Requirements Engineering - Software Engineering - Lecture Notes, Study notes of Software Engineering

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SE Fundamentals:

2. Requirements Engineering

Topics covered

 Functional and non-functional requirements

 The software requirements specification document

(SRS)

 Requirements specification

 Requirements engineering processes

 Requirements elicitation and analysis

 Requirements validation

 Requirements management

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.

 The requirements themselves are the descriptions of the

system services and constraints that are generated

during the requirements engineering process.

What is a requirement?

 It may range from a high-level abstract statement of a

service or of a system constraint to a detailed

mathematical functional specification.

 This is inevitable as requirements may serve a dual

function:

 May be the basis for a bid for a contract - therefore must be open

to interpretation;

 May be the basis for the contract itself - therefore must be

defined in detail;

Types of requirement

 User requirements

 Statements in natural language plus diagrams of the services the

system provides and its operational constraints. Written for

customers.

 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.

User and system requirements

Readers of different types of requirements

specification

Functional and non-functional requirements

 Functional requirements

 Statements of services the system should provide, how the

system should react to particular inputs and how the system

should behave in particular situations.

 May state what the system should not do.

 Non-functional requirements

 Constraints on the services or functions offered by the system

such as 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 domain of operation

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.

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.

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

 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.

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, it is impossible to produce a complete and

consistent requirements document

Non-functional requirements

 These define system properties and constraints e.g.

reliability, response time, and storage requirements.

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. If these are not met, the system

may be useless.

Types of nonfunctional requirement

Non-functional requirements implementation

 Non-functional requirements may affect the overall

architecture of a system rather than the individual

components.

 For example, to ensure that performance requirements are met,

you may have to organize the system to minimize

communications between components.

 A single non-functional requirement, such as a security

requirement, may generate a number of related

functional requirements that define system services that

are required.

 It may also generate requirements that restrict existing

requirements.

Non-functional classifications

 Product requirements

 Requirements which specify that the delivered product must

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

 Organizational requirements

 Requirements which are a consequence of organizational

policies and procedures e.g. process standards used,

implementation requirements, etc.

 External requirements

 Requirements which arise from factors which are external to the

system and its development process, e.g. interoperability

requirements, legislative requirements, etc.

Examples of nonfunctional requirements in the

MHC-PMS

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.

Metrics for specifying non-functional

requirements

Property Measure

Speed 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

The software requirements document

 The software requirements document is the official

statement of what is required of the system developers.

 Can 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 of WHAT the system should do rather than

HOW it should do it.

Agile methods and requirements

 Many agile methods argue that producing a

requirements document is a waste of time as

requirements change so quickly

 The document is therefore always out of date

 Methods such as XP use incremental requirements

engineering and express requirements as ‘user stories’

(discussed in Chapter 3).

 This is practical for business systems but problematic for

systems that require a lot of pre-delivery analysis (e.g.

critical systems) or systems developed by several teams

Users of a requirements document

The structure of a requirements document

Chapter Description Preface This should define the expected readership of the document and describe its version history, including a rationale for the creation of a new version and a summary of the changes made in each version. Introduction This should describe the need for the system. It should briefly describe the system’s functions and explain how it will work with other systems. It should also describe how the system fits into the overall business or strategic objectives of the organization commissioning the software. Glossary This should define the technical terms used in the document. You should not make assumptions about the experience or expertise of the reader. User requirements definition Here, you describe the services provided for the user. The nonfunctional system requirements should also be described in this section. This description may use natural language, diagrams, or other notations that are understandable to customers. Product and process standards that must be followed should be specified. System architecture This chapter should present a high-level overview of the anticipated system architecture, showing the distribution of functions across system modules. Architectural components that are reused should be highlighted.

The structure of a requirements document

Chapter Description System requirements specification This should describe the functional and nonfunctional requirements in more detail. If necessary, further detail may also be added to the nonfunctional requirements. Interfaces to other systems may be defined. System models This might include graphical system models showing the relationships between the system components and the system and its environment. Examples of possible models are object models, data-flow models, or semantic data models. System evolution This should describe the fundamental assumptions on which the system is based, and any anticipated changes due to hardware evolution, changing user needs, and so on. This section is useful for system designers as it may help them avoid design decisions that would constrain likely future changes to the system. Appendices These should provide detailed, specific information that is related to the application being developed; for example, hardware and database descriptions. Hardware requirements define the minimal and optimal configurations for the system. Database requirements define the logical organization of the data used by the system and the relationships between data. Index Several indexes to the document may be included. As well as a normal alphabetic index, there may be an index of diagrams, an index of functions, and so on.

Requirements specification

 The process of writing down the user and system

requirements in a requirements document

 User requirements have to be understandable by end-

users and customers who do not have a technical

background

 System requirements are more detailed requirements

and may include more technical information

 The requirements may be part of a contract for the

system development

 It is therefore important that these are as complete as possible

Ways of writing a system requirements

specification

Notation Description Natural language The requirements are written using numbered sentences in natural language. Each sentence should express one requirement. Structured natural language The requirements are written in natural language on a standard form or template. Each field provides information about an aspect of the requirement. Design description languages This approach uses a language like a programming language, but with more abstract features to specify the requirements by defining an operational model of the system. This approach is now rarely used although it can be useful for interface specifications. Graphical notations Graphical models, supplemented by text annotations, are used to define the functional requirements for the system; UML use case and sequence diagrams are commonly used. Mathematical specifications These notations are based on mathematical concepts such as finite-state machines or sets. Although these unambiguous specifications can reduce the ambiguity in a requirements document, most customers don’t understand a formal specification. They cannot check that it represents what they want and are reluctant to accept it as a system contract