Software Engineering

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Lecture 2: Software Life Cycles

Stefan Hallerstede

Århus School of Engineering

25 August 2011

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Contents

Naive Software Development Code & Fix

Towards A Software Process

Software Process Models Traditional Process Models Alternative Process Models Agile Process Models

Agile Development Methods – Two Examples

Concluding Remarks

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Contents

Concluding Remarks

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Developing Software

Question:

How to develop software?

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Contents

Concluding Remarks

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Developing Software

Code & Fix

?

SOFTWARE

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Developing Software

Code & Fix

?

SOFTWARE

code

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Developing Software

Code & Fix

?

SOFTWARE

code fix

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Developing Software

Code & Fix

?

code fix

SOFTWARE

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Developing Software

Code & Fix

?

code fix

SOFTWARE

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Contents

Concluding Remarks

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Software Process From The Internet (Source: xkcd)

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What Do We Expect Of A Software Process?

I What is theaimof having one?

I What is agoodsoftware process?

I What isgoodsoftware?

Correctness Reliability Efficiency Integrity Usability Maintainability Testability Flexibility Portability Reusability Interoperability

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What Do We Expect Of A Software Process?

I What is theaimof having one?

I What is agoodsoftware process?

I What isgoodsoftware?

Correctness Reliability Efficiency Integrity Usability Maintainability Testability Flexibility Portability Reusability Interoperability

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Contents

Concluding Remarks

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Contents

Concluding Remarks

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The Waterfall Model

Sequential¹

1Winston W. Royce (1970) Managing the Development of Large Software Systems. IEEE WESCON

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The Waterfall Model

Sequential¹

Royce notes that this process often cannot be followedin practice.

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The Waterfall Model

Iterative¹

Instead it is common to iterate between development phases.

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The Waterfall Model

V-Model²

2Barry W. Boehm (1988) A Spiral Model of Software Development and Enhancement. Computer 21(5), IEEE

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The Waterfall Model

V-Model²

Coding Analysis

Acceptance testing

Unit testing Detailed design

Architectural design Requirements

Integration testing System testing

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The Waterfall Model

V-Model²

Coding Analysis

Acceptance testing

Unit testing Detailed design

Architectural design Requirements

Integration testing System testing

The V-Model emphasises the need fortesting matching the development phases.

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Iterative Development

The Spiral Model³

3Barry W. Boehm (1988) A Spiral Model of Software Development and Enhancement. Computer 21(5), IEEE 14

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The Spiral Model³

1. Objective setting

• Definition of objectives for the phase

• Identification of risks and planning of alternative strategies 2. Risk assessment and reduction

• Analyse risks, and choose or adapt development model 3. Development and validation

4. Planning

• Review project

• Decide whether to continue with spiral

• Draw up plan for next phase

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The Spiral Model³

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The Spiral Model³

The Spiral Model takes into account:

risk analysis

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The Spiral Model³

The Spiral Model takes into account:

risk analysis and

project management

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Contents

Concluding Remarks

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Incremental Delivery

I System is not delivered as a whole but in smallincrements

I Customer is involved in decision on increments

Requirements specification

Assign requirements to increments

Design system architecture

Develop system increment

Validate increment

Integrate increment

Validate system

Final system

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Component-Based Software Engineering

Component analysis

Requirements modification

System design with reuse

Development and integration

System validation

I Targeted atreuse

I Process stages adapted to this aim:

I Component analysis: Search for component matching best the requirements

I Requirements modification: Modify requirements to reflect available component

I System design with reuse: Designers incorporate components to be reused, development of new software only if reuse is not possible

I Development and integration: Missing pieces of software are implemented, and reused components integrated

I Is this a reasonable approach?

I When could it be used?

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Software Design Process Without Reuse

I Turn requirements specification into an executable system

I Interleaved model of design from architecture to algorithms

Architectural design

Abstract specification

Interface design

Component design

Data structure design

Algorithm design

System architecture

Software specification

Interface architecture

Component specification

Data structure specification

Algorithm specification Design activities

Design products

Implementation

I Can this be used with incremental delivery?

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Contents

Concluding Remarks

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Agile Processes

Agile Manifesto⁴

“We are uncovering better ways of developing software by doing it and helping others do it. Through this work we have come to value:

I Individuals and interactionsover processes and tools

I Working softwareover comprehensive documentation

I Customer collaborationover contract negotiation

I Responding to changeover following a plan

That is, while there is value in the items on the right, we value the items on the left more.”

4Kent Beck, et al. (2001) Manifesto for Agile Software Development.

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Agile Processes

Unified Process⁵

Core Workflows:

Inception Elaboration Construction Transition PHASES

Business modeling

Requirements

Analysis and Design

Implementation

Test

Deployment

Organisation along time

ITERATIONS

Organisation along content Preliminary Iterations Iter#1 Iter#2 Iter#n Iter#n+1 Iter#n+2 Iter#m Iter#m+1

Supporting Workflows: Configuration and Change Management, Project Management,Environment

5Philippe Kruchten (2004) The rational unified process: an introduction, 3rd ediiton. Addison-Wesley.

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Agile Processes

Unified Process⁵

Core Workflows:

Inception Elaboration Construction Transition PHASES

Business modeling

Requirements

Analysis and Design

Implementation

Test

Deployment

Organisation along time

ITERATIONS

Organisation along content Preliminary Iterations Iter#1 Iter#2 Iter#n Iter#n+1 Iter#n+2 Iter#m Iter#m+1

Supporting Workflows: Configuration and Change Management, Project Management,Environment

5Philippe Kruchten (2004) The rational unified process: an introduction, 3rd ediiton. Addison-Wesley.

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Agile Processes

Unified Process

Six fundamental best practices:⁶ 1. Develop softwareiteratively.

2. Managerequirements.

3. Usecomponent-basedarchitectures. 4. Visuallymodelsoftware.

5. Verify softwarequality. 6. Control changesto software.

6Ian Sommerville (2007) Software Engineering, 8th edition. Addison-Wesley.

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Agile Processes

Unified Process

Plan increments of the system based on customer priorities and develop and deliver the highest priority system features early in the development process.

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Agile Processes

Unified Process

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Agile Processes

Unified Process

Explicitly document the customer’s requirements and keep track of changes to these requirements.

Analyse the impact of changes to the system before accepting them.

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Agile Processes

Unified Process

3. Usecomponent-basedarchitectures.

4. Visuallymodelsoftware. 5. Verify softwarequality. 6. Control changesto software.

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Agile Processes

Unified Process

Structure the system architecture into components and rely on reusable software components.

4. Visuallymodelsoftware. 5. Verify softwarequality. 6. Control changesto software.

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Agile Processes

Unified Process

4. Visuallymodelsoftware.

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Agile Processes

Unified Process

Use graphical UML models to present static and dynamic views of the software.

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Agile Processes

Unified Process

5. Verify softwarequality.

6. Control changesto software.

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Agile Processes

Unified Process

Ensure that the software meets the organisational quality standards.

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Agile Processes

Unified Process

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Agile Processes

Unified Process

Manage changes to the software using a change management system and configuration

management procedures and tools.

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Agile Processes

Unified Process

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Contents

Concluding Remarks

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Agile Processes

Two Popular Examples

I Extreme Programming:

Leverage developer creativity

and minimise administrative overhead

keywords: small releases, simple design, write tests first, refactoring, pair programming, continuous integration, coding standards

I Scrum:

Self-organising autonomous teams implement product increments in parallel

keywords: sprint, product backlog, prioritised requirements

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Agile Processes

Extreme Programming⁷

7Kent Beck (2000) Extreme Programming Explained. Embrace Change. Addison Wesley

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Agile Processes

Scrum⁸

8Ken Schwaber, Mike Beedle (2002) Agile software development with Scrum. Prentice Hall.

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Contents

Concluding Remarks

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Summing Up

Advantages And Disadvantages Of Different Software Processes⁹

Waterfall vs

Spiral vs Agile

9Eric J. Braude, Michael E. Bernstein (2011) Software Engineering – Modern Approaches. Wiley.

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The Waterfall Process

I Advantages

I Simple and easy to use

I Easy to allocate resources

I Works well for smaller projects

or when requirements are well understood

I Disadvantages

I Requirements must be known upfront

I Late feedback by users

I Lack of parallelism

I Inefficient use of resources

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The Spiral Process

I Advantages

I Risks are managed early and throughout the process

I Software evolves as the project progresses

I Planning is built into the process

I Disadvantages

I Complicated to use

I May be overkill for small projects

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The Agile Process

I Advantages

I The project always has demonstrable results

I Developers tend to be more motivated

I Customers are able to provide better requirements because they can see the evolving product

I Disadvantages

I Problematical for large application

I Problematical for large teams

I Documentation output is questionable

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