A.6 The fundamental ideas of software engineering are applicable to all types of
software system.
1.7 Excercises(Homework): P25
1.3．
Page 9 of 91
Chapter 2 (2) Software processes
2.1
Topics covered
1. Software process models 2. Process activities 3. Coping with change
2.2 The software process
2.2.1 Introduction
A.1 A structured set of activities required to develop a software system.
B.1
Many different software processes but all involve:
1. Specification – defining what the system should do;
2. Design and implementation – defining the organization of the system and implementing the
system;
3. Validation – checking that it does what the customer wants;
4. Evolution – changing the system in response to changing customer needs.
A.2 A software process model is an abstract representation of a process. It presents
a description of a process from some particular perspective.
2.2.2 Plan-driven and agile processes
1. Plan-driven processes are processes where all of the process activities are planned in advance
and progress is measured against this plan.
2. In agile processes, planning is incremental and it is easier to change the process to reflect
changing customer requirements.
3. In practice, most practical processes include elements of both plan-driven and agile
approaches.
4. There are no right or wrong software processes.
2.3 Software process models
2.3.1 Generic software process models A.1 The waterfall model
Plan-driven model. Separate and distinct phases of specification and development.
A.2 Incremental development
Specification, development and validation are interleaved. May be plan-driven or agile.
A.3 Reuse-oriented software engineering
Page 10 of 91
The system is assembled from existing components. May be plan-driven or agile.
A.4 In practice, most large systems are developed using a process that incorporates
elements from all of these models.
2.3.2 Waterfall model
B.1
1. 2. 3. 4. 5.
Waterfall model phases
Requirements analysis and definition System and software design Implementation and unit testing Integration and system testing Operation and maintenance
Waterfall model
RequirementsdefinitionSystem andsoftware designImplementationand unit testingIntegration andsystem testingOperation andmaintenance B.2
Waterfall model problems ? Inflexible partitioning of the project into distinct stages makes it difficult to respond to
changing customer requirements.
? Therefore, this model is only appropriate when the requirements are well-understood and
changes will be fairly limited during the design process.
? The waterfall model is mostly used for large systems engineering projects
2.3.3 Incremental development
Page 11 of 91
ConcurrentactivitiesSpecificationInitialversionOutlinedescriptionDevelopmentIntermediateversionsValidationFinalversion
A.1 Incremental development benefits
B.1
The cost of accommodating changing customer requirements is reduced.
The amount of analysis and documentation that has to be redone is much less than is required with the waterfall model.
B.2
It is easier to get customer feedback on the development work that has been done.
Customers can comment on demonstrations of the software and see how much has been implemented.
B.3
More rapid delivery and deployment of useful software to the customer is possible.
Customers are able to use and gain value from the software earlier than is possible with a waterfall process.
A.2 Incremental development problems
B.1
The process is not visible.
Managers need regular deliverables to measure progress. If systems are developed quickly, it is not cost-effective to produce documents that reflect every version of the system.
B.2 System structure tends to degrade as new increments are added.
Unless time and money is spent on refactoring to improve the software, regular change tends to corrupt its structure. Incorporating further software changes becomes increasingly difficult and costly.
2.3.4 Reuse-oriented software engineering
A.1 Based on systematic reuse where systems are integrated from existing
components or COTS (Commercial-off-the-shelf) systems.
A.2 Process stages
1. 2. 3. 4.
Component analysis;
Requirements modification; System design with reuse; Development and integration.
Page 12 of 91