Software Engineering

1
Software Engineering

• Sommerville, Ian (2001)
• Software Engineering, 6th edition
• Ch.1-3
• http//www.software-engin.com

2
What is software engineering?

• Software engineering is an engineering discipline
  which is concerned with all aspects of software
  production
• Software engineers should adopt a systematic and
  organised approach to their work and use
  appropriate tools and techniques depending on the
  problem to be solved, the development constraints
  and the resources available

3
What is the difference between software
engineering and system engineering?

• System engineering is concerned with all aspects
  of computer-based systems development including
  hardware, software and process engineering.
  Software engineering is part of this process
• System engineers are involved in system
  specification, architectural design, integration
  and deployment

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Problems of systems engineering

• Large systems are usually designed to solve
  'wicked' problems
• Systems engineering requires a great deal of
  co-ordination across disciplines
• Almost infinite possibilities for design
  trade-offs across components
• Mutual distrust and lack of understanding across
  engineering disciplines
• Systems must be designed to last many years in a
  changing environment

5
Software and systems engineering

• The proportion of software in systems is
  increasing. Software-driven general purpose
  electronics is replacing special-purpose systems
• Problems of systems engineering are similar to
  problems of software engineering
• Software is (unfortunately) seen as a problem in
  systems engineering. Many large system projects
  have been delayed because of software problems

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Emergent properties

• Properties of the system as a whole rather than
  properties that can be derived from the
  properties of components of a system
• Emergent properties are a consequence of the
  relationships between system components
• They can therefore only be assessed and measured
  once the components have been integrated into a
  system

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Examples of emergent properties

• The overall weight of the system
• This is an example of an emergent property that
  can be computed from individual component
  properties.
• The reliability of the system
• This depends on the reliability of system
  components and the relationships between the
  components.
• The usability of a system
• This is a complex property which is not simply
  dependent on the system hardware and software but
  also depends on the system operators and the
  environment where it is used.

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Influences on reliability

• Hardware reliability
• What is the probability of a hardware component
  failing and how long does it take to repair that
  component?
• Software reliability
• How likely is it that a software component will
  produce an incorrect output. Software failure is
  usually distinct from hardware failure in that
  software does not wear out.
• Operator reliability
• How likely is it that the operator of a system
  will make an error?

9
Systems and their environment

• Systems are not independent but exist in an
  environment
• Systems function may be to change its
  environment
• Environment affects the functioning of the system
  e.g. system may require electrical supply from
  its environment
• The organizational as well as the physical
  environment may be important

10
System hierarchies
11
What is software?

• Computer programs and associated documentation
• Software products may be developed for a
  particular customer or may be developed for a
  general market

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The software process

• A structured set of activities required to
  develop a software system
• Generic activities in all software processes are
• Specification
• Design
• Validation
• Evolution

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Generic software process models

• The waterfall model
• Separate and distinct phases of specification and
  development
• Evolutionary development
• Specification and development are interleaved

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Waterfall model
15
Waterfall model problems

• Inflexible partitioning of the project into
  distinct stages
• This makes it difficult to respond to changing
  customer requirements
• Therefore, this model is only appropriate when
  the requirements are well-understood

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Evolutionary development

• Exploratory development
• Objective is to work with customers and to evolve
  a final system from an initial outline
  specification. Should start with well-understood
  requirements
• Throw-away prototyping
• Objective is to understand the system
  requirements. Should start with poorly understood
  requirements

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Evolutionary development
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Evolutionary development

• Problems
• Lack of process visibility
• Systems are often poorly structured
• Special skills (e.g. in languages for rapid
  prototyping) may be required
• Applicability
• For small or medium-size interactive systems
• For parts of large systems (e.g. the user
  interface)
• For short-lifetime systems

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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
  for large systems
• Iteration can be applied to any of the generic
  process models
• Two (related) approaches
• Incremental development
• Spiral development

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Incremental development
21
Spiral development
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What is CASE (Computer-Aided Software Engineering)

• Software systems which are intended to provide
  automated support for software process
  activities. CASE systems are often used for
  method support
• Upper-CASE
• Tools to support the early process activities of
  requirements and design
• Lower-CASE
• Tools to support later activities such as
  programming, debugging and testing

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Vergelijk SE met bouw van woning

• Zoek locatie
• Wat voor woning
• Maken ontwerp (architect)
• Ontwerp ? bouwtekeningen
• Realiseren woning
• Oplevering woning
• Gebruik van de woning

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Beleid en plannen
strategisch beleid
informatiebeleid
organisatorisch- beleid of structuurbeleid
middelenbeleid
informatieplan
uitvoering van informatieplan
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Definitie en doel van informatieplanning

• Een informatieplan is een document op basis
  waarvan informatiesystemen ontwikkeld kunnen
  worden
• Een beschrijving van gewenste informatiesystemen
  in onderlinge relatie tot de organisatie.
• Een beschrijving van de weg waarlangs en de
  middelen waarmee de informatiesystemen ontwikkeld
  worden.
• Informatieplanning leidt tot het afstemmen van
  informatievoorziening en organisatie op elkaar
• Het aansluiten van de informatiesystemen op de
  doelstellingen, de structuur en de activiteiten
  in de organisatie.
• De ontwikkeling van gewenste informatiesystemen
  volgens gestelde prioriteiten.

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Informatieplanning

• Visievorming voor wat betreft de wijze waarop
  m.b.v. ICT bedrijfsdoelstellingen kunnen worden
  verwerkelijkt.
• Het mogelijk maken van een beheerste en
  planmatige ontwikkeling van de informatievoorzieni
  ng.

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Aanleiding informatieplanning

• Veranderingen in de doelstellingen van de
  organisatie.
• Veranderingen in marktbenaderingen.
• Knelpunten in het functioneren van de
  organisatie.
• Nieuwe ICT mogelijkheden.
• He vermijden van dubbele en/of strijdige
  werkzaamheden.
• De wens tot structurering van informatiseringproje
  cten en de beheersing van de kosten ervan.

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Informatieplan

• Een informatieplan vormt de basis voor
• Systeemontwikkeling (globale beschrijving
  functies van te ontwikkelen informatiesystemen,
  kosten/baten, prioriteiten).