1. CS 425/625 Software Engineering Software Processes
Based on Chapter 3 of the textbook [Somm00]:
Ian Sommerville, Software Engineering, 6th Ed., Addison-Wesley, 2000
and on Ch3 PowerPoint presentation from
September 3, 2003

2. Outline Software Process Models Software Process Activities
Waterfall model
Evolutionary development
Formal development
Reuse-oriented development
Incremental model
Spiral model
Software Process Activities
Specification
Design and implementation
Validation
Evolution
Automated Process Support

3. Software Process Models
Software process = organized set of activities aimed at building a software system
Software process model = an abstract representation of a software process
Fundamental software process activities:
Software specification
Software design and implementation
Software validation
Software evolution

4. Software Process Models: Waterfall..
The Waterfall model [Somm00, Fig 3.1]

5. Software Process Models: .Waterfall.
Main characteristics:
Also called classic software life cycle or sequential model
Process activities (phases/stages) are clearly separated
After a number of iterations, phases of the life cycle (such as specification and design) are “frozen”

6. Software Process Models: ..Waterfall
Advantages:
Organized approach, provides robust separation of phases
Reflects common engineering practice
Disadvantages:
Doesn’t cope well with changes required by the client
Development teams might wait for each other
A working version of the product is available only late
Applicability:
When requirements are well known and few changes are likely to be needed
Can be used also for parts of larger software systems

7. Software Process Models: Evolutionary Development…
Evolutionary Development model [Somm00, Fig 3.2]

8. Software Process Models: .Evolutionary Development..
Main characteristics:
The phases of the software construction are interleaved
Feedback from the user is used throughout the entire process
The software product is refined through many versions
Types of evolutionary development:
Exploratory development
Throw-away prototyping

9. Software Process Models: ..Evolutionary Development.
Advantages:
Deals constantly with changes
Provides quickly an initial version of the system
Involves all development teams
Disadvantages:
Quick fixes may be involved
“Invisible” process, not well-supported by documentation
The system’s structure can be corrupted by continuous change

10. Software Process Models: …Evolutionary Development
Disadvantages [cont’d]:
Special tools and techniques may be necessary
The client may have the impression the first version is very close to the final product and thus be less patient
Applicability:
When requirements are not well understood
When the client and the developer agree on a “rapid prototype” that will be thrown away
Good for small and medium-sized software systems

11. Software Process Models: Formal Development….
The Formal System Development model [Somm00, Fig 3.3]

12. Software Process Models: .Formal Development…
The Formal Transformation process [Somm00, Fig 3.4]

13. Software Process Models: ..Formal Development..
Main characteristics:
Approach based on formal, mathematics-based techniques for specification, refinement, and verification
A formal specification is produced, than transformed using mathematical methods into the software product’s design and code; formal verification is also used

14. Software Process Models: ...Formal Development.
Advantages:
Transformations between stages are sufficiently close to ensure better verification and less expensive proofs
Errors, omissions, ambiguities, and contradictions in specifications are more completely detected
The final product is more likely to satisfy the requirements
The product’s reliability is increased

15. Software Process Models: ….Formal Development
Disadvantages:
Requires knowledge of and experience with formal methods
Are resource consuming
Applicability:
Safety- and security-critical systems
Parts of the systems that must be thoroughly verified

16. Software Process Models: Reuse-Oriented Development…
The Reuse-oriented Development model [Somm00, Fig 3.5]

17. Software Process Models: .Reuse-Oriented..
Main characteristics:
Makes intensive use of existing reusable components
The focus is on integrating the components rather than on creating them from the scratch

18. Software Process Models: ..Reuse-Oriented.
Advantages:
Reduces considerably the software to be developed “in-house”
Allows faster delivery
In principle, more reliable systems, due to using previously tested components

19. Software Process Models: …Reuse-Oriented
Disadvantages:
Compromises in requirements are needed
Less control over the system’s evolution
Applicability:
When there is a pool of existing components that could satisfy the requirements of the new product

20. Software Process Models: Incremental Development…
The Incremental Model [Somm00, Fig 3.6]

21. Software Process Models: .Incremental..
Main characteristics:
Hybrid model that combines elements of the waterfall and evolutionary paradigms
The specification, design, and implementation phases are broken in smaller increments

22. Software Process Models: ..Incremental.
Advantages:
Provides better support for process iteration
Reduces rework in the software construction process
Some decisions on requirements may be delayed
Allows early delivery of parts of the system
Supports easier integration of sub-systems
Lower risk of project failure
Delivery priorities can be more easily set

23. Software Process Models: ...Incremental
Disadvantages:
Increments need be relatively small
Mapping requirements to increments may not be easy
Common software facilities may be difficult to identify
Applicability:
When it is possible to deliver the system “part-by-part”

24. Software Process Models: Spiral Model..
Boehm’s Spiral Model [Somm00, Fig 3.7]

25. Software Process Models: .Spiral Model.
Main characteristics:
Also a hybrid model that support process iteration
The process is represented as a spiral, each loop in the spiral representing a process phase
Four sectors per loop: objective setting, risk assessment and reduction, development and validation, planning
Risk is explicitly taken into consideration

26. Software Process Models: ..Spiral Model
Advantages:
Risk reduction mechanisms are in place
Supports iteration and reflects real-world practices
Systematic approach
Disadvantages:
Requires expertise in risk evaluation and reduction
Complex, relatively difficult to follow strictly
Applicable only to large systems
Applicability:
Internal development of large systems

27. Process Activities: Specification
Requirements engineering [Somm00, Fig 3.8]

28. Process Activities: Design & Implementation
A general model for design [Somm00, Fig 3.9]

29. Process Activities: Testing
The debugging process [Somm00, Fig 3.10]
The testing process [Somm00, Fig. 3.11]

30. Process Activities: Evolution
System evolution [Somm00, Fig 3.13]

31. Automated Process Support
Classification of CASE technology [Somm00, Fig 3.16]