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 describe various generic software process
  models and their pros and cons

3
Software Life Cycle

• The phases necessary to develop and maintain a
  software system. These phases include
• Requirements (Specification)
• Design
• Implementation (Coding)
• Testing (Validation)
• Maintenance (Evolution)
• A software process model is an abstract
  representation of how these phases can be
  addressed.

4
Requirements

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

5
A Generic Requirements Process
6
Design

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

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.

10
A Generic Testing Process
11
V-Model of Test 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
Software Process Models

• Waterfall model (Royce, 1970)
• Prototyping
• Throwaway
• Evolutionary
• Incremental development
• Spiral model (Boehm, 1988)

14
Waterfall Model
Requirements
Design
Implementation
Testing
Maintenance
15
Observations

• The following phase should not start until the
  previous phase has finished.
• In practice,
• Phases overlap
• May return to a previous phase
• Still widely used, especially on very large
  projects

16
Waterfall Model Pros and Cons

• Pros
• Cons

17
Prototyping
Requirements
Design
Design
Implementation
Implementation
Testing
Testing
Maintenance
18
Observations

• Used for requirements elicitation and validation
• A working model (prototype) of the final system
  is developed during requirements
• Is an iterative process
• Prototype can be thrown away or evolved into the
  final system (evolutionary prototyping)

19
Prototyping Pros and Cons

• Pros
• Cons

20
Throwaway vs. Evolutionary

• Throwaway pros
• Evolutionary pros

21
Incremental Development
22
Observations

• 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.
• Is an iterative process

23
Incremental Pros and Cons

• Pros
• Cons

24
Spiral Model
25
Observations

• 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.
• Uses prototyping

26
Things to Think About

• What about modifying existing software?
• What about using existing software?
• In-house modules
• COTS (Commercial Off-The-Shelf)