Software Processes
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 1
The software process
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Software process: A structured set of activities required to develop a
software system
Fundamental activities which are common to all software activities:
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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 -> Each process model provides only partial information
about that process.
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 2
Generic software process models
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The waterfall model
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Evolutionary development
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Specification, development and validation are interleaved.
An initial system is rapidly developed from abstract specifications.
Then, refined with customer input.
Component-based software engineering
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Seperates process phases such as requirements specifications, software
design, implementation and testing.
Based on the existence of a significant number of reusable components.
Development process focuses on integrating these components into a
system rather than developing them from scrach.
These three process models are widely used in current software engineering
practice.
There are many variants of these models.
They are often used together, especially for large systems development.
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 3
Waterfall model
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 4
Waterfall model
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The result of each phase is one or more documents that are approved.
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The following phase should not start until the previous phase has finished.
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The main drawback of the waterfall model is the difficulty of accommodating
change after the process is underway. One phase has to be complete before
moving onto the next phase.
Commitments must be made at an early stage in the process, which makes it
difficult to respond to changing requirements.
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 5
Waterfall model problems
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Inflexible partitioning of the project into distinct stages makes it difficult to
respond to changing customer requirements.
Therefore, this model is only appropriate when the requirements are wellunderstood and changes will be fairly limited during the design process.
Few business systems have stable requirements.
The waterfall model is mostly used for large systems engineering projects
where a system is developed at several sites.
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 6
Evolutionary development
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Evolutionary development: Developing an initial implementation, exposing
this to user comment and refining it through many versions until an adequate
system has been developed.
Two fundamental types of evolutionary development:
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Exploratory development
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Objective is to work with customers and to evolve a final system from an
initial outline specification. Should start with well-understood
requirements and add new features as proposed by the customer.
Throw-away prototyping
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Objective is to understand the system requirements. Should start with
poorly understood requirements to clarify what is really needed.
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 7
Evolutionary development
From an engineering and management perspective, the evolutionary approach
has two problems:
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Problems
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Lack of process visibility;
Systems are often poorly structured;
Special skills (e.g. in languages for rapid prototyping) may be required.
Applicability
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For small or medium-size interactive systems;
For parts of large systems (e.g. the user interface);
For short-lifetime systems.
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 8
Component-based software
engineering
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Based on systematic reuse where systems are integrated from existing
components or COTS (Commercial-off-the-shelf) systems.
While the initial requirements specification stage and the testing stage are
comparable with other processes, the intermediate stages are different.
Process stages
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Component analysis;
Requirements modification;
System design with reuse;
Development and integration.
This approach is becoming increasingly used as component standards have
emerged.
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 9
Reuse-oriented development
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 10
Process iteration
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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.
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Iteration can be applied to any of the generic process models.
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Two process models:
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Incremental delivery: Customers identify the services to be provided by
the system. They identify which of the systems are most important and
least important to them. Each increment provides a sub-set of the
system functionality.
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Spiral development: Rather than represent the software process as a
sequence of activities with some backtracking from one activity to
another, the process is represented as a spiral.
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 11
Process activities
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Software specification
Software design and implementation
Software validation
Software evolution
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in waterfall model -> activities are organised in sequence.
in evolutionary model -> activities are interleaved.
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No right or wrong way to organise these activities!
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©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 12
Software specification
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Software specification or requirements engineering: The process of:
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Understanding and defining what services are required from the system
Identifying the constraints on the system’s development and operation.
This process leads to the production of a requirements document that is a
specification for the system.
There are four main phases of requirements engineering process (next figure):
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Feasibility study;
Requirements elicitation and analysis;
Requirements specification;
Requirements validation.
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 13
The requirements engineering process
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 14
Software design and implementation
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Software design: Description of the structure of the software to be
implemented, the data which is part of the system, the interfaces
between system components and sometimes algorithm used.
Software implementation: Translation of this structure into an
executable program;
The activities of design and implementation are closely related and
may be inter-leaved.
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 15
Design process activities
The specific design processes are given as (next figure):
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Architectural design
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Abstract specification
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Interface design
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Component design
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Data structure design
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Algorithm design
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 16
The software design process
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 17
Structured methods
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A contrasting approach, structured methods for design: Producing graphical
models of the systems. In many cases, this structural design automatically
generates code from these models (ex: Unified Modeling Language (UML)).
A structured method includes a design process model, notations to represent
the design, report formats, rules and design guidelines.
They may support some or all of the following models of a system:
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Object model;
Sequence model;
State transition model;
Structural model;
Data-flow model.
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 18
Programming and debugging
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Programming is a personal activity and there is no general process that is
usually followed.
Programming: Translating a design into a program
Debugging: Removing errors from that program.
Debugging != Testing: Testing establishes the existence of defects. Debugging
is concerned with locating and correcting these defects.
Programmers carry out some program testing to discover faults in the program
and remove these faults in the debugging process.
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 19
The debugging process
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 20
Software validation
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Verification and validation (V & V) 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 (We will see the in
detail further).
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.
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 21
The testing process
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 22
Testing stages
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Component or unit testing
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System testing
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Individual components are tested independently;
Components may be functions or objects or coherent groupings of
these entities.
Testing of the system as a whole. Testing of emergent properties is
particularly important.
Acceptance testing
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Testing with customer data to check that the system meets the
customer’s needs.
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 23
Software evolution
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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.
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 24
System evolution
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 25
The Rational Unified Process
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The RUP: An example of a modern process model derived from the work on
the UML and associated process.
It is a good example of a hybrid process model.
It brings together elements from all the generic process models, it supports
iteration and it illustrates good practice in specification and design.
Normally described from 3 perspectives:
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A dynamic perspective that shows phases over time;
A static perspective that shows process activities;
A practice perspective that suggests good practice.
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 26
RUP phase model
Phase iteration
Inception
©Ian Sommerville 2006
Elaboration
Cons truction
Software Engineering, 8th edition. Chapter 4
Transition
Slide 27
RUP phases
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Inception
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Elaboration
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Develop an understanding of the problem domain and the system
architecture.
Construction
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Establish the business case for the system.
System design, programming and testing.
Transition
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Deploy the system in its operating environment.
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 28
RUP
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RUP is not a suitable process for all types of development but it does
represent a new generation of generic processes.
Most important innovations:
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The seperation of phases and workflows
Recognition that deploying software in a user’s environment is part of the process.
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 29
Static workflows
Wor kflow
Description
Busines s modelli ng
The bu sines s processes are modell ed using bu sines s use cases.
Requi rements
Actors who interact wit h the system are identified and use cases are
deve loped to model t he system requirements.
Ana lysis and de sign
A design model is created and do cumented using a rchit ectural
models, componen t models , object models and sequence mod els.
Implementation
The co mponents in the system are im plemented and structured into
im plementation sub- systems . Automatic code gen eration from design
models helps accelerate this process.
Test
Testing is an it erative p rocess that is carried out in con junc tion wit h
im plementation. System testing foll ows the completion of the
im plementation.
Dep loyment
A produc t release is created, dist ributed to us ers and install ed in their
workplace.
Confi guration and
chang e manag ement
This supporting workflow managed change s to the system (see
Chapter 29).
Project manag ement
This supporting workflow manage s the system deve lopment (see
Chapter 5).
Envi ronment
This workfl ow is concerned wit h ma king appropria te software tools
ava il able to the soft ware deve lopment team.
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 30
Key points
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Software processes are the activities involved in producing and evolving a
software system.
Software process models are abstract representations of these processes.
General activities are specification, design and implementation, validation and
evolution.
Generic process models describe the organisation of software processes.
Examples include the waterfall model, evolutionary development and
component-based software engineering.
Iterative process models describe the software process as a cycle of activities.
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.
The Rational Unified Process is a generic process model that separates
activities from phases.
©Ian Sommerville 2006
Software Engineering, 8th edition. Chapter 4
Slide 31