1. An Overview of Software Processes
Reference: Software Engineering, by Ian Sommerville, 6th edition, Chapter 3
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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
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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.
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4. Requirements The process of establishing
what services are required of the system
the constraints on the system’s operation and development
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5. A Generic Requirements Process
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6. Software Design
The process of converting the system specification (requirements) into a software structure that realizes that specification
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7. A Generic Software Design Process
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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.
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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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10. A Generic Testing Process
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11. Generic Testing Planning
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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.
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13. System Evolution
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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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15. Waterfall Model
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16. Waterfall Model Pros and Cons
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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.
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18. Evolutionary Development
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19. Exploratory Development Pros and Cons
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20. Throw-away Prototyping Pros and Cons
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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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22. Reuse-oriented Development
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23. Reuse-oriented Development Pros and Cons
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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
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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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26. Incremental Development
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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.
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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.
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29. Spiral Model of the Software Process
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30. Spiral Model Sectors Objective setting Risk assessment and reduction
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
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