Requirements Engineering: A Roadmap
Bashar Nuseibeh Steve Easterbrook
Department of Computing Department of Computer Science
Imperial College University of Toronto
180 Queen’s Gate 6 King’s College Road
London SW7 2BZ, U.K. Toronto, Ontario M5S 3H5, Canada
Email: [email protected] Email: [email protected]
ABSTRACT
This paper presents an overview of the field of software
systems requirements engineering (RE). It describes the
main areas of RE practice, and highlights some key open
research issues for the future.
1 Introduction
The primary measure of success of a software system is the
degree to which it meets the purpose for which it was
intended. Broadly speaking, software systems requirements
engineering (RE) is the process of discovering that purpose,
by identifying stakeholders and their needs, and
documenting these in a form that is amenable to analysis,
communication, and subsequent implementation. There are
a number of inherent difficulties in this process.
Stakeholders (including paying customers, users and
developers) may be numerous and distributed. Their goals
may vary and conflict, depending on their perspectives of
the environment in which they work and the tasks they wish
to accomplish. Their goals may not be explicit or may be
difficult to articulate, and, inevitably, satisfaction of these
goals may be constrained by a variety of factors outside
their control.
In this paper we present an overview of current research in
RE, presented in terms of the main activities that constitute
the field. While these activities are described independently
and in a particular order, in practice, they are actually
interleaved, iterative, and may span the entire software
systems development life cycle. Section 2 outlines the
disciplines that provide the foundations for effective RE,
while Section 3 briefly describes the context and
background needed in order to begin the RE process.
Sections 4 through 8 describe the core RE activities:
• eliciting requirements,
• modelling and analysing requirements,
• communicating requirements,
• agreeing requirements, and
• evolving requirements.
Section 9 discusses how these different activities may be
integrated into a single development process. We conclude
with a summary of the state of the art in RE, and offer our
view of the key challenges for future RE research.
2 Foundations
Before discussing RE activities in more detail, it is worth
examining the role of RE in software and systems
engineering, and the many disciplines upon which it draws.
Zave [83] provides one of the clearest definitions of RE:
“Requirements engineering is the branch of software
engineering concerned with the real-world goals for,
functions of, and constraints on software systems. It
is also concerned with the relationship of these
factors to precise specifications of software behavior,
and to their evolution over time and across software
families.”
This definition is attractive for a number of reasons. First, it
highlights the importance of “real-world goals” that
motivate the development of a software system. These
represent the ‘why’ as well as the ‘what’ of a system.
Second, it refers to “precise specifications”. These provide
the basis for analysing requirements, validating that they
are indeed what stakeholders want, defining what designers
have to build, and verifying that they have done so correctly
upon delivery. Finally, the definition refers to
specifications’ “evolution over time and across software
families”, emphasising the reality of a changing world and
the need to reuse partial specifications, as engineers often
do in other branches of engineering.
It has been argued that requirements engineering is a
misnomer. Typical textbook definitions of engineering refer
to the creation of cost-effective solutions to practical
problems by applying scientific knowledge [74]. Therefore,
the use of the term engineering in RE serves as a reminder
that RE is an important part of an engineering process,
being the part concerned with anchoring development
activities to a real-world problem, so that the
appropriateness and cost-effectiveness of the solution can
then be analysed. It also refers to the idea that specifications
themselves need to be engineered, and RE represents a
series of engineering decisions that lead from recognition of
a problem to be solved to a detailed specification of that
problem.
Note that the focus of Zave’s definition is on software
engineering. In reality, software cannot function in isolation
from the system in which it is embedded, and hence RE has
to encompass a systems level view. We therefore prefer to
characterise RE as a branch of systems engineering [76],
whose ultimate goal is to deliver some systems behaviour to
its stakeholders. The special consideration that software
systems requirements engineering has received is largely
due to the abstract and invisible nature of software, and the
vast range and variety of problems that admit to software
solutions.
Whether viewed at the systems level or the software level,
RE is a multi-disciplinary, human-centred process. The
tools and techniques used in RE draw upon a variety of
disciplines, and the requirements engineer may be expected
to master skills from a number of different disciplines.
In the context of software development, computer science
plays a particularly important role. Theoretical computer
science provides the framework to assess the feasibility of
requirements, while practical computer science provides the
tools by which software solutions are developed. Although
software engineering still lacks a mature science of
software behaviour on which to draw, requirements
engineers need such a science in order to understand how to
specify the required behaviour of software.
Since software is a formal description, analysis of its
behaviour is amenable to formal reasoning. Logic provides
a vehicle for performing such analysis [1]. In RE, logic can
be used to improve the rigour of the analysis performed,
and to make the reasoning steps explicit. Formal description
techniques have received considerable attention in RE
research, but have not yet been widely adopted into RE
practice. Since RE must span the gap between the informal
world of stakeholder needs, and the formal world of
software behaviour, the key question over the use of formal
methods is not whether to formalise, but when to formalise
[60]. Different logics may be used to express different
aspects of a required system. For example, temporal logic
can be used to describe timing information, deontic logic to
describe permissions and obligations, and linear logic to
describe resources and their use. A further advantage of
specification languages grounded in logic is that they are
potentially amenable to automated reasoning and analysis.
In the systems engineering context, an understanding and
application of systems theory and practice is also relevant
to RE [76]. This includes work on characterising systems,
identifying their boundaries and managing their
development life cycle [12; 81]. RE also encompasses work
on systems analysis, traditionally found in the information
systems world [68].
The context in which RE takes place is usually a human
activity system, and the problem owners are people.
Engagement in an RE process presupposes that some new
computer-based system could be useful, but such a system
will change the activities that it supports. Therefore, RE
needs to be sensitive to how people perceive and
understand the world around them, how they interact, and
how the sociology of the workplace affects their actions.
RE draws on the cognitive and social sciences to provide
both theoretical grounding and practical techniques for
eliciting and modelling requirements:
• Cognitive psychology provides an understanding of the
difficulties people may have in describing their needs [62]. For
example, problem domain experts often have large amounts of
tacit knowledge that is not amenable to introspection; hence
their answers to questions posed by requirements analysts may
not match their behaviour. Also, the requirements engineer may
need to model users’ understanding of software user interfaces,
rather than relying solely on implementers’ preferences.
• Anthropology provides a methodological approach to observing
human activities that helps to develop a richer understanding of
how computer systems may help or hinder those activities [29].
For example, the techniques of ethnomethodology [30] have
been applied in RE to develop observational techniques for
analysing collaborative work and team interaction.
• Sociology provides an understanding of the political and
cultural changes caused by computerisation. Introduction of a
new computer system changes the nature of the work carried
out within an organisation, may affect the structure and
communication paths within that organisation, and may even
change the original needs that it was built to satisfy [46]. A
requirements gathering exercise can therefore become
politicised. Approaches to RE that address this issue include the
“Scandanavian” approach, which aims to involve in the
requirements definition process those most affected by the
outcomes [36].
• Linguistics is important because RE is largely about
communication. Linguistic analyses have changed the way in
which the English language is used in specifications, for
instance to avoid ambiguity and to improve understandability.
Tools from linguistics can also be used in requirements
elicitation, for instance to analyse communication patterns
within an organisation [11].
Finally, there is an important philosophical element in RE.
RE is concerned with interpreting and understanding
stakeholder terminology, concepts, viewpoints and goals.
Hence, RE must concern itself with an understanding of
beliefs of stakeholders (epistemology), the question of what
is observable in the world (phenomenology), and the
question of what can be agreed on as objectively true
(ontology). Such issues become important whenever one
wishes to talk about validating requirements, especially
where stakeholders may have divergent goals and
incompatible belief systems. They also become important
when selecting a modelling technique, because the choice
of technique affects the set of phenomena that can be
modelled, and may even restrict what a requirements
engineer is capable of observing.
3 Context and Groundwork
RE is often regarded as a front-end activity in the software
systems development process. This is generally true,
although it is usually also the case that requirements change
during development and evolve after a system has been in
operation for some time. Therefore, RE plays an important
role in the management of change in software development.
Nevertheless, the bulk of the effort of RE does occur early
in the lifetime of a project, motivated by the evidence that
requirements errors, such as misunderstood or omitted
requirements, are more expensive to fix later in project
lifecycles [8; 56].
Before a project can be started, some preparation is needed.
Finkelstein [24] categorises such preparation as context and
groundwork. In the past, it was often the case that RE
methods assumed that RE was performed for a specific
customer, who could sign off a requirements specification.
However, RE is actually performed in a variety of contexts,
including market-driven product development and
development for a specific customer with the eventual
intention of developing a broader market. The type of
product will also affect the choice of method: RE for
information systems is very different from RE for
embedded control systems, which is different again from
RE for generic services such as networking and operating
systems.
For groundwork, some assessment of a project’s feasibility
and associated risks needs to be undertaken, and RE plays a
crucial role in making such an assessment. It is often
possible to estimate project costs, schedules and technical
feasibility from precise specifications of requirements. It is
also important that conflicts between high-level goals of an
envisioned system surface early, in order to establish a
system’s concept of operation and boundaries. Of course,
risk should be re-evaluated regularly throughout the
development lifetime of a system [58], since changes in the
environment can change the associated development risks.
Groundwork also includes the identification of a suitable
process for RE, and the selection of methods and techniques
for the various RE activities. We use the term process here
to denote an instance of a process model, which is an
abstract description of how to conduct a collection of
activities, describing the behaviour of one or more agents
and their management of resources. A technique prescribes
how to perform one particular activity - and, if necessary,
how to describe the product of that activity in a particular
notation. A method provides a prescription for how to
perform a collection of activities, focusing on how a related
set of techniques can be integrated, and providing guidance
on their use.
4 Eliciting Requirements
The elicitation of requirements is perhaps the activity most
often regarded as the first step in the RE process. The term
“elicitation” is preferred to “capture”, to avoid the
suggestion that requirements are out there to be collected
simply by asking the right questions [29]. Information
gathered during requirements elicitation often has to be
interpreted, analysed, modelled and validated before the
requirements engineer can feel confident that a complete
enough set of requirements of a system have been collected.
Therefore, requirements elicitation is closely related to
other RE activities – to a great extent, the elicitation
technique used is driven by the choice of modelling
scheme, and vice versa: many modelling schemes imply the
use of particular kinds of elicitation techniques.
4.1 Requirements to elicit
One of the most important goals of elicitation is to find out
what problem needs to be solved, and hence identify system
boundaries. These boundaries define, at a high level, where
the final delivered system will fit into the current
operational environment. Identifying and agreeing a
system’s boundaries affects all subsequent elicitation
efforts. The identification of stakeholders and user classes,
of goals and tasks, and of scenarios and use cases all
depend on how the boundaries are chosen.
Identifying stakeholders – individuals or organisations who
stand to gain or lose from the success or failure of a system
– is also critical. Stakeholders include customers or clients
(who pay for the system), developers (who design,
construct and maintain the system), and users (who interact
with the system to get their work done). For interactive
systems, users play a central role in the elicitation process,
as usability can only be defined in terms of the target user
population. Users themselves are not homogeneous, and
part of the elicitation process is to identify the needs of
different user classes, such as novice users, expert users,
occasional users, disabled users, and so on [73].
Goals denote the objectives a system must meet. Eliciting
high level goals early in the development process is crucial.
However, goal-oriented requirements elicitation [15] is an
activity that continues as development proceeds, as high-
level goals (such as business goals) are refined into lower-
level goals (such as technical goals that are eventually
operationalised in a system). Eliciting goals focuses the
requirements engineer on the problem domain and the
needs of the stakeholders, rather than on possible solutions
to those problems.
It is often the case that users find it difficult to articulate
their requirements. To this end, a requirements engineer can
resort to eliciting information about the tasks users
currently perform and those that they might want to
perform [42]. These tasks can often be represented in use
cases that can be used to describe the outwardly visible
requirements of systems [72]. More specifically, the
requirements engineer may choose a particular path through
a use case, a scenario, in order to better understand some
aspect of using a system [41].
4.2 Elicitation techniques
The choice of elicitation technique depends on the time and
resources available to the requirements engineer, and of
course, the kind of information that needs to be elicited. We
distinguish a number of classes of elicitation technique:
• Traditional techniques include a broad class of generic data
gathering techniques. These include the use of questionnaires
and surveys, interviews, and analysis of existing documentation
such as organisational charts, process models or standards, and
user or other manuals of existing systems.
• Group elicitation techniques aim to foster stakeholder
agreement and buy-in, while exploiting team dynamics to elicit
a richer understanding of needs. They include brainstorming
and focus groups, as well as RAD/JAD workshops (using
consensus-building workshops with an unbiased facilitator)
[52].
• Prototyping has been used for elicitation where there is a great
deal of uncertainty about the requirements, or where early
feedback from stakeholders is needed [17]. Prototyping can also
be readily combined with other techniques, for instance by
using a prototype to provoke discussion in a group elicitation
technique, or as the basis for a questionnaire or think-aloud
protocol.
• Model-driven techniques provide a specific model of the type of
information to be gathered, and use this model to drive the
elicitation process. These include goal-based methods, such as
KAOS [79] and I* [14], and scenario-based methods such as
CREWS [51].
• Cognitive techniques include a series of techniques originally
developed for knowledge acquisition for knowledge-based
systems [75]. Such techniques include protocol analysis (in
which an expert thinks aloud while performing a task, to
provide the observer with insights into the cognitive processes
used to perform the task), laddering (using probes to elicit
structure and content of stakeholder knowledge), card sorting
(asking stakeholders to sort cards in groups, each of which has
name of some domain entity), repertory grids (constructing an
attribute matrix for entities, by asking stakeholders for attributes
applicable to entities and values for cells in each entity).
• Contextual techniques emerged in the 1990’s as an alternative
to both traditional and cognitive techniques [30]. These include
the use of ethnographic techniques such as participant
observation. They also include ethnomethodogy and
conversation analysis, both of which apply fine grained analysis
to identify patterns in conversation and interaction [80].
To some extent, there is a fundamental methodological
disagreement between the proponents of contextual
techniques on the one hand, and the traditional and
cognitive techniques on the other. Contextual approaches
are based on the premise that local context is vital for
understanding social and organisational behaviour, and the
observer must be immersed in this local context in order to
experience how participants create their own social
structures. The emergence of contextual techniques in RE
in the early 1990’s paralleled their introduction as part of a
revolution in cognitive science and human-computer
interaction, where they reflected a blistering attack on the
attempt to build disembodied models of cognition [57]. In
their extreme forms, the two sides are incompatible:
traditional and cognitive approaches are based on the use of
abstracted models that are independent of context, whilst
the contextualists insist that context is paramount, and
completely resist any attempt to build generalisable models
of the phenomena they observe. However, it does seem that
the advantages of these alternative approaches are
complementary, and recent work has focussed on the
question of whether an integration is possible [63; 80].
4.3 The elicitation process
With a plethora of elicitation techniques available to the
requirements engineer, some guidance on their use is
needed. Methods provide one way of delivering such
guidance. Each method itself has its strengths and
weaknesses, and is normally best suited for use in particular
application domains. For example, the Inquiry Cycle [64]
and CREWS [51] provide alternative methods for eliciting
requirements using use cases and scenarios.
Of course, in some circumstances a full-blown method may
be neither required nor necessary. Instead, the requirements
engineer needs simply to select the appropriate technique or
techniques most suitable for the elicitation process in hand.
In such situations, technique-selection guidance is more
appropriate than a rigid method [52].
5 Modelling and Analysing Requirements
Modelling – the construction of abstract descriptions that
are amenable to interpretation – is a fundamental activity in
RE. So much so that a number of RE textbooks (e.g., [18;
81]) focus almost entirely on modelling methods and their
associated analysis techniques. Models can be used to
represent a whole range of products of the RE process.
Moreover, many modelling approaches are used as
elicitation tools, where the modelling notation and partial
models produced are used as drivers to prompt further
information gathering.
The key question to ask for any modelling approach is
“what is it good for?”, and the answer should always be in
terms of the kind of analysis and reasoning it offers. We
suggest below some general categories of RE modelling
approaches, and give some example techniques under each
category. We then suggest some analysis techniques that
can be used to generate useful information from the models
produced.
5.1 Enterprise Modelling
The context of most RE activities and software systems is
an organisation in which development takes place or in
which a system will operate. Enterprise modelling and
analysis deals with understanding an organisation’s
structure; the business rules that affect its operation; the
goals, tasks and responsibilities of its constituent members;
and the data that it needs, generates and manipulates.
Enterprise modelling is often used to capture the purpose of
a system, by describing the behaviour of the organisation in
which that system will operate [47]. This behaviour can be
expressed in terms of organisational objectives or goals and
associated tasks and resources [82]. Others prefer to model
an enterprise in terms of its business rules, workflows and
the services that it will provide [33].
Modelling goals is particularly useful in RE. High-level
business goals can be refined repeatedly as part of the
elicitation process, leading to requirements that can then be
operationalised [15].
5.2 Data Modelling.
Large computer-based systems, especially information
systems use and generate large volumes of information.
This information needs to be understood, manipulated and
managed. Careful decisions need to be made about what
information the system will need to represent, and how the
information held by the system corresponds to the real
world phenomena being represented. Data modelling
provides the opportunity to address these issues in RE.
Traditionally, Entity-Relationship-Attribute (ERA)
modelling is used for this type of modelling and analysis.
However, object-oriented modelling, using class and object
hierarchies, are increasingly supplanting ERA techniques.
5.3 Behavioural Modelling
Modelling requirements often involves modelling the
dynamic or functional behaviour of stakeholders and
systems, both existing and required. The distinction
between modelling an existing system, and modelling a
future system is an important one, and is often blurred by
the use of the same modelling techniques for both. Early
structured analysis methods suggested that one should start
by modelling how the work is currently carried out (the
current physical system), analyse this to determine the
essential functionality (the current logical system), and
finally build of model of how the new system ought to
operate (the new logical system). Explicitly constructing all
three models may be overkill, but it is nevertheless useful to
distinguish which of these is being modelled.
A wide range of modelling methods are available, from
structured to object-oriented methods, and from soft to
formal methods. These methods provide different levels of
precision and are amenable to different kinds of analysis.
Formal methods (for example, based on Z) can be difficult
to construct, but are also amenable to automated analysis
[71]. On the other hand, soft methods provide rich
representations [63] that non-technical stakeholders find
appealing, but are often difficult to check automatically.
5.4 Domain Modelling.
A significant proportion of the RE process is about
developing domain descriptions [40]. A model of the
domain provides an abstract description of the world in
which an envisioned system will operate. Building explicit
domain models provides two key advantages: they permit
detailed reasoning about (and therefore validation of) what
is assumed about the domain, and they provide
opportunities for requirements reuse within a domain.
Domain-specific models have also been shown to be
essential for building automated tools, because they permit
tractable reasoning over a closed world model of the system
interacting with its environment; e.g., [67].
5.5 Modelling Non-Functional Requirements (NFRs)
Non-functional requirements (also known as quality
requirements) are generally more difficult to express in a
measurable way, making them more difficult to analyse. In
particular, NFRs tend to be properties of a system as a
whole, and hence cannot be verified for individual
components. Recent work by both researchers [14] and
practitioners [69] has investigated how to model NFRs and
to express them in a form that is measurable or testable.
There also is a growing body of research concerned with
particular kinds of NFRs, such as safety [49; 55], security
[13], reliability [19], and usability [42].
5.6 Analysing Requirements Models
A primary benefit of modelling requirements is the
opportunity this provides for analysing them. Analysis
techniques that have been investigated in RE include
requirements animation [32], automated reasoning (e.g.,
analogical and case-based reasoning [54] and knowledge-
based critiquing [23]), consistency checking (e.g., model
checking [37]), and a variety of techniques for validation
and verification (V&V) that we discuss in Section 7.
6 Communicating Requirements
RE is not only a process of discovering and specifying
requirements, it is also a process of facilitating effective
communication of these requirements among different
stakeholders. The way in which requirements are
documented plays an important role in ensuring that they
can be read, analysed, (re-)written, and validated.
The focus of requirements documentation research is often
on specification languages and notations, with a variety of
formal, semi-formal and informal languages suggested for
this purpose [18; 81]. From logic [3] to natural language
[2], different languages have been shown to have different
expressive and reasoning capabilities.
What is increasingly recognised as crucial, however, is
requirements management – the ability, not only to write
requirements but also to do so in a form that is readable and
traceable by many, in order to manage their evolution over
time. One attempt to achieve readability has been the
development of a variety of documentation standards that
provide guidelines for structuring requirements documents
[78]. However, some authors, such as Kovitz [44], argue
that standards or templates cannot in themselves provide a
general structuring mechanism for requirements. Rather, he
argues that the structure has to be developed for the
particular context or problem in hand. Nevertheless, it is
often the case that projects with rigid contractual constraints
demand conformance to standards. Kovitz suggests some
heuristics for focusing on the small details of writing
requirements documentation, which can improve the quality
of the requirements documentation, regardless of the format
in which requirements are expressed.
Requirements traceability (RT) is another major factor that
determines how easy it is to read, navigate, query and
change requirements documentation. Gotel [31] defines
requirements traceability as “the ability to describe and
follow the life of a requirement in both forwards and
backwards direction (i.e., from its origins, through its
development and specification, to its subsequent
deployment and use, and through all periods of on-going
refinement and iteration in any of these phases)”. RT lies at
the heart of requirements management practice in that it can
provide a rationale for requirements and is the basis for
tools that analyse the consequences and impact of change.
Providing RT in requirements documentation is a means of
achieving integrity and completeness of that
documentation, and has an important role to play in
managing change, which will be discussed in Section 8.
7 Agreeing Requirements
As requirements are elicited and modelled, maintaining
agreement with all stakeholders can be a problem,
especially where stakeholders have divergent goals. Recall
that validation is the process of establishing that the
requirements and models elicited provide an accurate
account of stakeholder requirements. Explicitly describing
the requirements is a necessary precondition not only for
validating requirements, but also for resolving conflicts
between stakeholders.
Techniques such as inspection and formal analysis tend to
concentrate on the coherence of the requirements
descriptions: are they consistent, and are they structurally
complete? The formal method SCR [35] illustrates this
approach. The SCR tool provides automated checking that
the formal model is syntactically consistent and complete.
In contrast, techniques such as prototyping, specification
animation, and the use of scenarios are geared towards
testing a correspondence with the real world problem. For
example, have all the aspects of the problem that the
stakeholders regard as important been covered?
Requirements validation is difficult for two reasons. The
first reason is philosophical in nature, and concerns the
question of truth and what is knowable. The second reason
is social, and concerns the difficulty of reaching agreement
among different stakeholders with conflicting goals. We
will briefly examine each of these in turn.
We can compare the problem of validating requirements
with the problem of validating scientific knowledge. Many
requirements engineers adopt a logical positivist approach –
essentially the belief that there is an objective world that
can be modelled by building a consistent body of
knowledge grounded in empirical observation. In RE, this
view says that the requirements describe some objective
problem that exists in the world, and that validation is the
task of making sufficient empirical observations to check
that this problem has been captured correctly. Popper’s
observations on the limitations of empirical observation
apply here : that scientific theories can never be proved
correct through observation, they can only be refuted [61].
For RE, this view suggests that validation should adopt the
same stance that software testers take: it should devise
experiments to attempt to refute the current statement of
requirements. Jackson [39] argues that descriptions used in
RE should be refutable – those that are not refutable are
vague, and should only be treated as rough sketches.
Logical positivism was severely criticised in the latter part
of the twentieth century [5]. For example, Kuhn [45]
observed that science tends to move through paradigm
shifts, where the dominant paradigm determines the nature
of current scientific theories. This leads to the realisation
that observation is not value-free, rather it is theory-driven,
and is biased by the current paradigm. For requirements
engineers, the methods and tools they use dominate the way
that they see and describe problems. In the extreme case,
this shifts the problem of validating requirements
statements to a problem of convincing stakeholders that the
chosen representation for requirements models is
appropriate. Jackson captures this perspective through his
identification of problem frames [39]. If stakeholders do not
agree with the choice of problem frame, it is unlikely that
they will ever agree with any statement of the requirements.
Ethnomethodologists attempt to avoid the problem
altogether, by refusing to impose modelling constructs on
the stakeholders [30]. By discarding traditional problem
analysis tools, they seek to apply value-free observations of
stakeholder activities, and therefore circumvent the
requirements validation issue altogether.
The second essential difficulty in requirements validation
centres on the problem of disagreement among
stakeholders. Recent approaches that explicitly model
stakeholders’ goal hierarchies make the problem clear:
stakeholders may have goals that conflict with one another
[79]. Requirements negotiation attempts to resolve conflicts
between stakeholders without necessarily weakening
satisfaction of each stakeholder’s goals. Early approaches to
requirements negotiation focused on modelling each
stakeholder’s contribution separately rather than trying to
fit their contributions into a single consistent model [20]
and on the importance of establishing common ground [70].
Boehm introduced the win-win approach [7] in which the
win conditions for each stakeholder are identified, and the
software process is managed and measured to ensure that
all the win conditions are satisfied, through negotiation
among the stakeholders.
The theory underlying these negotiation models is the same
in each case: identify the most important goals of each
participant, and ensure these goals are met. This approach is
used in other RE techniques to promote agreement, without
necessarily making the goals explicit [43]. For example, in
Quality Function Deployment (QFD) [34], matrices are
constructed to compare functional requirements with one
another and rate their importance, but without explicitly
identifying stakeholder goals.
We have described some essential difficulties in agreeing
and validating requirements. These difficulties are
compounded by a number of contextual issues, including
contractual and procurement issues, and the fact that the
political and social milieu in which the introduction of a
new computer system changes the nature of work and the
organisations [46].
8 Evolving Requirements
Successful software systems always evolve as the
environment in which these systems operate changes and
stakeholder requirements change. Therefore managing
change is a fundamental activity in RE [9].
Changes to requirements documentation need to be
managed. Minimally, this involves providing techniques
and tools for configuration management and version control
[22], and exploiting traceability links to monitor and control
the impact of changes in different parts of the
documentation. Typical changes to requirements
specifications include adding or deleting requirements, and
fixing errors. Requirements are added in response to
changing stakeholder needs, or because they were missed in
the initial analysis. Requirements are deleted usually only
during development, to forestall cost and schedule
overruns, a practice known as requirements scrubbing [6].
In any case, managing inconsistency [28] in requirements
specifications as they evolve is a major challenge.
Inconsistencies arise both as a result of mistakes, and
because of conflicts between requirements. Each
inconsistency implies that some action is needed, to identify
the cause and seek a resolution [38].
While traceability links help to scope the possible impact of
change, they do not support automated reasoning about
change, because the links carry little semantic information.
One attempt to address this problem is the ViewPoints
framework, in which consistency relationships between
chunks (‘viewpoints’) of a specification are expressed
operationally, so that automated support for propagation of
change becomes possible [21].
Managing changing requirements is not only a process of
managing documentation, it is also a process of recognising
change through continued requirements elicitation, re-
evaluation of risk, and evaluation of systems in their
operational environment. In software engineering, it has
been demonstrated that focusing change on program code
leads to a loss of structure and maintainability [4]. Thus,
each proposed change needs to be evaluated in terms of
existing requirements and architecture so that the trade-off
between the cost and benefit of making a change can be
assessed.
Finally, the development of software system product
families has become an increasingly important form of
development activity. For this purpose, there is a need to
develop a range of software products that share similar
requirements and architectural characteristics, yet differ in
certain key requirements. The process of identifying core
requirements in order to develop architectures that are (a)
stable in the presence of change, and (b) flexible enough to
be customised and adapted to changing requirements, is one
of the key research issues in software engineering [27].
9 Integrated Requirements Engineering
RE is a multi-disciplinary activity, deploying a variety of
techniques and tools at different stages of development and
for different kinds of application domains. Methods provide
a systematic approach to combining different techniques
and notations, and method engineering [10] plays an
important role in designing the RE process to be deployed
for a particular problem or domain. Methods provide
heuristics and guidelines for the requirements engineer to
deploy the appropriate notation or modelling technique at
different stages of the process.
A variety of approaches have been suggested to manage
and integrate different RE activities and products. Jacskon,
for example, uses problem frames to structure different
kinds of elementary and composite problems [39]. His
argument is that identifying well-understood problems
offers the possibility of selecting corresponding,
appropriate, well-understood, solutions.
An alternative approach to organising, selecting and
tailoring multiple methods is through the use of multiple
perspectives or views of requirements [16; 26]. This
approach can facilitate requirements partitioning and
subsequent modelling and analysis. For example, a
viewpoint can be treated as an encapsulation of an
individual technique, with a defined notation, a set of
actions that can be performed on that notation, and a set of
rules for consistency relationships with other viewpoints. In
this way, the design and integration of multiple methods
can be supported as a process of creating and tailoring
viewpoint templates [59].
Finally, to enable effective management of an integrated
RE process, automated tool support is essential.
Requirements management tools, such as DOORS [65],
Requisite Pro [66], Cradle [77], and others, provide
capabilities for documenting requirements, managing their
change, and integrating them in different ways depending
on project needs.
10 A Requirements Engineering Roadmap
This paper has set out a roadmap, and we feel that no
roadmap is complete without a big arrow labelled “you are
here”
1
. By way of providing such a marker, we will
summarise the important developments in RE during the
last decade, and give our predictions about what will be
important in RE research for the coming decade.
The 1990’s saw several important and radical shifts in the
understanding of RE. By the early 1990’s, RE had emerged
as a field of study in its own right, as witnessed by the
emergence of two series of international meetings – the
IEEE sponsored conference and symposium, held in
alternating years – and the establishment of an international
journal published by Springer [48]. By the late 1990’s, the
field had grown enough to support a large number of
additional smaller meetings and workshops in various
countries.
During this period, we can discern the emergence of three
radical new ideas that challenged and overturned the
orthodox views of RE. These three ideas are closely
interconnected:
• The idea that modelling and analysis cannot be performed
adequately in isolation from the organisational and social
context in which any new system will have to operate. This
view emphasised the use of contextualised enquiry techniques,
including ethnomethodology and participant observation [29;
63].
• The notion that RE should not focus on specifying the
functionality of a new system, but instead should concentrate on
modelling indicative and optative properties of the environment
[84]
2
. Only by describing the environment, and expressing what
the new system must achieve in that environment, we can
capture the system’s purpose, and reason about whether a given
design will meet that purpose. This notion has been
accompanied by a shift in emphasis away from modelling
information flow and system state, and towards modelling
stakeholders’ goals [15] and scenarios that illustrate how goals
are (or can be) achieved [51].
• The idea that the attempt to build consistent and complete
requirements models is futile, and that RE has to take seriously
the need to analyse and resolve conflicting requirements, to
support stakeholder negotiation, and to reason with models that
contain inconsistencies [28].
Having identified these trends from the past decade, we
now turn our attention to the future. We believe the
following represent major challenges for RE in the years
ahead:
1. Development of new techniques for formally modelling and
analysing properties of the environment, as opposed to the
1 Sadly, this is an infeasible requirement for most portable road maps!
2 Indicative descriptions express things that are currently true (and will be
true irrespective of the introduction of a new system), while optative
descriptions express the things that we wish the new system to make
true [84].
behaviour of the software. Such techniques must take into
account the need to deal with inconsistent, incomplete, and
evolving models. We expect such approaches will better support
areas where RE has been weak in the past, including the
specification of the expectations that a software component has
of its environment. This facilitates migration of software
components to different software and hardware environments,
and the adaptation of products into product families.
2. Bridging the gap between requirements elicitation approaches
based on contextual enquiry and more formal specification and
analysis techniques. Contextual approaches, such as those based
on ethnographic techniques, provide a rich understanding of the
organisational context for a new software system, but do not
map well onto existing techniques for formally modelling the
current and desired properties of problem domains. This
includes the incorporation of a wider variety of media, such as
video and audio, into behavioural modelling techniques.
3. Richer models for capturing and analysing non-functional
requirements. These are also known as the “ilities” and have
defied a clear characterisation for decades [50].
4. Better understanding of the impact of software architectural
choices on the prioritisation and evolution of requirements.
While work in software architectures has concentrated on how
to express software architectures and reason about their
behavioural properties, there is still an open question about how
to analyse what impact a particular architectural choice has on
the ability to satisfy current and future requirements, and
variations in requirements across a product family [27].
5. Reuse of requirements models. We expect that in many domains
of application, we will see the development of reference models
for specifying requirements, so that the effort of developing
requirements models from scratch is reduced. This will help
move many software projects from being creative design to
being normal design [50], and will facilitate the selection of
commercial off-the-shelf (COTS) software [25; 53].
6. Multidisciplinary training for requirements practitioners. In this
paper, we have used the term “requirements engineer” to refer
to any development participant who applies the techniques
described in the paper to elicit, specify, and analyse
requirements. While many organisations do not even employ
such a person, the skills that such a person or group should
possess is a matter of critical importance. The requirements
engineer must possess both the social skills to interact with a
variety of stakeholders, including potentially non-technical
customers, and the technical skills to interact with systems
designers and developers.
Many delivered systems do not meet their customers’
requirements due, at least partly, to ineffective RE. RE is
often treated as a time-consuming, bureaucratic and
contractual process. This attitude is changing as RE is
increasingly recognised as a critically important activity in
any systems engineering process. The novelty of many
software applications, the speed with which they need to be
developed, and the degree to which they are expected to
change, all play a role in determining how the systems
development process should be conducted. The demand for
better, faster, and more usable software systems will
continue, and RE will therefore continue to evolve in order
to deal with different development scenarios. We believe
that effective RE will continue to play a key role in
determining the success or failure of projects, and in
determining the quality of systems that are delivered.
Acknowledgements. Thanks to Dan Berry, Anthony Finkelstein,
Olly Gotel, Sophia Guerra, and Axel van Lamsweerde for their
feedback on earlier drafts of this paper. This work was partially
funded by the UK EPSRC projects MISE (GR/L 55964) and
VOICI (GR/M 38582).
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