An Overview of Software Processes

1
An Overview ofSoftware Processes

• Reference Software Engineering, by Ian
  Sommerville, 6th edition, Chapter 3

2
Objectives

• To introduce the general phases of the software
  development life cycle
• To introduce the software process model concept
• To describe different generic process models and
  their pros and cons

3
The Software Process

• A structured set of activities required to
  develop a software system. These activities
  include
• Requirements (Specification)
• Design
• Implementation (Coding)
• Testing (Validation)
• Maintenance (Evolution)
• A software process model is an abstract
  representation of a process.

4
Requirements

• The process of establishing
• what services are required of the system
• the constraints on the systems operation and
  development

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A Generic Requirements Process
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Software Design

• The process of converting the system
  specification (requirements) into a software
  structure that realizes that specification

7
A Generic Software Design Process
8
Implementation

• 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 activities of design and implementation are
  closely related and may be interleaved.

9
Testing

• 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.

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A Generic Testing Process
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Generic Testing Planning
12
System Maintenance

• Software is inherently flexible and can change
  (as opposed to hardware).
• In the past, there has been a demarcation between
  development and evolution (maintenance). This is
  increasingly irrelevant as fewer and fewer
  systems are completely new.
• Software engineering should be thought of as an
  evolutionary process where software is
  continually changed over its lifetime in response
  to customer needs.

13
System Evolution
14
Generic Software Process Models

• The Waterfall model
• Separate, non-overlapping phases of specification
  and development
• Evolutionary development
• Specification and development are interleaved
• Reuse-based development
• The system is assembled from some (most likely)
  or all existing components

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Waterfall Model
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Waterfall Model Pros and Cons

• Pros
• Cons

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Evolutionary Development

• Two general types
• Exploratory development
• Objective is to work with the customers to evolve
  a final system from an initial outline
  specification. Process starts with the
  well-understood requirements.
• Throw-away prototyping
• Objective is to understand the system
  requirements. Process starts with the poorly
  understood requirements.

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Evolutionary Development
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Exploratory Development Pros and Cons

• Pros
• Cons

20
Throw-away Prototyping Pros and Cons

• Pros
• Cons

21
Reuse-oriented Development

• Based on systematic reuse where systems are
  integrated from existing components or COTS
  (commercial-off-the-shelf) systems
• This approach is becoming more important, but
  there is still limited experience with it.

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Reuse-oriented Development
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Reuse-oriented Development Pros and Cons

• Pros
• Cons

24
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, especially for large systems.
• Iteration can be applied to any of the generic
  process models.
• Examples of two iterative approaches
• Incremental development
• Spiral development

25
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 prioritized 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.

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Incremental Development
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Incremental Development Advantages

• Customers do not have to wait until the entire
  system is delivered until they can gain value
  from it.
• 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
• Risks are explicitly assessed and resolved
  throughout the process.

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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