Transcript TCS 2411 Software Engineering

Software Life Cycle
“What happens in the ‘life’ of software”
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Lecture Objectives
To look at the ‘life cycle’ of a software
To understand the software process and
its related elements
To relate to the different software
engineering paradigms
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Software Engineering
Layers
tools
methods
process model
a “quality” focus
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Software Process
Activities in software projects
Characterised by a common process
framework
Framework activities - task sets
Umbrella activities
“Process maturity” enables development
of quality software products
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Common Process
Framework
Common process framework
Framework activities
work tasks
work products
milestones & deliverables
QA checkpoints
Umbrella Activities
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Umbrella Activities
Software project management
Formal technical reviews
Software quality assurance
Software configuration management
Document preparation and production
Reusability management
Measurement
Risk management
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Process as Problem
Solving
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The Process Model:
Adaptability
the framework activities will always
be applied on every project ... BUT
the tasks (and degree of rigor) for
each activity will vary based on:
the type of project (an “entry point” to
the model)
characteristics of the project
common sense judgment; concurrence
of the project team
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The Primary Goal:
High Quality
Remember:
High quality = project timeliness
Why?
Less rework!
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The Systematic Process
Analysis
Design
Problem
Models
Development
Testing
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Generic Phases
Definition Phase
Focus on ‘what’ the software is
Development Phase
Focus on ‘how’ the software works
Maintenance Phase
Focus on ‘change’ to the software
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Definition Phase
Identify information to be processed
Identify system behaviour - functions and
performance
Determine constraints, interfaces,
validation criteria
Major tasks:
System engineering
Software project planning
Requirements analysis
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Development Phase
Define data structures, function
implementation, procedural details, interfaces
Translate design to programming language
How testing is performed
Major tasks:
Software design
Code generation
Software testing
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Maintenance Phase
Reapplies definition and development
phases to existing software
Types of changes:
Correction
Adaptation
Enhancement
Prevention
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Waterfall Model
Syst em
Engineering
Analysis
Design
Code
Test ing
M aint enance
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Waterfall Model
Characteristics
The classic life cycle - oldest and most
widely used paradigm
Activities ‘flow’ from one phase to another
If there are corrections, return to a
previous phase and ‘flow’ from there again
Major advantages: Good for planning and
well-defined/repeated projects
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Problems of Waterfall
Model
Real projects often follow the sequence
All requirements may not be stated
explicitly by customer
Customer only sees the results after some
time
Developers are often delayed at certain
phases
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Prototyping Model
Start
Requirements
gathering and
refinement
Engineer
product
Quick
design
Building
prototype
Refining Customer
prototype evaluation
Stop
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Prototyping Model
Characteristics
Developer and customer determine
objectives and draft requirements
Prototype quickly produced and evaluated
by customer
Prototype then refined, and re-evaluated
Process iterated, before final product
development
Advantages: Customer participation and
better requirements
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Problems of Prototyping
Model
Problem 1: Customer may see prototype
as working model and expects fast results
Problem 2: Developer compromised when
producing prototype quickly, e.g. different
operating system or programming
language
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Rapid Application
Development (RAD)
Team #2
Business
Modeling
Team #1
Data
Business
Modeling
Modeling
Process
Data
Modeling
Application
Modeling
Generation
Process
Testing &
Turnover
Modeling
Application
Generation
Testing &
Turnover
Time period
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RAD Characteristics
“High-speed” version of waterfall model
Primarily for information systems applications
Requirements well-understood, fully functional
system produced in short time
The application modularised - major functions
can be completed in 3 months
Separate teams complete the functions, then
integrated as a whole
Requires human resource and commitment
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Incremental Model
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Incremental Model
Characteristics
Software separated into different
“increments” - complete working portions
Focus on delivery of operational product
with each increment - can be evaluated
Useful when insufficient staff and can be
planned to manage technical risks, e.g.
waiting for new hardware
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Spiral Model
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Spiral Model
Characteristics
Originally proposed by Boehm, couples
iterative nature of prototyping and the
systematic aspects of waterfall model
Software is developed in series of
incremental releases
Each iteration produces a more complete
product
Better management through risk analysis
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Problems of Spiral
Model
May be difficult to convince customers
that evolution is controllable
Demands risk assessment expertise major risk will cause problems if not
identified
Relatively new and not widely used cannot determine performance
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Component Assembly
Model
Extract
components
Identify
candidate
components
Look up
components
in library
yes
Available?
Construct
System
no
Build
components
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Component Assembly
Characteristics
Use of object-oriented technology
Components - classes that encapsulate
both data and algorithms
Components developed to be reusable
Paradigm similar to spiral model, but
engineering activity involves components
System produced by assembling the
correct components
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Fourth Generation
Techniques (4GT)
Requirement s
gat hering
"Design"
Strat egy
Imp lement at ion
us ing 4GL
Test ing
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4GT Characteristics
Use of software tools that allow software
engineer to specify s/w characteristics at
higher level
The tools generate codes based on
specification
More time in design and testing - increase
productivity
Tools may not be easy to use, codes
generated may not be efficient
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Other Process Models
Component assembly model—the process to
apply when reuse is a development objective
Concurrent process model—recognizes that
different part of the project will be at different
places in the process
Formal methods—the process to apply when a
mathematical specification is to be developed
Cleanroom software engineering—
emphasizes error detection before testing
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Conclusion
The paradigm used for development of
software depends on a number of factors
People - staff & users
Software product
Tools available
Environment
Existing models makes development
process clearer, but they can be evolved
to become new paradigms
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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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