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©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 1
Chapter 6
Requirements Engineering
Process

Requirements Engineering Processes

Processes used to discover,
analyse and validate system
requirements

Objectives

To describe the principal requirements
engineering activities

To introduce techniques for requirements
elicitation and analysis

To describe requirements validation

To discuss the role of requirements
management in support of other requirements
engineering processes

Topics covered

Feasibility studies

Requirements elicitation and analysis

Requirements validation

Requirements management

Requirements engineering processes

The processes used for RE vary widely depending
on the application domain, the people involved
and the organisation developing the requirements

However, there are a number of generic activities
common to all processes
• Requirements elicitation
• Requirements analysis
• Requirements validation
• Requirements management

The requirements engineering process

Feasibility studies

A feasibility study decides whether or not the
proposed system is worthwhile

A short focused study that checks
• If 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

Feasibility study implementation

Based on information assessment (what is
required), information collection and report
writing

Questions for people in the organisation
• What if the system wasn’t implemented?
• What are current process problems?
• How will the proposed system help?
• What will be the integration problems?
• Is new technology needed? What skills?
• What facilities must be supported by the proposed system?

Elicitation and analysis

Sometimes called requirements elicitation or
requirements discovery

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

Problems of requirements
analysis

Stakeholders don’t know what they really want

Stakeholders express requirements in their own terms

Different stakeholders may have conflicting
requirements

Organisational and political factors may influence the
system requirements

The requirements change during the analysis process.
New stakeholders may emerge and the business
environment change

The requirements analysis
process

Process activities

Domain understanding

Requirements collection

Classification

Conflict resolution

Prioritisation

Requirements checking

System models

Different models may be produced during the
requirements analysis activity

Requirements analysis may involve three
structuring activities which result in these different
models
• Partitioning. Identifies the structural (part-of) relationships
between entities
• Abstraction. Identifies generalities among entities
• Projection. Identifies different ways of looking at a problem

System models covered in Chapter 7

Viewpoint-oriented elicitation

Stakeholders represent different ways of looking
at a problem or problem viewpoints

This multi-perspective analysis is important as
there is no single correct way to analyse system
requirements

Banking ATM system

The example used here is an auto-teller system
which provides some automated banking services

I use a very simplified system which offers some
services to customers of the bank who own the
system and a narrower range of services to other
customers

Services include cash withdrawal, message
passing (send a message to request a service),
ordering a statement and transferring funds

Autoteller viewpoints

Bank customers

Representatives of other banks

Hardware and software maintenance engineers

Marketing department

Bank managers and counter staff

Database administrators and security staff

Communications engineers

Personnel department

Types of viewpoint

Data sources or sinks
• Viewpoints are responsible for producing or consuming data.
Analysis involves checking that data is produced and consumed
and that assumptions about the source and sink of data are valid

Representation frameworks
• Viewpoints represent particular types of system model. These may
be compared to discover requirements that would be missed using
a single representation. Particularly suitable for real-time systems

Receivers of services
• Viewpoints are external to the system and receive services from it.
Most suited to interactive systems

External viewpoints

Natural to think of end-users as receivers of
system services

Viewpoints are a natural way to structure
requirements elicitation

It is relatively easy to decide if a viewpoint is
valid

Viewpoints and services may be sued to
structure non-functional requirements

Method-based analysis

Widely used approach to requirements analysis.
Depends on the application of a structured
method to understand the system

Methods have different emphases. Some are
designed for requirements elicitation, others are
close to design methods

A viewpoint-oriented method (VORD) is used as
an example here. It also illustrates the use of
viewpoints

The VORD method

VORD process model

Viewpoint identification
• Discover viewpoints which receive system services and identify
the services provided to each viewpoint

Viewpoint structuring
• Group related viewpoints into a hierarchy. Common services are
provided at higher-levels in the hierarchy

Viewpoint documentation
• Refine the description of the identified viewpoints and services

Viewpoint-system mapping
• Transform the analysis to an object-oriented design

VORD standard forms

Viewpoint identification

Viewpoint service information

Viewpoint data/control

Viewpoint hierarchy

Customer/cash withdrawal
templates

Scenarios

Scenarios are descriptions of how a system is
used in practice

They are helpful in requirements elicitation as
people can relate to these more readily than
abstract statement of what they require from a
system

Scenarios are particularly useful for adding detail
to an outline requirements description

Scenario descriptions

System state at the beginning of the scenario

Normal flow of events in the scenario

What can go wrong and how this is handled

Other concurrent activities

System state on completion of the scenario

Event scenarios

Event scenarios may be used to describe how a
system responds to the occurrence of some
particular event such as ‘start transaction’

VORD includes a diagrammatic convention for
event scenarios.
• Data provided and delivered
• Control information
• Exception processing
• The next expected event

Event scenario - start transaction

Notation for data and control
analysis

Ellipses. data provided from or delivered to a
viewpoint

Control information enters and leaves at the top
of each box

Data leaves from the right of each box

Exceptions are shown at the bottom of each box

Name of next event is in box with thick edges

Exception description

Most methods do not include facilities for
describing exceptions

In this example, exceptions are
• Timeout. Customer fails to enter a PIN within the allowed time
limit
• Invalid card. The card is not recognised and is returned
• Stolen card. The card has been registered as stolen and is
retained by the machine

Use cases

Use-cases are a scenario based technique in the
UML which identify the actors in an interaction
and which describe the interaction itself

A set of use cases should describe all possible
interactions with the system

Sequence diagrams may be used to add detail
to use-cases by showing the sequence of event
processing in the system

Lending use-case

Library use-cases

Catalogue management

Social and organisational factors

Software systems are used in a social and
organisational context. This can influence or
even dominate the system requirements

Social and organisational factors are not a single
viewpoint but are influences on all viewpoints

Good analysts must be sensitive to these factors
but currently no systematic way to tackle their
analysis

Example

Consider a system which allows senior
management to access information without
going through middle managers
• Managerial status. Senior managers may feel that they are too
important to use a keyboard. This may limit the type of system
interface used
• Managerial responsibilities. Managers may have no
uninterrupted time where they can learn to use the system
• Organisational resistance. Middle managers who will be made
redundant may deliberately provide misleading or incomplete
information so that the system will fail

Ethnography

A social scientists spends a considerable time
observing and analysing how people actually work

People do not have to explain or articulate their
work

Social and organisational factors of importance
may be observed

Ethnographic studies have shown that work is
usually richer and more complex than suggested
by simple system models

Focused ethnography

Developed in a project studying the air traffic
control process

Combines ethnography with prototyping

Prototype development results in unanswered
questions which focus the ethnographic analysis

Problem with ethnography is that it studies
existing practices which may have some
historical basis which is no longer relevant

Ethnography and prototyping

Scope of ethnography

Requirements that are derived from the way that
people actually work rather than the way I which
process definitions suggest that they ought to
work

Requirements that are derived from cooperation
and awareness of other people’s activities


Concerned with demonstrating that the
requirements define the system that the
customer really wants

Requirements error costs are high so validation
is very important
• Fixing a requirements error after delivery may cost up to 100
times the cost of fixing an implementation error

Requirements checking

Validity. Does the system provide the functions
which best support the customer’s needs?

Consistency. Are there any requirements conflicts?

Completeness. Are all functions required by the
customer included?

Realism. Can the requirements be implemented
given available budget and technology

Verifiability. Can the requirements be checked?

techniques

Requirements reviews
• Systematic manual analysis of the requirements

Prototyping
• Using an executable model of the system to check
requirements. Covered in Chapter 8

Test-case generation
• Developing tests for requirements to check testability

Automated consistency analysis
• Checking the consistency of a structured requirements
description

Requirements reviews

Regular reviews should be held while the
requirements definition is being formulated

Both client and contractor staff should be
involved in reviews

Reviews may be formal (with completed
documents) or informal. Good communications
between developers, customers and users can
resolve problems at an early stage

Review checks

Verifiability. Is the requirement realistically
testable?

Comprehensibility. Is the requirement properly
understood?

Traceability. Is the origin of the requirement
clearly stated?

Adaptability. Can the requirement be changed
without a large impact on other requirements?

Automated consistency checking

Requirements management

Requirements management is the process of
managing changing requirements during the
requirements engineering process and system
development

Requirements are inevitably incomplete and
inconsistent
• New requirements emerge during the process as business needs
change and a better understanding of the system is developed
• Different viewpoints have different requirements and these are
often contradictory

Requirements change

The priority of requirements from different
viewpoints changes during the development
process

System customers may specify requirements
from a business perspective that conflict with
end-user requirements

The business and technical environment of the
system changes during its development

Requirements evolution

Enduring and volatile
requirements

Enduring requirements. Stable requirements
derived from the core activity of the customer
organisation. E.g. a hospital will always have
doctors, nurses, etc. May be derived from
domain models

Volatile requirements. Requirements which
change during development or when the system
is in use. In a hospital, requirements derived
from health-care policy

Classification of requirements

Mutable requirements
• Requirements that change due to the system’s environment

Emergent requirements
• Requirements that emerge as understanding of the system develops

Consequential requirements
• Requirements that result from the introduction of the computer
system

Compatibility requirements
• Requirements that depend on other systems or organisational
processes

Requirements management planning

During the requirements engineering process,
you have to plan:
• Requirements identification
» How requirements are individually identified
• A change management process
» The process followed when analysing a requirements change
• Traceability policies
» The amount of information about requirements relationships that is
maintained
• CASE tool support
» The tool support required to help manage requirements change

Traceability

Traceability is concerned with the relationships
between requirements, their sources and the
system design

Source traceability
• Links from requirements to stakeholders who proposed these
requirements

Requirements traceability
• Links between dependent requirements

Design traceability
• Links from the requirements to the design

A traceability matrix

CASE tool support

Requirements storage
• Requirements should be managed in a secure, managed data
store

Change management
• The process of change management is a workflow process
whose stages can be defined and information flow between
these stages partially automated

Traceability management
• Automated retrieval of the links between requirements

Requirements change management

Should apply to all proposed changes to the
requirements

Principal stages
• Problem analysis. Discuss requirements problem and propose
change
• Change analysis and costing. Assess effects of change on
other requirements
• Change implementation. Modify requirements document and
other documents to reflect change

Requirements change management

Key points

The requirements engineering process includes a
feasibility study, requirements elicitation and
analysis, requirements specification and
requirements management

Requirements analysis is iterative involving domain
understanding, requirements collection,
classification, structuring, prioritisation and validation

Systems have multiple stakeholders with different
requirements

Key points

Social and organisation factors influence system
requirements

Requirements validation is concerned with checks
for validity, consistency, completeness, realism
and verifiability

Business changes inevitably lead to changing
requirements

Requirements management includes planning
and change management

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