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©Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 1
Software Engineering
• Software Engineering is the science and art ofbuilding significant software systems that are:
1) on time2) on budget3) with acceptable performance4) with correct operation.
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©Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 2
The economies of all developed nations are dependent on software.
More and more systems are software controlled. Software engineering is concerned with theories,
methods and tools for professional software development.
Software engineering expenditure represents a significant fraction of the GNP of developed countries.
Software Engineering
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©Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 3
Software costs often dominate system costs. The costs of software on a PC are often greater than the hardware cost.
Software costs more to maintain than it does to develop.
Software engineering is concerned with cost-effective software development.
Software Costs
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©Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 4
Software Products
Generic products:– Stand-alone systems which are produced by a development
organization and sold on the open market to any customer.
Customized products:– Systems which are commissioned by a specific customer and
developed specially by some contractor.
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©Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 5
Software Product Attributes
Maintainability Dependability Efficiency Usability
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©Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 6
Importance of Product Characteristics
The relative importance of these characteristics depends on the product and the environment in which it is to be used.
In some cases, some attributes may dominate– In safety-critical real-time systems, key attributes may be
dependability and efficiency.
Costs tend to rise exponentially if very high levels of any one attribute are required.
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©Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 7
Efficiency CostsCost
Ef ficiency
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©Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 8
The Software Process
Structured set of activities required to develop a software system– Specification– Design– Validation– Evolution
Activities vary depending on the organization and the type of system being developed.
Must be explicitly modeled if it is to be managed.
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©Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 9
Engineering Process Model Specification: Set out the requirements and
constraints on the system. Design: Produce a model of the system. Manufacture: Build the system. Test: Check the system meets the required
specifications. Install: Deliver the system to the customer and
ensure it is operational. Maintain: Repair faults in the system as they
are discovered.
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©Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 10
Software Engineering is Different
Normally, specifications are incomplete. Very blurred distinction between specification,
design and manufacture. No physical realization of the system for testing. Software does not wear out - maintenance
does not mean component replacement.
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©Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 11
Generic Software Process Models
Waterfall– Separate and distinct phases of specification and development
Evolutionary– Specification and development are interleaved
Formal Transformation– A mathematical system model is formally transformed to an
implementation
Reuse-based– The system is assembled from existing components
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©Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 12
Waterfall Process ModelRequirements
definition
System andsoftware design
Implementationand unit testing
Integration andsystem testing
Operation andmaintenance
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©Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 13
Evolutionary Process Model
ValidationFinal
version
DevelopmentIntermediate
versions
SpecificationInitial
version
Outlinedescription
Concurrentactivities
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©Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 14
Process Model Problems Waterfall
– High risk for new systems because of specification and design problems.
– Low risk for well-understood developments using familiar technology.
Prototyping– Low risk for new applications because specification and
program stay in step.
– High risk because of lack of process visibility.
Transformational– High risk because of need for advanced technology and
staff skills.
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©Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 15
Hybrid Process Models
Large systems are usually made up of several sub-systems.
The same process model need not be used for all subsystems.
Prototyping for high-risk specifications. Waterfall model for well-understood
developments.
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©Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 16
Spiral Process Model
Riskanalysis
Riskanalysis
Riskanalysis
Riskanalysis Proto-
type 1
Prototype 2Prototype 3
Opera-tionalprotoype
Concept ofOperation
Simulations, models, benchmarks
S/Wrequirements
Requirementvalidation
DesignV&V
Productdesign Detailed
design
CodeUnit test
IntegrationtestAcceptance
testService Develop, verifynext-level product
Evaluate alternativesidentify, resolve risks
Determine objectivesalternatives and
constraints
Plan next phase
Integrationand test plan
Developmentplan
Requirements planLife-cycle plan
REVIEW
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©Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 17
Spiral Model Advantages
Focuses attention on reuse options. Focuses attention on early error elimination. Puts quality objectives up front. Integrates development and maintenance. Provides a framework for hardware/software
development.
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©Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 18
Spiral Model Problems
Contractual development often specifies process model and deliverables in advance.
Requires risk assessment expertise.
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©Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 19
Process Visibility
Software systems are intangible so managers need documents to assess progress.
Waterfall model is still the most widely used model.
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©Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 20
Waterfall Model Documents
Activity Output documentsRequirements analysis Feasibility study, Outline requirementsRequirements definition Requirements documentSystem specification Functional specification, Acceptance test plan
Draft user manualArchitectural design Architectural specification, System test planInterface design Interface specification, Integration test planDetailed design Design specification, Unit test planCoding Program codeUnit testing Unit test reportModule testing Module test reportIntegration testing Integration test report, Final user manualSystem testing System test reportAcceptance testing Final system plus documentation
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©Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 21
Process Model Visibility
Process model Process visibilityWaterfall model Good visibility, each activity produces some
deliverableEvolutionarydevelopment
Poor visibility, uneconomic to producedocuments during rapid iteration
Formaltransformations
Good visibility, documents must be producedfrom each phase for the process to continue
Reuse-orienteddevelopment
Moderate visibility, it may be artificial toproduce documents describing reuse andreusable components.
Spiral model Good visibility, each segment and each ringof the spiral should produce some document.
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©Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 22
Professional Responsibility
Software engineers should not just be concerned with technical considerations. They have wider ethical, social and professional responsibilities.
No clear rights and wrongs about many of these issues:– Development of military systems
– Whistle blowing
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©Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 23
Ethical Issues
Confidentiality Competence Intellectual property rights Computer misuse