cs223: software engineering lecture 6: requirement engineering

CS223: Software EngineeringLecture 6: Requirement Engineering

Recap

• Software development process model

• Iterative Model

• Timeboxing Model

Objective

After the end of the class students should be able to

o Define software requirement

o Identify components of software requirement

o Differentiate between functional and non-functional requirement

Introduction

• It is concerned with o Elicitation, o Analysis, o Specification, and o Validation of software requirements

• Management of requirements during the whole life cycle of the software product

• Software projects are critically vulnerable when the requirements related activities are poorly performed

Topics of discussionSoftware

Requirement

Process

Model

Management

Quality

Elicitation Analysis

Classification

Design

Negotiation

Analysis

Specification

SRS

Validation

Review

Verification

Test

Considerations

Change management

Tracing

Measurement

Definition

• It is a property that must be exhibited by something

• To solve some problem in the real world

• A complex combination from various people at different levels of an organization

• Objects connected with this feature from the environment in which the software will operate

Few important terminologies

• Product and process requirement

• Functional and Nonfunctional Requirements

• Emergent requirement

• Quantifiable requirement

• Avoid vague and unverifiable requirements

System and Software requirement

• International Council on Software and Systems Engineering (INCOSE)

• Software (user) requirementso Statements, in a natural language plus diagrams

o Services the system is expected to provide to system users and the constraints under which it must operate.

• System requirementso Detailed descriptions of the software system’s functions, services,

and operational constraints.

o Define exactly what is to be implemented (Functional specification)

Example

User requirement specification

• Generate monthly report

System requirement specification

• On the last working day of the month

• Automatically generate after 17.30 hrs.

• Created for each office

• Restricted access to valid user

Different Users

User Requirements

Client managers

System end-users

Client engineers

Contractor managers

System Architects

System Requirements

System end-users

Client engineers

System architects

Software developers

Functional and Non-functional requirements

• Functional

o statements of services the system should provide

o System reaction to a particular input

o System behaviour at a particular situation

o What a system should not do (optional)

• Non-Functional

o Constraints on the services or functions offered by the system

timing constraints,

constraints on the development process,

constraints imposed by standards

o Applicable to the entire system, not on parts

Functional requirements

• Describe functionality or system services.

• Depend on the type of software, expected users and the type of

system where the software is used.

• Functional user requirements may be high-level statements of what

the system should do.

• Functional system requirements should describe the system services

in detail.

A case study

• A user shall be able to search the appointments lists for all clinics.

• The system shall generate each day, for each clinic, a list of

patients who are expected to attend appointments that day.

• Each staff member using the system shall be uniquely identified by

his or her 8-digit employee number.

Requirements imprecision

• Problems arise when requirements are not precisely stated.

• Ambiguous requirements may be interpreted in different ways by

developers and users.

• Consider the term ‘search’ in requirement 1

o User intention – search for a patient name across all

appointments in all clinics;

o Developer interpretation – search for a patient name in an

individual clinic. User chooses clinic then search.

Requirements completeness and consistency

• Requirements should be both complete and consistent.

• Complete

oThey should include descriptions of all facilities required.

• Consistent

oThere should be no conflicts or contradictions in the descriptions

of the system facilities.

• It is a challenging task to meet both the conditions

Non-functional requirements

• These define system properties and constraints

o E.g. reliability, response time and storage requirements.

o Constraints are I/O device capability, system representations, etc.

• Process requirements may also be specified mandating a particular

IDE, programming language or development method.

• Non-functional requirements may be more critical than functional

requirements.

o If these are not met, the system may be useless.

Types of nonfunctional requirement Non-functional Requirement

Product

UsabilityEfficiency

Performance

Space

Dependability Security

Organizational

Environmental

Operational

Development

External

Regulatory Ethical Legislative

Accounting

Safety/Security

Non-functional requirements implementation• Non-functional requirements may affect the overall architecture of a

system rather than the individual components.

o Minimize communications between components.

• A single non-functional requirement may generate a number of related functional requirements that define system services that are required.

o It may also generate requirements that restrict existing

requirements.

Non-functional classifications

• Product requirementso Delivered product must behave in a particular way

o e.g. execution speed, reliability, etc.

• Organisational requirementso Consequence of organisational policies and procedures

o e.g. process standards used, implementation requirements, etc.

• External requirementsoRequirements which arise from factors which are external to the

system and its development process

o e.g. interoperability requirements, legislative requirements, etc.

Examples of nonfunctional requirements

Product requirement• The MHC-PMS shall be available to all clinics during normal

working hours (Mon–Fri, 0830–17.30). • Downtime within normal working hours shall not exceed five

seconds in any one day.

Organizational requirement• Users of the MHC-PMS system shall authenticate themselves

using their health authority identity card.

External requirement• The system shall implement patient privacy provisions as set out

in HStan-03-2006-priv.

Goals and requirements

• Non-functional requirements may be very difficult to state precisely

• Imprecise requirements may be difficult to verify.

• Goal

o A general intention of the user such as ease of use.

• Verifiable non-functional requirement

o A statement using some measure that can be objectively tested.

• Goals are helpful to developers as they convey the intentions of the

system users.

Usability requirements

• Should be easy to use by medical staff and should be organized in such a way that user errors are minimized. (Goal)

• Medical staff shall be able to use all the system functions after four hours of training.

• After this training, the average number of errors made by experienced users shall not exceed two per hour of system use. (Testable non-functional requirement)

Metrics for specifying nonfunctional requirements

Property MeasureSpeed • Processed transactions/second

• User/event response time• Screen refresh time

Size • Mbytes• Number of ROM chips

Ease of use • Training time• Number of help frames

Reliability • Mean time to failure• Probability of unavailability• Rate of failure occurrence• Availability

Robustness • Time to restart after failure• Percentage of events causing failure• Probability of data corruption on failure

Portability • Percentage of target dependent statements• Number of target systems

Domain requirements

• The system’s operational domain imposes requirements on the

system.

oFor example, a train control system has to take into account the

braking characteristics in different weather conditions.

• Domain requirements be new functional requirements, constraints

on existing requirements or define specific computations.

• If domain requirements are not satisfied, the system may be

unworkable.

Train protection system

• This is a domain requirement for a train protection system:

• The deceleration of the train shall be computed as:

o Dtrain = Dcontrol + Dgradient

o Dgradient is 9.81ms2 * compensated gradient/ alpha

o The values of 9.81ms2 /alpha are known for different types of

train.

• It is difficult for a non-specialist to understand the implications of

this and how it interacts with other requirements.

Domain requirements problems

• Understandability

o Requirements are expressed in the language of the application

domain;

o This is often not understood by software engineers developing

the system.

• Implicitness

o Domain specialists understand the area so well that they do not

think of making the domain requirements explicit.

The software requirements document

• The software requirements document is the official statement of

what is required of the system developers.

• Include both

o A definition of user requirements

o A specification of the system requirements.

• It is NOT a design document.

• As far as possible, it should set of WHAT the system should do

rather than HOW it should do it.

Key points

• Requirements for a software system set out what the system should do and

define constraints on its operation and implementation.

• Functional requirements are

o statements of the services that the system must provide or are

descriptions of how some computations must be carried out.

• Non-functional requirements

o Constrain the system being developed and the development process

being used.

• They often relate to the emergent properties of the system and therefore

apply to the system as a whole.

Thank youNext Lecture: Requirement Engineering