Software Process Models in Software Engineering

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 1
Software Processes

Coherent sets of activities for
specifying, designing, implementing
and testing software systems

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

Topics covered

Software process models

Process iteration

Software specification

Software design and implementation

Software validation

Software evolution

Automated process support

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

Generic software process models

The waterfall model
• Separate and distinct phases of specification and development

Evolutionary development
• Specification and development are interleaved

Formal systems development
• A mathematical system model is formally transformed to an
implementation

Reuse-based development
• The system is assembled from existing components

Waterfall model
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Waterfall model phases

Requirements analysis and definition

System and software design

Implementation and unit testing

Integration and system testing

Operation and maintenance

The drawback of the waterfall model is the
difficulty of accommodating change after the
process is underway

Waterfall model problems

Inflexible partitioning of the project into distinct
stages

This makes it difficult to respond to changing
customer requirements

Therefore, this model is only appropriate when
the requirements are well-understood


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

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
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


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

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Formal transformations
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
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

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

Reuse-oriented development
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Process iteration

System requirements ALWAYS evolve in the
course of a project so 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 development
• Spiral development

Incremental development

Rather than deliver the system as a single delivery,
the 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

Once the development of an increment is started, the
requirements are frozen though requirements for later
increments can continue to evolve

Incremental development
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Incremental development 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

Extreme programming

New approach to development based on the
development and delivery of very small
increments of functionality

Relies on constant code improvement, user
involvement in the development team and
pairwise programming

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

Risks are explicitly assessed and resolved
throughout the process

Spiral model of the software process
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Spiral model sectors

Objective setting
• Specific objectives for the phase are identified

Risk assessment and reduction
• Risks are assessed and activities put in place to reduce the key risks

Development and validation
• 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

Software specification

The process of establishing what services are
required and the constraints on the system’s
operation and development

Requirements engineering process
• Feasibility study
• Requirements elicitation and analysis
• Requirements specification
• Requirements validation

The requirements engineering process
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Software design and implementation

The process of converting the system
specification into an executable system

Software design
• Design a software structure that realises the specification

Implementation
• Translate this structure into an executable program

The activities of design and implementation are
closely related and may be inter-leaved

Design process activities

Architectural design

Abstract specification

Interface design

Component design

Data structure design

Algorithm design

The software design process
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Design methods

Systematic approaches to developing a software
design

The design is usually documented as a set of
graphical models

Possible models
• Data-flow model
• Entity-relation-attribute model
• Structural model
• Object models

Programming and debugging

Translating a design into a program and removing
errors from that program

Programming is a personal activity - there is no
generic programming process

Programmers carry out some program testing to
discover faults in the program and remove these
faults in the debugging process

The debugging process
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Software validation

Verification and validation is intended to show
that a system conforms to its specification and
meets the requirements of the system customer

Involves checking and review processes and
system testing

System testing involves executing the system
with test cases that are derived from the
specification of the real data to be processed by
the system

The testing process
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Testing stages

Unit testing
• Individual components are tested

Module testing
• Related collections of dependent components are tested

Sub-system testing
• Modules are integrated into sub-systems and tested. The focus here should
be on interface testing

System testing
• Testing of the system as a whole. Testing of emergent properties

Acceptance testing
• Testing with customer data to check that it is acceptable

Testing phases
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Software evolution

Software is inherently flexible and can change.

As requirements change through changing
business circumstances, the software that
supports the business must also evolve and
change

Although there has been a demarcation between
development and evolution (maintenance) this is
increasingly irrelevant as fewer and fewer
systems are completely new

System evolution
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Automated process support (CASE)

Computer-aided software engineering (CASE) is
software to support software development and
evolution processes

Activity automation
• Graphical editors for system model development
• Data dictionary to manage design entities
• Graphical UI builder for user interface construction
• Debuggers to support program fault finding
• Automated translators to generate new versions of a program

Case technology

Case technology has led to significant
improvements in the software process though not
the order of magnitude improvements that were
once predicted
• Software engineering requires creative thought - this is not
readily automatable
• Software engineering is a team activity and, for large projects,
much time is spent in team interactions. CASE technology does
not really support these

CASE classification

Classification helps us understand the different types
of CASE tools and their support for process activities

Functional perspective
• Tools are classified according to their specific function

Process perspective
• Tools are classified according to process activities that are supported

Integration perspective
• Tools are classified according to their organisation into integrated
units

Functional tool classification

Activity-based classification
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CASE integration

Tools
• Support individual process tasks such as design consistency
checking, text editing, etc.

Workbenches
• Support a process phase such as specification or design,
Normally include a number of integrated tools

Environments
• Support all or a substantial part of an entire software process.
Normally include several integrated workbenches

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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

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

Software Process Models in Software Engineering

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