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 system’s operation and development

5. A Generic Requirements Process

6. 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.

10. A Generic Testing Process

11. 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

15. Waterfall Model

16. Waterfall Model Pros and Cons Pros Cons

17. 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.

18. Evolutionary Development

19. 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.

22. Reuse-oriented Development

23. 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.

26. Incremental Development

27. 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.

28. 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.

29. Spiral Model of the Software Process

30. 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