02 req ts collection

8/12/2019 02 Req Ts Collection

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ESEEinfhrung in Software Engineering

2. Requirements Collection

Prof. O. Nierstrasz


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Oscar Nierstrasz

ESE Requirements Collection

ESE 2.2

http://en.wikipedia.org/wiki/Fair_use


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ESE 2.3

Roadmap

> The Requirements Engineering Process> Use Cases

> Functional and non-functional requirements

> Evolutionary and throw-away prototyping

> Requirements checking and reviews


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ESE 2.4

Sources

> Software Engineering, I. Sommerville, 7th Edn., 2004.> Software Engineering A Practitioners Approach, R. Pressman,

Mc-Graw Hill, 5th Edn., 2001.

> Objects, Components and Frameworks with UML, D. D'Souza, A.Wills, Addison-Wesley, 1999


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ESE 2.5

Roadmap

> The Requirements Engineering Process> Use Cases





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ESE 2.6

Kommission:

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3 4 5 6 7 88:00 - 9:00

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Bemerkungen: Unterschrift:

Zeitschema

Jan 2002 Feb 2002

Bitte ankreuzen wo Sie keinenfalls mitmachen knnen, und senden Sie das ausgefllte Formular bis ___________ ans Dekanat zurck.


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ESE 2.7

Electronic Time Schedule

So,basically we need a form for the time schedule that can be distributed by eMail, aplace (html) where I can deposit these forms after they have been filled out, and an

algorithm that calculates a few possible meeting times, possibly setting priorities tocertain persons of each committee (since there will always be some time scheduleoverlaps). It would also be great if there were a way of checking whether everybody ofthe relevant committee has really sent their time schedule back and at the same timelisting all the ones who have failed to do so. An automatic invitation letter for thecommittee meeting to all the persons involved, generated through this programme, wouldbe even a further asset.

How can we transform this description into a requirements specification?




ESE 2.8

The Requirements Engineering Process

Feasibilitystudy

Requirementselicitation and

analysisRequirementsspecification

Requirementsvalidation

Feasibilityreport

Systemmodels

User and system

requirements

Requirements

document

Ian Sommerville 2000




ESE 2.9

Requirements Engineering Activities

Feasibi l i ty studyDetermine if the user needscan be satisfiedwith the available technologyand budget.

Requirements

analysis

Find out what system stakeholders require

from the system.Requirements

def in i t ion

Define the requirementsin a formunderstandable to the customer.

Requirements

specif icat ion

Define the requirements in detail. (Written asa contract between client and contractor.)

Requirements are for users;

specifications are for analysts

and developers.




ESE 2.10

Requirements Analysis

Sometimes called requirements elicitationor requirementsdiscovery

Technical staff work with customers to determine

> the application domain,> the servicesthat the system should provide and

> the systems operational constraints.

Involves various stakeholders:> e.g., end-users, managers, engineers involved in

maintenance, domain experts, trade unions, etc.




ESE 2.11http://en.wikipedia.org/wiki/Fair_use




ESE 2.12

Problems of Requirements Analysis

Various problems typically arise:

Stakeholders dont knowwhat they really want

Stakeholders express requirements in their own terms

Different stakeholders may have conflicting requirements

Organisational and political factorsmay influence the systemrequirements

The requirements changeduring the analysis process.

New stakeholdersmay emerge.


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ESE 2.13

Impedance Mismatches

As Management

requested it

As the Project

Leader defined it

As Systems

designed it

As Programming

developed it

As

Operations

installed it

What the

User wanted


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ESE 2.14

Requirements evolution

> Requirements always evolveas a better understanding

of user needs is developed and as the organisationsobjectives change

> It is essential toplan for changein the requirements as

the system is being developed and used


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ESE 2.15

The Requirements Analysis Process

Requirementsvalidation

Domain

understanding Prioritization

Requirements

collection

Conflict

resolution

Classification

Requirementsdefinition andspecification

Processentry



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ESE 2.16

Roadmap

> The Requirements Engineering Process

> Use Cases





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ESE 2.17

Use Cases and Scenarios

A use caseis the specificationof a sequence of actions, includingvariants, that a system (or other entity) can perform, interacting withactorsof the system.

e.g., buy a DVD through the internet

A scenariois aparticular trace of action occurrences, starting from aknown initial state.

e.g., connect to myDVD.com, go to the search page

...


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ESE 2.18

Use Cases and Viewpoints ...

Stakeholdersrepresent different problem viewpoints.

Interview as many differentkinds of stakeholders aspossible/necessary

Translate requirements into use casesor stories about thedesired system involving a fixed set of actors (users and systemobjects)

For each use case, capture both typical and exceptionalusagescenarios

Userstend to think about systems in terms of features.

You must get them to tell you storiesinvolving those features.

Use cases and scenarios can tell you if the requirements arecomplete and consistent!


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ESE 2.19

Unified Modeling Language

Class Diagramsvisualize logical structureof system

in terms of classes, objects and relationships

Use Case Diagramsshow external actors and use casestheyparticipate in

Sequence Diagramsvisualize temporal message orderingof aconcrete scenarioof a use case

Collaborat ion

(Communicat ion)

Diagrams

visualize relationshipsof objects exchangingmessages in a concrete scenario

State Diagramsspecify the abstract statesof an object andthe transitionsbetween the states

UML is the industry standard for documenting OO models


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ESE 2.20

Use Case Diagrams

More on

this later


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ESE 2.21

Sequence Diagrams


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ESE 2.22

Writing Requirements Definitions

Requirements definitions usually consist of natural language,supplemented by (e.g., UML) diagrams and tables.

Three types of problems can arise:

Lack of clarity: It is hard to write documents that are bothprecise andeasy-to-read.

Requirements confusion: Functional and non-functional requirementstend to be intertwined.

Requirements amalgamation: Several different requirementsmay beexpressed together.


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ESE 2.23

Roadmap


> Use Cases





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ESE 2.24

Functional and Non-functionalRequirements

Functional requirementsdescribe system servicesorfunctionsCompute sales tax on a purchase

Update the database on the server ...

Non-functional requirementsare constraintson the systemor the development process

Non-functional requirements may be more critical than functionalrequirements.

If these are not met, the system is useless!


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ESE 2.25

Non-functional Requirements

Product

requirements:

specify that the delivered product must behavein a particular way

e.g. execution speed, reliability, etc.

Organisat ional

requirements:

are a consequence of organisational policiesand procedures

e.g. process standards used, implementationrequirements, etc.

External

requirements:

arise from factors which are externalto thesystem and its development process

e.g. interoperability requirements, legislativerequirements, etc.


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ESE 2.26

Types of Non-functional Requirements

Performancerequirements

Spacerequirements

Usability

requirements

Efficiencyrequirements

Reliabilityrequirements

Portabilityrequirements

Interoperabilityrequirements

Ethicalrequirements

Legislative

requirements

Implementation

requirements

Standards

requirements

Delivery

requirements

Safetyrequirements

Privacyrequirements

Productrequirements

Organizationalrequirements

Externalrequirements

Non-functionalrequirements


S C


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ESE 2.27

Examples of Non-functionalRequirements

Product

requirement

It shall be possible for all necessary communicationbetween the APSE and the user to be expressed inthe standard Ada character set.

Organisat ional

requirement

The system development processand deliverabledocuments shall conform to the process anddeliverables defined inXYZCo-SP-STAN-95.

External

requirement

The system shall provide facilities that allow any

user to check if personal data is maintained on thesystem.A procedure must be defined and supportedin the software that will allow users to inspect

personal dataand to correct any errors in that data.

ESE R i C ll i


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ESE 2.28

Requirements Verifiability

Requirements must be written so that they can beobjectively verified.

Imprecise:

The system should be easy to useby experienced controllersand should be organised in such a way that user errors areminimised.

Terms like easy to use and errors shall be minimised areuseless as specifications.

Verif iable: Experienced controllers should be able to use all the system

functions after a total of two hours training. After this training,the average number of errors made by experienced usersshould not exceed two per day.


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ESE Req irements Collection


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ESE 2.30

Precise Requirements Measures (II)

Property Measure

Reliability

Mean time to failure

Probability of unavailabilityRate of failure occurrence

RobustnessTime to restart after failurePercentage of events causing failureProbability of data corruption on failure

PortabilityPercentage of target dependent statementsNumber of target systems



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ESE 2.31

Roadmap


> Use Cases






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ESE 2.32

Prototyping Objectives

The objective of evolutionary prototypingis to deliver aworking systemto end-users.

Development starts with the requirements that are bestunderstood.

The objective of throw-away prototypingis to validate orderive the system requirements.

Prototyping starts with that requirements that arepoorlyunderstood.



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ESE 2.33

Evolutionary Prototyping

> Must be used for systems where the specification cannotbe developed in advance.

e.g., AI systems and user interface systems

> Based on techniques which allow rapid system iterations.e.g., executable specification languages, VHL languages, 4GLs,

component toolkits

> Verification is impossibleas there is no specification.Validationmeans demonstrating the adequacy of the system.



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ESE 2.34

Throw-away Prototyping

> Used to reduce requirements riskThe prototype is developedfrom an initial specification, delivered

for experiment then discarded

> The throw-away prototype should notbe considered as afinal systemSome system characteristics may have been left out

(e.g., platform requirements may be ignored)

There is no specification for long-term maintenance

The system will be poorly structured and difficult to maintain



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ESE 2.35

Roadmap


> Use Cases






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ESE 2.36

Requirements Checking

ValidityDoes the system provide the functionswhich best supportthe customers needs?

Consistency Are there any requirements conflicts?

CompletenessAre all functionsrequired by the customerincluded?

Real ismCan the requirements be implementedgiven available budget and technology?



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ESE 2.37

Requirements Reviews

> Regular reviewsshould be held while the requirementsdefinition is being formulated

> Both client and contractorstaff should be involved inreviews

> Reviews may be formal(with completed documents) orinformal.

Good communicationsbetween developers, customers andusers can resolve problems at an early stage


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ESE 2.39

Traceability

To protect against changes you should be able to trace back from everysystem component to the original requirementthat caused itspresence

C1 C2 Cmreq1 x

req2 x

x

x

reqn x x

A software processshould help you keepthis virtual table up-to-date

Simple techniquesmay be quitevaluable (namingconventions, ...)



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ESE 2.40

What you should know!

> What is the difference between requirements analysisand specification?

> Why is it hard to define and specify requirements?

> What are use cases and scenarios?> What is the difference between functional and non-

functional requirements?

> Whats wrong with a requirement that says a product

should be user-friendly?> Whats the difference between evolutionary and throw-

away prototyping?



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Oscar Nierstrasz ESE 2.41

Can you answer the followingquestions?

> Why isnt it enough to specify requirements as a set ofdesired features?

> Which is better for specifying requirements: naturallanguage or diagrams?

> How would you prototype a user interface for a web-based ordering system?

> Would it be an evolutionary or throw-away prototype?

> What would you expect to gain from the prototype?

> How would you check a requirement for adaptability?



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