Requirements Engineering
Processes
Slide 1
Something to Think About …
The indispensable first step to getting the
things you want out of life: decide what
you want.
—Ben Stein
Slide 2
Objectives
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To describe the principal requirements
engineering activities and their relationships
To introduce techniques for requirements
elicitation and analysis
To describe requirements validation and the
role of requirements reviews
Slide 3
Requirements engineering processes
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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
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Requirements elicitation;
Requirements analysis;
Requirements validation;
Requirements management.
Slide 4
Feasibility studies
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A feasibility study decides whether or not the
proposed system is worthwhile.
A short focused study that checks
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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.
Slide 5
Feasibility study implementation
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Based on information assessment (what is required),
information collection and report writing.
Questions for people in the organisation
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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?
Slide 6
Elicitation and analysis
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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.
Slide 7
Problems of requirements analysis
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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.
Slide 8
Process activities
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Requirements discovery
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Requirements classification and organisation
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Groups related requirements and organises them into
coherent clusters.
Prioritisation and negotiation
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Interacting with stakeholders to discover their
requirements. Domain requirements are also discovered
at this stage.
Prioritising requirements and resolving requirements
conflicts.
Requirements documentation
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Requirements are documented and input into the next
round of the spiral.
Slide 9
Requirements discovery
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The process of gathering information about
the proposed and existing systems and
distilling the user and system requirements
from this information.
Sources of information include
documentation, system stakeholders and the
specifications of similar systems.
Slide 10
ATM stakeholders
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Bank customers
Representatives of other banks
Bank managers
Counter staff
Database administrators
Security managers
Marketing department
Hardware and software maintenance engineers
Banking regulators
Slide 11
Viewpoints
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Viewpoints are a way of structuring the
requirements to represent the perspectives
of different stakeholders. Stakeholders may
be classified under different viewpoints.
This multi-perspective analysis is important
as there is no single correct way to analyse
system requirements.
Slide 12
Types of viewpoint
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Interactor viewpoints
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Indirect viewpoints
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People or other systems that interact directly with the
system. In an ATM, the customer’s and the account
database are interactor VPs.
Stakeholders who do not use the system themselves but
who influence the requirements. In an ATM, management
and security staff are indirect viewpoints.
Domain viewpoints
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Domain characteristics and constraints that influence the
requirements. In an ATM, an example would be standards
for inter-bank communications.
Slide 13
Viewpoint identification
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Identify viewpoints using
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Providers and receivers of system services;
Systems that interact directly with the system
being specified;
Regulations and standards;
Sources of business and non-functional
requirements.
Engineers who have to develop and maintain
the system;
Marketing and other business viewpoints.
Slide 14
LIBSYS viewpoint hierarchy
All VPs
In direct
Library
manager
Finan ce
Studen ts
In teractor
Article
providers
Staff
Users
Ex ternal
Slide 15
Do main
Library
staff
Sy stem
managers
UI
standards
Catalo guers
Classification
sy stem
Interviewing
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In formal or informal interviewing, the RE
team puts questions to stakeholders about
the system that they use and the system to
be developed.
There are two types of interview
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Closed interviews where a pre-defined set of
questions are answered.
Open interviews where there is no pre-defined
agenda and a range of issues are explored with
stakeholders.
Slide 16
Interviews in practice
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Normally a mix of closed and open-ended
interviewing.
Interviews are good for getting an overall
understanding of what stakeholders do and how
they might interact with the system.
Several issues need to be addressed before
conducting interviews for understanding domain
requirements
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Requirements engineers cannot understand specific
domain terminology;
Some domain knowledge is so familiar that people find it
hard to articulate or think that it isn’t worth articulating.
Slide 17
Effective interviewers
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Interviewers should be open-minded, willing
to listen to stakeholders and should not have
pre-conceived ideas about the requirements.
They should prompt the interviewee with a
question or a proposal and should not simply
expect them to respond to a question such
as ‘what do you want’.
Slide 18
Use Cases - Background
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Alistair Cockburn claims to have
encountered over 18 different definitions of
use case, given by different, each expert,
teachers and consultants.
They differed along 4 dimensions:
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Purpose
Contents
Plurality
Structure.
Slide 19
Use Cases - Purpose
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Is the purpose of use cases to gather user
stories, or build requirements?
Dimension Values:
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Stories
Requirements
Slide 20
Use Cases - Contents
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Are the contents of the use case required to
be consistent, or can they be selfcontradicting?
If consistent, are they in plain prose or are
they in a formal notation?
Dimension Values:
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Contradicting
Consistent prose
Formal content
Slide 21
Use Cases - Plurality
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Is a use case really just another name for a
scenario, or does a use case contain more
than one scenario?
Dimension Values:
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One
Multiple
Slide 22
Use Cases - Structure
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Does a collection of use cases have a formal
structure, an informal structure, or do they
form an unstructured collection?
Dimension Values:
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Unstructured
Semi-formal
Formal structure
Slide 23
Cockburn in Use Case Space
Purpose = requirements
Contents = consistent prose
Plurality = multiple scenarios per use case
Structure = semi-formal
Slide 24
Actors
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External Actors
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Internal Actors
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May be the system in design, a subsystem or an object.
The system in design consists of subsystems, which
consist of objects.
Actors have behavior(s).
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A person, a group of people or a system of any kind.
The top-level behavior is a responsibility.
A responsibility contains goals, which contain
actions.
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An action triggers an interaction.
The interaction is one actor’s goal calling upon another
actors (or its own) responsibility.
Slide 25
Scenario
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A sequence of interactions happening under
certain conditions, to achieve the initiating
actor’s goal, and having a particular result
with respect to that goal.
The interactions start from the triggering
action and continue until the goal is delivered
or abandoned, and the system completes
whatever responsibilities it has with respect
to the interaction.
AKA: Use Case Instance
Slide 26
Use Case
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A collection of possible scenarios between
the system under discussion (SuD) and
initiating actor.
Characterized by the goal the initiating actor
has toward the SuD’s declared
responsibilities.
Shows how the initiating actor’s goal might
be delivered or might fail.
Slide 27
Scenario/Use Case Bounds
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All the interactions relate to the same goal.
Interactions start at the triggering event and
end when the goal is delivered or
abandoned, and the SuD completes its
responsibilities with respect to the
interaction.
Slide 28
Use Case
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A set of use-case instances, where each instance is
a sequence of actions a system performs that yields
an observable result of value to a particular actor
A key attitude in use case work is to focus on the
question
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“How can using the system provide observable value to
the user, or fulfill their goals?”
As opposed to thinking of system requirements in terms
of a “laundry list” of features or functions.
Slide 29
LIBSYS use cases
Slide 30
LIBSYS Use Case (1)
Initial assumption: The user has logged on to the LIBSYS system and has located the journal containing
the copy of the article.
Normal: The user selects the article to be copied. He or she is then prompted by the system to ei ther
provide subscriber information for the journal or to indicate how they will pay for the article. Alternative
payment me thods are by credit card or by quoting an organisational account number.
The user is then asked to fill in a copyright form that ma intains details of the transaction and they then
submit this to the LIBSYS system.
The copyright form is c hecked and, if OK, the PDF version of the article is d ownloaded to the LIBSYS
working area on the userÕscomputer and the user is informed that it is available. The user is asked to select
a printer and a copy of the article is printed. If the article has been flagged as Ōprint-onlyÕit i s deleted from
the userÕs system o nce the user has confirmed that printing is complete.
Slide 31
LIBSYS Use Case (2)
What can go wrong: The user may fail to fill in the copyright form correctly. In this case, the form should
be re-presented to the user for correction. If the resubmitted form is s till incorrect then the userÕsrequest
for the article is rejected.
The payment ma y be rejected by the system. The userÕs er quest for the article is rejected.
The article download may fail. Retry until successful or the user terminates the session.
It may not be possible to print the article. If t he article is not flagged as Ōprint-onlyÕthen it is held in the
LIBSYS workspace. Otherwise, the article is d eleted and the userÕs account credited with the cost of the
article.
Other activities: Simultaneous downloads of other articles.
System state on completion: User is logged on. The downloaded article has been deleted from LIBSYS
workspace if it has been flagged as print-only.
Slide 32
Requirements validation
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Concerned with demonstrating that the
requirements define the system that the
customer really wants.
Requirements error costs are high so
validation is very important
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Fixing a requirements error after delivery may
cost up to 100 times the cost of fixing an
implementation error.
Slide 33
Requirements checking
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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?
Slide 34
Requirements validation techniques
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Requirements reviews
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Prototyping
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Systematic manual analysis of the
requirements.
Using an executable model of the system to
check requirements.
Test-case generation
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Developing tests for requirements to check
testability.
Slide 35
Requirements reviews
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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.
Slide 36
Review checks
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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?
Slide 37
Requirements management
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Requirements management is the process of
managing changing requirements during the
requirements engineering process and system
development.
Requirements are inevitably incomplete and
inconsistent
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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.
Slide 38
Requirements change
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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.
Slide 39
Enduring and volatile requirements
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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
Slide 40
Requirements classification
Requirement
Type
Description
Mutable
requirements
Requirements that change because of changes to the environme nt in which the
organisation is operating. For example, in hospital systems , the funding of patient
care ma y change and thus require different treatment information to be collected.
Emergent
requirements
Requirements that emerge as the customer's understanding of the system develops
during the system development. The design process may reveal new emergent
requirements.
Consequential
requirements
Requirements that result from the introduction of the comp uter system. Introducing
the computer system may change the organisations processes and open up new ways
of working which generate new system requirements
Compatibility
requirements
Requirements that depend on the particular systems or b usiness processes within an
organisation. As these change, the comp atibility requirements on the commissioned
or delivered system m ay also have to evolve.
Slide 41
Requirements management planning
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During the requirements engineering process, you
have to plan:
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Requirements identification
• How requirements are individually identified;
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A change management process
• The process followed when analysing a requirements
change;
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Traceability policies
• The amount of information about requirements relationships
that is maintained;
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CASE tool support
• The tool support required to help manage requirements
change;
Slide 42
Traceability
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Traceability is concerned with the relationships
between requirements, their sources and the system
design
Source traceability
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Requirements traceability
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Links from requirements to stakeholders who proposed
these requirements;
Links between dependent requirements;
Design traceability
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Links from the requirements to the design;
Slide 43
CASE tool support
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Requirements storage
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Change management
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Requirements should be managed in a secure, managed
data store.
The process of change management is a workflow
process whose stages can be defined and information
flow between these stages partially automated.
Traceability management
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Automated retrieval of the links between requirements.
Slide 44
Requirements change management
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Should apply to all proposed changes to the
requirements.
Principal stages
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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.
Slide 45
Key points
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The requirements engineering process
includes a feasibility study, requirements
elicitation and analysis, requirements
specification and requirements management.
Requirements elicitation and analysis is
iterative involving domain understanding,
requirements collection, classification,
structuring, prioritisation and validation.
Systems have multiple stakeholders with
different requirements.
Slide 46
Key points
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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.
Slide 47
Use Cases – the Big Picture
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Many high-level use cases start with simple
situations and straightforward, naturallanguage questions.
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This is Jill.
This is her phone.
What does Jill want to do with her phone?
What steps must she take to achieve this goal?
How will her pathway toward this goal follow the
business logic fundamental to our project's
success?
Slide 48
Use Cases – the Big Picture
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Use cases tend to account for various kinds of
failure modes and defaults in user flow.
But the very context in which these failure modes
occur tends to be rather cooked and artificial.
Many start with a neatly conventional circumstance
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"Jill wants to buy a new ringtone"
They end in a similarly pat fulfillment
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"Jill successfully downloads and installs the ringtone"
Slide 49
Use Cases – the Big Picture
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What about use cases that start out like:
"Greta wants to sneak out and meet her lover
Patrick, without making her husband Bertrand
suspicious."
"Kenji wants his private contact information to be
more available to his close friends than the random
boys he picks up clubbing."
"Claudia wants to play games on her computer at
work, while making it seem as if she's busy getting
things done."
Slide 50
Use Cases – the Big Picture
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Experience tells us that these are the kinds of things
people actually do with technology.
Human beings are endlessly creative, contrary, even
perverse.
Our motives are not always noble.
We figure out ways to use whatever technology
happens to be at hand to further our goals
•
whether or not those goals bear any resemblance to what
we're "supposed to be" doing.
Slide 51
Use Cases – the Big Picture
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Consider the uses foreseen by designers,
manufacturers and retailers for a new
technology
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Inevitably featured in the advertising and
marketing campaigns
They often turn out to be much less
interesting than what people actually do with
them.
Slide 52
Use Cases – the Big Picture
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Fault lines are all the gaps between the assumptions
and the reality
Fault lines are places where emergent patterns of
use expose
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incorrect assumptions on the part of the designers
imperfect models of the target audience on the part of
marketers
social realities that might otherwise have remained latent
Fault lines also crop up where tech-savvy
developers forget to take off their expert hat.
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They consequently fail to understand the mental models
users carry around with them regarding how technology
works.
Slide 53
Use Cases – the Big Picture
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There is good business sense in attending
carefully to these fault lines.
That is where the truly useful products and
services wait to be born.
Slide 54
Example
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Cingular Wireless, US.
Offers a service called "Escape-a-Date.”
Provides its subscribers an emergency exit from bad
dates and similarly awkward social circumstances.
The subscriber schedules a "rescue" phone call for a
pre-set time.
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When their phone rings they are guided through a script
that gives them a convenient excuse to get up and leave.
A social discomfort is circumvented,
Everybody gets to save face.
Slide 55
Example
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We laugh, but this is a real circumstance, something
we can all recognize.
Someone, somewhere, has identified a genuine
social need and devised response to it.
There is the “side effect” of shifting some packets
and producing some revenue for the service
provider!
A definite improvement on use cases which imagine
or fabricate "needs" nobody actually ever had, which
produce "features" and "conveniences" nobody ever
uses.
Slide 56
Case in Point
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Camera phones.
At some point in the last five years, bundling a camera and a
mobile phone became technically feasible.
Soon thereafter, it became a potential source of profit
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thus the current proliferation of models.
But there never was a realistic scenario in which these
cameras become part of the everyday pattern of use for the
majority of users.
That's why we see the use profile we almost invariably do.
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interest in the phone's camera spikes immediately after
purchase, as customers explore the novelty and seek to justify
the extra expenditure
then tapers off asymptotically to zero.
Slide 57
A Basic Problem with Use Cases
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They generally fail to anticipate the larger social
context inside which all technology exists.
Another example:
We have any number of good models for why and
how a user would go about sending an email, or
using a mobile phone.
But no such model could have predicted the
insidious way in which always-on availability that
arrives alongside them affects even non-users.
Slide 58
A Basic Problem with Use Cases
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What scenario would have predicted the
emerging etiquette of "reachability“ that says
serious adults must both own a mobile
phone and carry it at all times?
What use case would have accounted for the
resentment towards those who either refuse
or simply forget to do so?
Slide 59
A Basic Problem with Use Cases
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What designer would have captured and accurately
modeled the annoyance your correspondents
experience when you don't answer their messages
in something closely approximating real time?
Remember, one of the original selling points of email
was precisely that it was asynchronous.
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One didn't have to be sitting in front of one's computer to
benefit from it.
How things have changed!
Slide 60
The Moral of the Story
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... is not to abandon use cases!!
They'll remain vital tools for anyone seeking to
understand the delicate interactions between human
beings and the things we use.
They will probably assume even greater importance
over time, as our artifacts become ever more
complicated.
But it would be both unexpected and fun if some of
the features and functionality we are sure to be
offered were based on a fuller, more robust
appreciation of everyday life.
Slide 61