Chapter 4 – Requirements Engineering

Lecture 1The hardest part of the software task is arriving at a complete

and consistent specification, and much of the essence of building a program is in fact the debugging of the

specification.F. Brooks

Computers do not solve problems -computers carry out solutions, specified by people, to problems.

D. D. Spencer 1Chapter 4 Requirements engineering

Where we are

Requirements

Chapter 4 Requirements engineering 2

Requirementsanalysis

though we will be following an agile process rather than a waterfall model, we will be going through these phases.

Topics covered

Functional and non-functional requirements The software requirements document Requirements specification Requirements engineering processes Requirements elicitation and analysis Requirements validation Requirements management

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Requirements engineering

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

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What is a requirement?

description of the system services or constraints range from

a high-level abstract statement of a service or system constraint to

a detailed mathematical functional specification. requirements serve a dual function

the basis for a bid for a contract - therefore must be open to interpretation;

the basis for the contract itself - therefore must be defined in detail;

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Types of requirement

User requirements Statements in natural language from the users viewpoint. Written for customers or users of the system.

System requirements A structured document setting 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. Written for developers.

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Readers of different types of requirements specification

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User and system requirements

8Chapter 4 Requirements engineeringwhat’s the difference?

Functional and non-functional requirements

Both User and System requirements have both functional and non-functional requirements.

Functional requirements Statements of services the system should provide, how the system should react to particular inputs how the system should behave in particular situations. May state what the system should not do.

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Functional and non-functional requirements

Non-functional requirements Constraints on the services or functions offered by the system

• timing constraints, constraints on the development process, standards, etc.

Often apply to the system as a whole rather than individual features or services.

Domain requirements Constraints on the system from the user area of operation

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Functional requirements

Describe functionality or system services. Depend on

type of software, expected users type of system where the software is used.

Functional user requirements high-level statements of what the system should do.

Functional system requirements describe the system services in detail.

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Functional requirements for the MHC-PMS

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.

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Requirements ambiguity

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 User intention – search for a patient name across all

appointments in all clinics; Developer interpretation – search for a patient name in an

individual clinic. User chooses clinic then search.

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Requirements completeness and consistency

In principle, requirements should be both complete and consistent.

Complete They should include descriptions of all facilities required.

Consistent There should be no conflicts or contradictions in the descriptions

of the system facilities.

In practice impossible

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Non-functional requirements

These define system properties and constraints reliability response time storage requirements I/O device capability system representations

Process requirements mandating a particular IDE programming language development method

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Non-functional requirements implementation

affects overall architecture of a system example: performance

• may have to organize the system to minimize communications between components.

One non-functional requirement may generate many functional requirements e.g., a security requirement

• many ways/places in a system to implement security

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Non-functional classifications

Product requirements the delivered product must behave in a particular way e.g.

execution speed, reliability, etc.

Organisational requirements a consequence of organisational policies and procedures e.g. process standards used, implementation requirements, etc.

External requirements arise from factors which are external to the system and its

development process e.g. interoperability requirements, legislative requirements, etc.

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Types of nonfunctional requirement

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Examples of nonfunctional requirements in the MHC-PMS

Product requirementThe 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 requirementUsers of the MHC-PMS system shall authenticate themselves using their health authority identity card.

External requirementThe system shall implement patient privacy provisions as set out in HStan-03-2006-priv.

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Writing non-functional requirements

Even at user level, non-functional requirements must be testable. Non-testable user requirement:The system should be easy to use by medical staff and should be organized in such a way that user errors are minimized.

Testable user requirement: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.

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could the testable user requirement be used as a system requirement without changing it?

Metrics for specifying nonfunctional requirements

Property MeasureSpeed Processed transactions/second

User/event response timeScreen refresh time

Size MbytesNumber 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

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Domain requirements

The system’s operational domain imposes requirements on the system. For example, a train control system has to take into account the braking characteristics in different weather conditions.

Domain requirements may be new functional requirements, constraints on existing requirements or define specific computations. If domain requirements are not satisfied, the system may be unworkable.

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Train protection system

This is a domain requirement for a 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.

It is difficult for a non-specialist to understand the implications of this and how it interacts with other requirements.

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Domain requirements problems

Understandability Requirements are expressed in the language of the application domain; This is often not understood by software engineers developing the system.

Implicitness Domain specialists understand the area so well that they do not think of making the domain requirements explicit.

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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 statements of the services that the system must provide or are descriptions of how some computations must be carried out.

Non-functional requirements often 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.

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