available earlier.
Early increments act as a prototype to help elicit requirements for later increments.
Lower risk of overall project failure. The highest priority system services tend to
receive the most testing.
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Spiral Development Process is represented as a spiral
rather than as a sequence of activities with backtracking.
Each loop in the spiral represents a phase in the process.
No fixed phases such as specification or design - loops in the spiral are chosen depending on what is required.
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Spiral Development Model
Riskanalysis
Riskanalysis
Riskanalysis
Riskanalysis Proto-
type 1
Prototype 2
Prototype 3Opera-tionalprotoype
Concept ofOperation
Simulations, models, benchmarks
S/Wrequirements
Requirementvalidation
DesignV&V
Productdesign Detailed
design
Code
Unit test
IntegrationtestAcceptance
testService Develop, verifynext-level product
Evaluate alternatives,identify, resolve risks
Determine objectives,alternatives and
constraints
Plan next phase
Integrationand test plan
Developmentplan
Requirements planLife-cycle plan
REVIEW
(1) Development & Validation(2) Plan next phase!!!!!
Risks are explicitly assessed and resolved throughout the process.
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Key Points Software processes are the activities involved in
producing and evolving a software system. General activities are specification, design and
implementation, validation and evolution. Generic process models describe the organisation
of software processes. For example waterfall model, evolutionary development; and component-based software engineering.
Iterative process models describe the software process as a cycle of activities.