Lect3 ch15-unit2

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Software Engineering

By

Mobeen Mustafa

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Course outline

• Unit 1: Software Engineering Basics Unit 2: Process Models and Software Life Cycles• Unit 3: Software Requirements • Unit 4: Unified Modeling Language (UML)• Unit 5: Design Basics and Software Architecture• Unit 6: OO Analysis and Design • Unit 7: Design Patterns• Unit 8: Testing and Reliability • Unit 9: Software Engineering Management and Economics

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Reference

• These slides are based on:– Lecture slides by Ian Summerville, see

http://www.comp.lancs.ac.uk/computing/resources/ser/

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Overview

• Build-and-fix model• Waterfall model• Rapid prototyping model• Incremental model• Evolutionary• Synchronize-and-stabilize model• Spiral model

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Software Life-Cycle Models

• Life-cycle model (also, process model)• The software development and operation

activities and their ordering– Requirements elicitation– Specification– Design– Implementation– Integration– Maintenance phase– Retirement– …

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New or changed

requirements

New or changed

system

Software EngineeringProcess

What Is a Software Engineering Process?

A process defines Who is doing What, When and How in the development of a software system– Roles and workflows

– Workproducts

– Milestones

– Guideline

– …

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Process vs. Product

Processmodel

Project

Product

People

Tools

Template

Participants

Result

Automation

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An Effective Process ...

• Provides guidelines for efficient development of quality software

• Reduces risk and increases predictability • Captures and presents best practices

– Learn from other’s experiences– Mentor on your desktop– Extension of training material

• Promotes common vision and culture• Provides roadmap for applying tools• Delivers information on-line, at your finger tips

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Lightweight vs. Heavyweight Processes

Document drivenElaborate workflow definitionsMany different rolesMany checkpointsHigh management overheadHighly bureaucratic

Focus on working coderather than documentationFocus on direct communication(between developers andbetween developers and the customer)Low management overhead

Heavyweighte.g., V-Process

CustomizableFrameworke.g., RationalUnifiedProcess (RUP)

Agile (Lightweight)e.g., eXtremeProgramming (XP)

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• Process used should depend on type of product which is being developed– For large systems, management is usually the

principal problem so you need a strictly managed process. For smaller systems, more informality is possible.

• High costs may be incurred if you force an inappropriate process on a development team

Process choice

©Ian Sommerville 1995 [modified]

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Build and Fix Model

• Properties– No planning or analysis– The working program is the

only workproduct• Advantage

– Appropriate for small programs written by one person

• Disadvantage– Understandability and

maintainability decrease rapidly with increasing program size

– Totally unsatisfactory– Need a life-cycle model

• “Game plan” • Phases• Milestones

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Waterfall Model

• Characterized by– Sequential steps (phases)– Feedback loops (between two phases in

development)– Documentation-driven

• Advantages – Documentation– Maintenance easier

• Disadvantages– Complete and frozen specification document

up-front often not feasible in practice– Customer involvement in the first phase only– Sequential and complete execution of phases

often not desirable– Process difficult to control– The product becomes available very late in

the process

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Rapid Prototyping Model

• Rapid prototyping phase followed by waterfall– Do not turn the rapid

prototype into the product– Rapid prototyping may replace

the specification phase—never the design phase

• Comparison:– Waterfall model—try to get it

right the first time– Rapid prototyping—frequent

change, then discard

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Advantages and Disadvantages

• Advantages– Requirements better specified and validated– Early feasibility analysis– Strong involvement of the customer in the

prototyping phase

• Disadvantage– Higher development effort– Danger that due to schedule slip, the prototype

becomes part of the product

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Incremental

Design Coding Test Deployment



Requirem

ents

Release 1

Release 2

Release 3

Each release adds more functionality, i.e., a new increment

(Some call it iterative)

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Incremental Model (contd)

• Waterfall, rapid prototyping models– Operational quality complete product at end

• Incremental model– Operational quality portion of product within weeks

• Less traumatic• Smaller capital outlay, rapid return on investment

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Evolutionary

Design Coding Test DeploymentRequirements



Feedback

Version 1

Version 1

Version 1

New versions implement new and evolving requirements

(Some call it iterative)

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Evolutionary Model (contd)

• Advantages– Constant customer involvement and validation– Allows for good risk management

• Disadvantages– Build-and-fix danger– Contradiction in terms

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Spiral model

• Waterfall model plus risk analysis preceding each phase and evaluation following each phase

• Prototyping for high-risk specifications• Radial dimension: cumulative cost to date• Angular dimension: progress through the spiral• If all risks cannot be resolved, the project is

immediately terminated• Appropriate only for big projects (high

management overhead)

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Spiral model

Riskanalysis

Riskanalysis

Riskanalysis

RiskanalysisProto-

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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Process model risk 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

• Evolutionary and Spiral– Middle ground between waterfall and prototyping

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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• Taylor the process to a problem

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Use of the Models in Practice

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• The fundamental problem with software– The software process is badly managed

• Understanding existing processes

• Introducing process changes to achieve organisational objectives which are usually focused on quality improvement, cost reduction and schedule acceleration

Process improvement


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Reference

• These slides are based on:– Lecture slides by Ian Summerville, see

http://www.comp.lancs.ac.uk/computing/resources/ser/

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Rational Unified Process – Main Characteristics

• Iterative and incremental• Use-case-driven• Architecture-centric• Uses UML as its modeling notation• Process framework

– Comprehensive set of document templates, process workflow templates, and process guidelines

– Distributed by IBM/Rational on a CD

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Rational Unified Process Is Use-Case-Driven

• Use cases are concise, simple, and understandable by a wide range of stakeholders– End users, developers and acquirers understand functional

requirements of the system

• Use cases drive numerous activities in the process:– Creation and validation of the design model– Definition of test cases and procedures of the test model– Planning of iterations– Creation of user documentation– System deployment

• Use cases help synchronize the content of different models

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Rational Unified Process Is Architecture-Centric

• Architecture is the focus of the elaboration phase– Building, validating, and baselining the architecture constitute the

primary objective of elaboration

• The Architectural Prototype validates the architecture and serves as the baseline for the rest of development

• The Software Architecture Description is the primary artifact that documents the architecture chosen

• Other artifacts derive from architecture:– Design guidelines including use of patterns and idioms– Product structure– Team structure

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Representing Architecture: The 4+1 View Model

Process View

Deployment View

Logical View

Implementation View

Programmers Software management

PerformanceScalabilityThroughput

System IntegratorsSystem topology

Delivery, installationcommunication

System Engineering

Use-Case View

Structure

Analysts/Designers End-user

Functionality

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InceptionInception ElaborationElaboration ConstructionConstruction TransitionTransition

Process Architecture - Lifecycle Phases

The Rational Unified Process has four phases:– Inception - Define the scope of project

– Elaboration - Plan project, specify features, baseline architecture

– Construction - Build the product– Transition - Transition the product into end user community

time

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Phase Boundaries Mark Major Milestones

Lifecycle Objective Milestone

Lifecycle Architecture

Milestone

Initial Operational Capability Milestone

Product Release

time

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Iterations and Phases

An An iterationiteration is a distinct sequence of activities with an is a distinct sequence of activities with an established plan and evaluation criteria, resulting in established plan and evaluation criteria, resulting in an executable release (internal or external)an executable release (internal or external)

PreliminaryPreliminaryIterationIteration

Architect.Architect.IterationIteration

Architect.Architect.IterationIteration

Devel. Devel. IterationIteration



TransitionTransitionIterationIteration

TransitionTransitionIterationIteration


Minor Milestones: Releases

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Major Workflows Produce Models

Analysis & Analysis & DesignDesign

DesignModel

ImplementationModel

TestModel

realized by

implemented by

verified by

RequirementsRequirements

ImplementationImplementation

TestTest

Use-CaseModel

BusinessBusinessModelingModeling

Business Model

supported by

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RUP Overview

Management

Environment

Business Modeling

Implementation

Test

Architecture & Design

Preliminary Iteration(s)

Iter.#1

PhasesProcess Workflows

Iterations

Supporting Workflows

Iter.#2

Iter.#n

Iter.#n+1

Iter.#n+2

Iter.#m

Iter.#m+1

Deployment

Configuration Mgmt

Requirements

Elaboration TransitionInception Construction

Workflows group

activities logically

In an iteration,you walk throughall workflows

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XP Overview

Characteristics

• Evolutionary development• Collection of 12 „Best

Practices“• Focus on working code that

implements customer needs (rather than documents)

• Testing is a crucial element of the process

• Focus on flexibility and efficiency of the process

• Designed for small teams (<10)

Write tests

Planning

Test

Pair Programming+ Refactoring

IntegrationMin.daily

Every 2-3weeks

Release

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XP Practices (I)

• The planning game– Stakeholder meeting to plan the next iteration

– Business people decide on business value of features

– Developers on the technical risk of features and predicted effort per feature

• Small releases– Start with the smallest useful feature set; release early and often,

adding a few features each time

• Metaphor– Each project has an organizing metaphor, a providing easy to

remember naming conventions

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XP Practices (II)

• Simple Design– Always use the simplest possible design that gets the job done

(runs the tests and states intentions of the programmer)

– No speculative genericity

• Testing– Test-first: write test, then implement it

– Programmers write unit tests and customers write acceptance tests

• Refactoring– Refactoring is done continuously; the code is always kept clean

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XP Practices (III)

• Pair programming– All production code written by two programmers– One programmer is thinking about implementing the current

method, the other is thinking strategically about the whole system– Pairs are put together dynamically

• Collective code ownership– Any programmer that sees an opportunity to add value to any

portion of the code is required to do so at any time

• Continuous integration– Use of version and configuration management (e.g., CVS)– All changes are integrated into the code-base at least daily– The tests have to run 100% before and after the integration

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XP Practices (IV)

• 40-h week– Programmers go home on time– Overtime is a symptom of a serious problem– No errors by tired developers; better motivated developers

• On-site customer– Development team has continuous access to a real life

customer/user

• Coding standards– Everyone codes to the same standards– Ideally, you should not be able to tell by looking at it who has

written a specific piece of code

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XP Advantages

• Integrated, simple concept• Low management overhead (no complicated procedures to

follow, no documentation to maintain, direct communication, pair programming)

• Continuous risk management (early feedback from the customer)

• Continuous effort estimation

• Emphasis on testing; tests help in evolution and maintenance

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XP Disadvantages

• Appropriate for small teams (up to 10 developers) only (does not scale)

• Large development groups may require more structures and documents

• If maintainers are not the people that developed the code, good documentation is necessary

• Generic design may be necessary to enable expected future development

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Reading

• RUP– Craig Larman, Applying UML and Patterns: An Introduction to Object-

Oriented Analysis and Design and the Unified Process, Prentice-Hall, 2002 (2nd edition)

– Kendall Scott. The Unified Process Explained. Addison Wesley, 2001• Agile development

– Kent Beck, Extreme Programming: Explained, Addison-Wesley, 1999– R. Jeffries, C. Hendrikson, A. Anderson, Extreme Programming Installed,

Addison-Wesley, 2001• http://member.netease.com/~wooce/tip/se/

– Alistair Cockburn, Agile Software Development, Addison-Wesley 2002

• Online-Ressourcen– http://www.xprogramming.com– http://www.xprogramming.org– http://groups.yahoo.com/group/extremeprogramming– http://c2.com

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Process improvement stages

• Process analysis– Model and analyse (quantitatively if possible) existing

processes

• Improvement identification– Identify quality, cost or schedule bottlenecks

• Process change introduction– Modify the process to remove identified bottlenecks

• Process change training– Train staff involved in new process proposals

• Change tuning– Evolve and improve process improvements

©Ian Sommerville 1995

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Software process improvement initiatives

• Capability maturity model (CMM)– http://www.sei.cmu.edu/cmm/cmms/cmms.html

• ISO 9000-series

• ISO/IEC 15504 – Standard for Software Process Assessment (SPICE)– http://www-sqi.cit.gu.edu.au/spice/

©Steven Schach 2002 [modified]

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Capability Maturity Model

• A set of strategies for improving the software process– SW–CMM for software– P–CMM for human resources (“people”)– SE–CMM for systems engineering– IPD–CMM for integrated product development– SA–CMM for software acquisition

• These strategies are being unified into CMMI (capability maturity model integration)

• Developed by Software Engineering Institute (SEI)

©Steven Schach 2002 [modified]

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SW–CMM

• A strategy for improving the software process• Put forward in 1986 by the SEI • Fundamental ideas:

– Improving the software process leads to• Improved software quality• Delivery on time, within budget

– Improved management leads to• Improved techniques

• Five levels of “maturity” are defined• Organization advances stepwise from level to

level©Steven Schach 2002

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Level 1. Initial Level

• Ad hoc approach– Entire process is unpredictable– Management consists of responses to crises

• Most organizations world-wide are at level 1

©Steven Schach 2002

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Level 2. Repeatable Level

• Basic software management– Management decisions should be made on the basis

of previous experience with similar products– Measurements (“metrics”) are made– These can be used for making cost and duration

predictions in the next project– Problems are identified, immediate corrective action

is taken


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Level 3. Defined Level

• The software process is fully documented– Managerial and technical aspects are clearly defined– Continual efforts are made to improve quality,

productivity– Reviews are performed to improve software quality– CASE tools are applicable now (and not at levels 1 or

2)


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Level 4. Managed Level

• Quality and productivity goals are set for each project– Quality, productivity are continually monitored– Statistical quality controls are in place


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Level 5. Optimizing Level

• Continuous process improvement– Statistical quality and process controls– Feedback of knowledge from each project to

the next


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SW–CMM Summary


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• Any type of measurement which relates to a software system, process or related documentation– Lines of code in a program, number of person-days

required to develop a component

• Allow the software and the software process to be quantified

• Should be captured automatically and monitored if possible

Software metrics


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• A quality metric should be a predictor of product quality

• Most quality metrics are design quality metrics and are concerned with measuring the coupling or the complexity of a design

• The relationship between these metrics and quality has to be judged by a human (no automatic connection to quality possible)

• Outliers may point to problems• There are no “magic thresholds,” rather the trend

of metrics over time needs to be monitored

Product quality metrics


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Traditional Software Metics

• Coupling metrics (associated with a structure chart in Yourdon's Structured Design)– High number of calling functions or called functions suggests high

coupling• Cyclomatic complexity is a measure of control structure complexity

– Metric has two drawbacks• It is inaccurate for data-driven programs as it is only

concerned with control constructs.• It places the same weight on nested and non-nested loops.

Deeply nested structures, however, are usually harder to understand.

• Oviedo's metric modifies this to take data references into account– C = aE +bN

(with N external data entities, E edges to the data entities and constants a,b)


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Metrics for Object-Oriented Software

• Traditional (still usable)– Cyclomatic complexity (CC)

• New OO metrics– Coupling

• Coupling between objects (CBO)• Depth of inheritance tree (DIT)• …

– Cohesion• Lack of cohesion of methods (LCOM)• …

– Complexity• Weighted methods per class (WMC)• …

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• Time taken for process activities to be completed– E.g. Calendar time or effort to complete an activity

or process

• Resources required for processes or activities– E.g. Total effort in person-days

• Number of occurrences of a particular event– E.g. Number of defects discovered

• Process improvement requires process measurement!

Classes of process measurement (Process Metrics)


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• Goals– What is the organisation trying to achieve? The

objective of process improvement is to satisfy these goals

• Questions– Questions about areas of uncertainty related to the

goals. You need process knowledge to derive these

• Metrics– Measurements to be collected to answer the

questions

Goal-Question-Metric Paradigm
