Transcript Software Processes - Pennsylvania State University

Chapter 3
Software Processes
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 1
Software Processes

Coherent sets of activities for
specifying, designing, implementing
and testing software systems
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 2
Objectives




To introduce software process models
To describe a number of different process models
and when they may be used
To describe outline process models for
requirements engineering, software development,
testing and evolution
To introduce CASE technology to support
software process activities
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 3
Topics covered







Software process models
Process iteration
Software specification
Software design and implementation
Software validation
Software evolution
Automated process support
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 4
The software process

A structured set of activities required to develop a
software system
•
•
•
•

Specification
Design
Validation
Evolution
A software process model is an abstract
representation of a process. It presents a
description of a process from some particular
perspective
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 5
Generic software process models

The waterfall model and V model
•

Evolutionary development
•

Specification and development are interleaved
Formal systems development
•

Separate and distinct phases of specification and development
A mathematical system model is formally transformed to an
implementation
Reuse-based development
•
The system is assembled from existing components
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 6
Waterfall model
Requirements
definition
System and
software design
Implementation
and unit testing
Integr ation and
system testing
Operation and
maintenance
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 7
Waterfall model problems



Inflexible partitioning of the project into distinct
stages
Difficult to accommodate changing customer
requirements after the process is underway
Appropriate only when the requirements are wellunderstood
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 8
V Model
Requirement
Analysis
Operation &
Maintenance
Validate Requirement
Acceptance
Testing
System
Design
Verify Design
Program
Design
System
Testing
Unit & Integration Testing
Coding
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 9
Waterfall model
Requirements
definition
System and
software design
Implementation
and unit testing
Integr ation and
system testing
Prototyping
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Operation and
maintenance
Slide 10
Evolutionary development

Exploratory development
•

Objective is to work with customers and to evolve a final
system from an initial outline specification. Should start with
well-understood requirements
Throw-away prototyping
•
Objective is to understand the system requirements. Should start
with poorly understood requirements
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 11
Evolutionary development
Concurr ent
activities
Outline
description
©Ian Sommerville 2000
Specification
Initial
version
Development
Intermediate
versions
Validation
Final
version
Software Engineering, 6th edition. Chapter 3
Slide 12
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
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 13
Formal systems development



Based on the transformation of a mathematical
specification through different representations to
an executable program
Transformations are ‘correctness-preserving’ so it
is straightforward to show that the program
conforms to its specification
Embodied in the ‘Cleanroom’ approach to
software development
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 14
Formal systems development
Requirements
definition
©Ian Sommerville 2000
Formal
specification
Formal
transformation
Software Engineering, 6th edition. Chapter 3
Integration and
system testing
Slide 15
Formal transformations
Formal transformations
T1
Formal
specification
T2
R1
P1
T3
R2
P2
T4
Executable
program
R3
P3
P4
Proofs of transformation correctness
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 16
Formal systems development

Problems
•
•

Need for specialised skills and training to apply the
technique
Difficult to formally specify some aspects of the system
such as the user interface
Applicability
•
Critical systems especially those where a safety or
security case must be made before the system is put into
operation
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 17
Reuse-oriented development


Based on systematic reuse where systems are
integrated from existing components or COTS
(Commercial-off-the-shelf) systems
Process stages
•
•
•
•

Component analysis
Requirements modification
System design with reuse
Development and integration
This approach is becoming more important but
still limited experience with it
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 18
Reuse-oriented development
Requirements
specification
Component
analysis
Requirements
modification
System design
with reuse
Development
and integration
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
System
validation
Slide 19
Topics covered







Software process models
Process iteration
Software specification
Software design and implementation
Software validation
Software evolution
Automated process support
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 20
Process iteration



Iteration means earlier stages are reworked in the
process for large systems
Iteration can be applied to any of the generic
process models
Two (related) approaches
•
•
Incremental development
Spiral development
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 21
Incremental development



System development is decomposed into increments
and each delivers a proportion of the system.
Increments are developed based on their
requirement priorities.
When the development of an increment is started,
its requirement is fixed until the development of the
next increment.
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 22
Incremental development
Define outline
requirements
Develop system
increment
Assign requirements
to increments
Valida te
increment
Design system
architecture
Integrate
increment
Valida te
system
Final
system
System incomplete
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 23
Incremental development advantages

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Some system functionalities are available earlier
Early increments help elicit requirements for later
increments
Lower risk of overall project failure
The high priority system services receive more
testing
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 24
Spiral model sectors

Objective setting
•

Risk assessment and reduction
•

Risks are assessed and activities put in place to reduce the key
risks
Development and validation
•

Specific objectives for the phase are identified
A development model for the system is chosen which can be
any of the generic models
Planning
•
The project is reviewed and the next phase of the spiral is
planned
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 25
Spiral model of the software process
Determine objectives
alternatives and
constraints
Evaluate alternatives
identify, resolve risks
Risk
analysis
Risk
analysis
Risk
analysis
REVIEW
Requirements plan
Life-cycle plan
Development
plan
Plan next phase
©Ian Sommerville 2000
Integration
and test plan
Prototype 3
Prototype 2
Risk
a nayl sis Prototype 1
Operational
protoype
Simulations, models, benchmarks
Concept of
Operation
S/W
requirements
Requirement
validation
Product
design
Detailed
design
Code
Unit test
Design
V&V
Integr ation
test
Acceptance
test
Develop, verify
Service
next-level product
Software Engineering, 6th edition. Chapter 3
Slide 26
Spiral development
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Process is represented as a spiral
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
Risks are explicitly assessed and resolved
throughout the process
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 27
Topics covered







Software process models
Process iteration
Software specification
Software design and implementation
Software validation
Software evolution
Automated process support
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 28
Software specification


Define required services and constraints for
system development
Requirements engineering process (Ch. 6)
•
•
•
•
Feasibility study
Requirements elicitation and analysis
Requirements specification
Requirements validation
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 29
The requirements engineering process
Feasibility
study
Requirements
elicitation and
analysis
Requir ements
specification
Feasibility
report
Requirements
validation
System
models
User and system
requirements
Requirements
document
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 30
Topics covered







Software process models
Process iteration
Software specification
Software design and implementation
Software validation
Software evolution
Automated process support
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 31
Software design and implementation




The process of converting the system
specification into an executable system
Software design - design software structure
Implementation - translate structure into an
executable program
The activities of design and implementation can
be interleaved
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 32
Design process activities
Re quire me nts
spec if ication
Design a cti
vitie s
Arc hitec tura l
design
Abstra ct
spec if ication
Inte rf ac e
design
Com ponent
design
Data
structure
design
Algor ithm
design
System
a rc hitec ture
Softwa re
spec if ication
Inte rf ac e
spec if ica
tion
Com ponent
spec if ication
Data
structure
spec if ication
Algor ithm
spec if ica
tion
Design pr
oducts
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 33
Design methods

Systematic approaches to developing a software
design (Ch. 7)
•
Data-flow model
•
Entity-relation-attribute model
•
Structural model
•
Object models
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 34
Topics covered







Software process models
Process iteration
Software specification
Software design and implementation
Software validation
Software evolution
Automated process support
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 35
Software validation


Validate user requirements and verify designs
(Ch. 8, 20)
Review processes and test system
•
Testing is to execute system with test cases that
are derived from the specification, or real user
data
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 36
The testing process
Unit
testing
Module
testing
Sub-system
testing
System
testing
Acceptance
testing
Component
testing
©Ian Sommerville 2000
Integration testing
Software Engineering, 6th edition. Chapter 3
User
testing
Slide 37
Topics covered







Software process models
Process iteration
Software specification
Software design and implementation
Software validation
Software evolution
Automated process support
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 38
System evolution
Define system
requirements
Assess existing
systems
Propose system
changes
Existing
systems
©Ian Sommerville 2000
Modify
systems
New
system
Software Engineering, 6th edition. Chapter 3
Slide 39
Topics covered







Software process models
Process iteration
Software specification
Software design and implementation
Software validation
Software evolution
Automated process support
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 40
Automated process support (CASE)


CASE is software to support software
development and evolution processes
Activity automation
•
•
•
•
•
Graphical editors for system model development
Graphical UI builder for user interface construction
Debuggers to support program fault finding
:
:
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 41
CASE classification

Functional perspective
•

Process perspective
•

Tools are classified according to their specific function
(Editing, Planning, etc.)
Tools are classified according to process activities that
are supported (Design, Prototyping, Testing, etc.)
Integration perspective
•
Tools are classified according to their organization into
integrated units (Version management, system building
tools)
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 42
Key points




Software processes are the activities involved in
producing and evolving a software system. They
are represented in a software process model
General activities are specification, design and
implementation, validation and evolution
Generic process models describe the organisation
of software processes
Iterative process models describe the software
process as a cycle of activities
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 43
Key points





Requirements engineering is the process of
developing a software specification
Design and implementation processes transform
the specification to an executable program
Validation involves checking that the system
meets to its specification and user needs
Evolution is concerned with modifying the
system after it is in use
CASE technology supports software process
activities
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 3
Slide 44