Chapter 6+7 in textbook

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Chapter 4Software Requirements

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Objectives

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To understand the concept of software requirements

To understand the difference between functional and non functional requirements

Understand the importance of getting it right.Understand how requirements are

documented (the SRS document) Understand the requirements engineering

processUnderstand how to discover requirementsUnderstand how to validate requirementsUnderstand how to manage requirements

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Overview

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What are software requirements? Functional requirements Non functional requirements Domain requirements User and system requirements Interface specification Why is it important to get it right? The SRS document The requirements engineering process. Feasibility study Requirements elicitation Requirements specification Requirements validation Requirements management

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What are Software Requirements

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“The descriptions of the system services and constraints that are generated during the requirements engineering process.”

Developed during the first phase in the software development life cycle.

The LIBSYS case study

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A library system that provides a single interface to a number of databases of articles in different libraries.

Users can search for, download and print these articles for personal study.

Requirements types

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1. Functional requirementsStatements of services the system should provide,

how the system should react to particular inputs and how the system should behave in particular situations.

2. Non-functional requirementsconstraints on the services or functions offered by the

system such as timing constraints, constraints on the development process, standards, etc.

3. Domain requirementsRequirements that come from the application domain

of the system and that reflect characteristics of that domain.

Functional Requirements

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Describe functionality or system services.Depend on the type of software, expected

users and the type of system where the software is used.

Functional requirements can be User requirements are high-level statements

of what the system should System requirements should describe the

system services in detail.

Examples

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1. The user shall be able to search either all of the initial set of databases or select a subset from it.

2. The system shall provide appropriate viewers for the user to read documents in the document store.

3. Every order shall be allocated a unique identifier (ORDER_ID) which the user shall be able to copy to the account’s permanent storage area.

Problems

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Problems arise when requirements are not precisely stated (Ambiguous)

Ambiguous requirements may be interpreted in different ways

by developers and users.

Consider the term ‘appropriate viewers’User intention - special purpose viewer for each

different document type;Developer interpretation - Provide a text viewer

that shows the contents of the document.

Non-Functional Requirements

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These define system properties, constraints,

and process requirements

System Properties

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Reliability,Response time Maintainability Storage requirements.

Constraints

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I/O device capability, Data representations

Process Requirements

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Mandating a particular CASE system, Programming language or Development method.

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Which do you think is more critical ,

A functional or non-functional requirement and why?

Have a look at the following video

Europeana Project

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Non-functional classifications

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Product requirementsRequirements which specify that the delivered product

must behave in a particular way e.g. execution speed, reliability, etc.

Organisational requirementsRequirements which are a consequence of

organisational policies and procedures e.g. process standards used, implementation requirements, etc.

External requirementsRequirements which arise from factors which are

external to the system and its development process e.g. interoperability requirements, legislative requirements, etc.

Types of Non-functional requirements

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Performancerequirements

Spacerequirements

Usabilityrequirements

Efficiencyrequirements

Reliabilityrequirements

Portabilityrequirements

Interoperabilityrequirements

Ethicalrequirements

Legislativerequirements

Implementationrequirements

Standardsrequirements

Deliveryrequirements

Safetyrequirements

Privacyrequirements

Productrequirements

Organisationalrequirements

Externalrequirements

Non-functionalrequirements

Examples

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Product requirement The user interface for LIBSYS shall be

implemented as simple HTML without frames or Java applets.

Organisational requirementThe system development process and deliverable documents shall conform to the process and deliverables defined in XYZCo-SP-STAN-95.

External requirementThe system shall not disclose any personal information about customers apart from their name and reference number to the operators of the system.

Verifiable Non-functional requirements

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Non-functional requirements may be very difficult to state precisely

Imprecise requirements may be difficult to verify.

Therefore we need a statement using some measure that can be objectively tested.

Example

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The system shall be easy to use

Better expressed as:Experienced users shall 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 users shall not exceed two per day.

Requirements measures

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Property Measure

Speed Processed transactions/secondUser/Event response timeScreen refresh time

Size M BytesNumber of ROM chips

Ease of use Training timeNumber of help frames

Reliability Mean time to failureProbability of unavailabilityRate of failure occurrenceAvailability

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

Portability Percentage of target dependent statementsNumber of target systems

Domain Requirements

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Derived from the application domain and describe system characteristics and features that reflect the domain.

Domain requirements may be new functional requirements, constraints on existing requirements or define specific computations.

Library system domain requirements

There shall be a standard user interface to all databases which shall be based on the Z39.50 standard (library standard -design constraint ).

Because of copyright restrictions, some documents must be deleted immediately on arrival. Depending on the user’s requirements, these documents will either be printed locally on the system server for manually forwarding to the user or routed to a network printer.

Train protection system

The deceleration of the train shall be computed as:Dtrain = Dcontrol + Dgradient

where Dgradient is 9.81ms2 * compensated gradient/alpha and where the values of 9.81ms2 /alpha are known for different types of train.

Class Activity

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Working with your team, identify the type of requirements listed on the sheet.

When you are done discuss your decisions with the rest of the class.

Check each requirement, if you got it right give yourself 1 point.

Compute your total. The winning teams will get a 0.5 (out of 2

in-class marks)

User Requirements

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Written for customersStatements in natural languageDescribe the services the system

provides and its operational constraints.May include diagrams or tablesShould describe functional and non-

functional requirements Should be understandable by system

users who don’t have detailed technical knowledge.

We provide a definition for a user requirement.

System Requirements

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Statements that set out detailed descriptions of the system’s functions, services and operational constraints.

Defines what should be implemented so may be part of a contract between client and contractor.

Intended to be a basis for designing the system.

Can be illustrated using system models

We provide a specification for a system requirement.

Guidelines for writing requirements

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Invent a standard format and use it for all requirements.

Use language in a consistent way. Use shall for mandatory requirements,

should for desirable requirements.Use text highlighting to identify key parts

of the requirement.Avoid the use of computer jargon.

Definition and Specifications

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1. The software must provide a means of representing and1. accessing external files created by other tools.

1.1 The user should be provided with facilities to define the type of1.2 external files.1.2 Each external file type may have an associated tool which may be1.2 applied to the file.1.3 Each external file type may be represented as a specific icon on1.2 the user’s display.1.4 Facilities should be provided for the icon representing an1.2 external file type to be defined by the user.1.5 When a user selects an icon representing an external file, the1.2 effect of that selection is to apply the tool associated with the type of1.2 the external file to the file represented by the selected icon.

User requirement definition

System requirements specification

Interface Specification

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Most systems must operate with other systems and the operating interfaces must be specified as part of the requirements.

Why is it important to get it right?

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If you don’t do it right you will build a very elegant software solution that solves the wrong problem.

the result is project failure (wasted time, and money, personnel frustration, and customer dissatisfaction.

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Representing Requirements The SRS document

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The Software Requirements Specification (SRS)

document is the official statement of what is

required of the system developers.

The SRS document

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Should include both a definition of user requirements and

a specification of the system requirements.

It is NOT a design document. As far as possible, it should set WHAT the system should do rather than HOW it should do it.

Users of the SRS

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Use the requirements todevelop validation tests for

the system

Use the requirementsdocument to plan a bid forthe system and to plan the

system development process

Use the requirements tounderstand what system is to

be developed

System testengineers

Managers

Systemengineers

Specify the requirements andread them to check that they

meet their needs. Theyspecify changes to the

requirements

Systemcustomers

Use the requirements to helpunderstand the system and

the relationships between itsparts

Systemmaintenanceengineers

Structure of the SRS

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PrefaceIntroductionGlossaryUser requirements definitionSystem architecture (high level)System requirements specificationSystem modelsSystem evolutionAppendicesIndex

Requirements Engineering Process

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“The process of establishing the services that the customer requires from a system and the constraints under which it operates and is developed.”

The result is a specification :“representing the requirements in a manner that ultimately leads to successful software implementation.

Requirements engineering

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Involves the following processesFeasibility Study Feasibility Report

Requirements elicitation System Models

Requirements Specification user and system requirements

Requirements validation Requirements document (SRS)

+ Requirements Management

Requirements Engineering Process

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Feasibilitystudy

Requirementselicitation and

analysisRequirementsspecification

Requirementsvalidation

Feasibilityreport

Systemmodels

User and systemrequirements

Requirementsdocument

The Feasibility Study

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A feasibility study decides whether or not the proposed system is worthwhile.

A short focused study that checksIf the system contributes to organisational

objectives;If the system can be engineered using

current technology and within budget;If the system can be integrated with other

systems that are used.

Requirements Elicitation

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Involves technical staff working with customers to find out about the application domain, the services that the system should provide and the system’s operational constraints.

May involve end-users, managers, engineers involved in maintenance, domain experts, trade unions, etc. These are called stakeholders.

“refer to any person or group who will be affected by the system, directly or in directly.”

Requirements discovery

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Sources of informationDocumentationSystem stakeholders

Interviews (close, open)Understand and critique real-life examples. (scenarios , prototypes)

Observation (ethnography)Specifications of similar systems

Stakeholders for LIBSYS

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Can you identify possible stakeholders for the LIBSYS?

Library managerArticle providersStudentsStaff (library , finance, maintenance etc..)

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Articleproviders

FinanceLibrarymanager

Librarystaff

Users

InteractorIndirect

All VPs

Classificationsystem

UIstandards

Domain

ExternalStaffStudents CataloguersSystem

managers

Analysis : System models

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Why?The model aids the analyst in understanding the system, thereby makes the requirements analysis easier and more systematic.

The model becomes the focal point of review.

The model becomes foundation for design.

Produced during requirements analysis.More on modeling in chapter 5.

Requirements specification

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It is the final work product produced by the requirements engineer.

The SRS documentIt serves as a foundation for the software design and implementation.

Requirements Validation

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The process of ensuring that the requirements actually define the system that the customer wants.

Why is it important? The cost of fixing a requirements

problem by making a system change is much greater than repairing design or coding errors.

Validation checks

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Validity checks◦ Is this what the user wants?◦ Does the system provide the functions which best support

the customer’s needs?Consistency checks

◦ Requirements should not conflictCompleteness checks

◦ Requirements should define all functions and constraints◦ Are all functions required by the customer included?

Realism checks◦ Ensure they could be implemented ◦ Can the requirements be implemented given available

budget and technologyVerifiability

◦ Requirements should be written so that they are verifiable, you should be able to write tests for each requirement.

Validation techniques

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Requirements reviewsA team of reviewers manually check the

requirements. Prototyping

An executable model of the system is demonstrated to customers.

Test-case generationRequirements should be testable. If tests are

difficult or impossible to design, this means that the requirements will be difficult to implement.

Developing tests before any code is written is used in ----?

Requirements Management

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Why?Requirements for large software systems are

always changing. Because the problem can not be fully specified, the requirements are usually incomplete and bound to change.

During the software process the stakeholders understanding of the problem is constantly changing

After the system is installed, new requirements will emerge.

What? It is the process of understanding and

controlling changes to system requirements.

Requirements Management Planning

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Requirements identificationEach requirement must be uniquely

identifiedA change management process

Assess the impact and cost of changeTraceability policies

Define the relationship between requirements

CASE tool support

Class Activity

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Working with your team, list the most important characteristics of good requirements.

When you are done discuss your decisions with the rest of the class.

Check each characteristic, if you got it right give yourself 1 point.

Compute your total. The winning teams will get a 0.5 (out of 2

in-class marks)

Good RequirementsCharacteristic Explanation

Complete Everything the software is supposed to do and responses of the software to all classes of input data are specified in the SRS

Consistent The requirement does not contradict any other requirement

Correct Every requirement in the SRS represents something required in the final system.

Unambiguous The requirement is concisely stated without recourse to technical jargon, acronyms (unless defined elsewhere in the Requirements document). Every requirement has one and only one interpretation.

Verifiable There is a cost effective method that can check whether the final software meets the requirement.