Software Process Models

1
Software Process Models

• Software ProcessA structured set of activities
  and associated results leading to the production
  of a software product
• Software Process Model
• An abstract representation of a process

2
Generic software process models

• The waterfall model
• Process phases are distinct and separate
• Evolutionary development
• Process phases 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

3
Generic software process models

• The waterfall model
• Process phases are distinct and separate
• Evolutionary development
• Process phases 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

4
Waterfall model phases

• Five phases
• Requirements analysis and definition
• System and software design
• Implementation and unit testing
• Integration and system testing
• Operation and maintenance
• Formal sign-off at end of each phase
• Documentation as product of each phase

5
Waterfall model
6
Waterfall ModelAdvantages

• State of progress clear
• Good for project management
• Engineers know their tasks
• Development is (relatively) slow and deliberate
• Results in solid, well-constructed systems

7
Waterfall ModelProblems

• Difficult (expensive) to accommodate change after
  process is underway
• Inflexible partitioning of the project into
  distinct stages
• E.g., difficult to respond to changing customer
  requirements
• Therefore, model is most appropriate when
  requirements are well-understood

8
Generic software process models

• The waterfall model
• Process phases are distinct and separate
• Evolutionary development
• Process phases 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

9
Evolutionary development
10
Evolutionary development

• Basic idea
• Build prototype version of system, seek user
  comments, and keep refining until done
• Other side of formality spectrum from Waterfall
  Model

11
Evolutionary development

• Two flavors
• Exploratory development
• Work with customers and evolve a final system
  from an initial outline specification. Assumes
  well-understood requirements
• Throw-away prototyping
• Build straw-man as system to study. Used to
  help develop better requirements.

12
Evolutionary developmentAdvantages

• Systems developed faster than with Waterfall
• Particularly useful when requirements less-well
  understood
• Customer input throughout development yields
  system closer to their (immediate) needs

13
Evolutionary developmentProblems

• Lack of process visibility
• Systems 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

14
Evolutionary developmentApplicability

• For small or medium-size interactive systems
• For parts of large systems (e.g. the user
  interface)
• For short-lifetime systems

15
Generic software process models

• The waterfall model
• Process phases are distinct and separate
• Evolutionary development
• Process phases 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

16
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

17
Formal systems development
18
Formal transformations
19
Formal systems developmentAdvantages

• Transformations are small, making verification
  tractable
• Resulting implementations are proven to meet
  specifications, so testing (of components)
  unnecessary
• Although, overall system characteristics (e.g.
  performance, reliability) still need verification

20
Formal systems developmentProblems

• Need for specialised skills and training
• Difficult to formally specify some aspects of the
  system, e.g. user interfaces
• Development time high (usually not worth the
  time/cost)

21
Formal systems developmentApplicability

• Critical systems
• especially those where a safety or security case
  must be made before the system is put into
  operation
• Critical components within large systems

22
Generic software process models

• The waterfall model
• Process phases are distinct and separate
• Evolutionary development
• Process phases 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

23
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

24
Reuse-oriented development
25
Summary

• Software processes
• the activities involved in producing and evolving
  a software system
• represented in a software process model
• General activities
• specification, design and implementation,
  validation, evolution
• Generic process models
• Describe the organization of software processes
• Examples waterfall model, evolutionary
  development, formal systems development,
  reuse-oriented development