Software Engineering: A Practitioner’s Approach, 6/e
Chapter 7
Requirements Engineering
copyright © 1996, 2001, 2005
R.S. Pressman & Associates, Inc.
NOTE: Some slides referenced from: Ian Sommerville Slides for Software Engineering.
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
1
What is a requirement?
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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
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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
Both these statements may be called requirements
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
2
Requirements Definition
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Should specify external behavior of the system so the
requirements should not be defined using a
computational model
Includes functional and non-functional requirements
 Functional requirements are statements of the
services that the system should provide
 Non-functional requirements are constraints on
the services and functions offered by the system
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
3
Wicked Problems
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Most large software systems address wicked
problems
Problems which are so complex that they can never
be fully understood and where understanding
develops during the system development
Therefore, requirements are normally both
incomplete and inconsistent
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
4
Requirements Engineering-I
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Inception—ask a set of questions that establish …
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basic understanding of the problem
the people who want a solution
the nature of the solution that is desired, and
the effectiveness of preliminary communication and collaboration between
the customer and the developer
Elicitation—elicit requirements from all stakeholders
Elaboration—create an analysis model that identifies data, function
and behavioral requirements
Negotiation—agree on a deliverable system that is realistic for
developers and customers
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
5
Requirements Engineering-II
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Specification—can be any one (or more) of the following:
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Validation—a review mechanism that looks for
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A written document
A set of models
A formal mathematical
A collection of user scenarios (use-cases)
A prototype
errors in content or interpretation
areas where clarification may be required
missing information
inconsistencies (a major problem when large products or systems
are engineered)
conflicting or unrealistic (unachievable) requirements.
Requirements management
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
6
Inception
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Identify stakeholders
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“who else do you think I should talk to?”
Recognize multiple points of view
Work toward collaboration
The first questions
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Who is behind the request for this work?
Who will use the solution?
What will be the economic benefit of a successful solution
Is there another source for the solution that you need?
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
7
Eliciting Requirements
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meetings are conducted and attended by both software
engineers and customers
rules for preparation and participation are established
an agenda is suggested
a "facilitator" (can be a customer, a developer, or an outsider)
controls the meeting
a "definition mechanism" (can be work sheets, flip charts, or wall
stickers or an electronic bulletin board, chat room or virtual
forum) is used
the goal is
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to identify the problem
propose elements of the solution
negotiate different approaches, and
specify a preliminary set of solution requirements
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
8
Eliciting Requirements
Conduc t FA ST
m eet ings
Make list s of
f unc t ions, c las ses
Mak e lis t s of
c onst raint s , et c.
f orm al priorit iz at ion?
El i c i t re q u i re m e n t s
no
yes
Us e QFD t o
priorit iz e
requirem ent s
def ine ac t ors
inf orm ally
priorit ize
requirem ent s
draw use-cas e
diagram
writ e s cenario
Creat e Us e-cases
com plet e t em plat e
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
9
Elicitation Work Products
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a statement of need and feasibility.
a bounded statement of scope for the system or product.
a list of customers, users, and other stakeholders who
participated in requirements elicitation
a description of the system’s technical environment.
a list of requirements (preferably organized by function)
and the domain constraints that apply to each.
a set of usage scenarios that provide insight into the use of
the system or product under different operating conditions.
any prototypes developed to better define requirements.
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
10
Use-Cases
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A collection of user scenarios that describe the thread of usage of a system
Each scenario is described from the point-of-view of an “actor”—a person or
device that interacts with the software in some way
Each scenario answers the following questions:
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Who is the primary actor, the secondary actor (s)?
What are the actor’s goals?
What preconditions should exist before the story begins?
What main tasks or functions are performed by the actor?
What extensions might be considered as the story is described?
What variations in the actor’s interaction are possible?
What system information will the actor acquire, produce, or change?
Will the actor have to inform the system about changes in the external environment?
What information does the actor desire from the system?
Does the actor wish to be informed about unexpected changes?
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
11
Use-Case Diagram
Arms/ disarms
syst em
Accesses syst em
via Int ernet
sensors
homeow ner
Responds t o
alarm event
Encount ers an
error condit ion
syst em
administ rat or
Reconf igures sensors
and relat ed
syst em f eat ures
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
12
Building the Analysis Model
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Elements of the analysis model
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Scenario-based elements
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Class-based elements
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Implied by scenarios
Behavioral elements
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Functional—processing narratives for software functions
Use-case—descriptions of the interaction between an
“actor” and the system
State diagram
Flow-oriented elements
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Data flow diagram
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
13
Class Diagram
From the SafeHome system …
Sensor
name/id
type
location
area
characteristics
identify()
enable()
disable()
reconfigure ()
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
14
State Diagram
Reading
commands
Init ializat ion
t urn copier
“on“
syst em st at us=“not ready”
display msg = “please wait ”
display st at us = blinking
subsyst ems
ready
ent ry/ swit ch machine on
do: run diagnost ics
do: init iat e all subsyst ems
not jammed
syst em st at us=“Ready”
display msg = “ent er cmd”
display st at us = st eady
paper f ull
ent ry/ subsyst ems ready
do: poll user input panel
do: read user input
do: int erpret user input
t urn copier “of f ”
st art copies
Making copies
copies complet e
syst em st at us=“Copying”
display msg= “copy count =”
display message=#copies
display st at us= st eady
ent ry/ st art copies
do: manage copying
do: monit or paper t ray
do: monit or paper f low
paper t ray empt y
paper jammed
problem diagnosis
syst em st at us=“Jammed”
display msg = “paper jam”
display message=locat ion
display st at us= blinking
load paper
syst em st at us=“load paper”
display msg= “load paper”
display st at us= blinking
ent ry/ paper empt y
do: lower paper t ray
do: monit or f ill swit ch
do: raise paper t ray
not jammed
ent ry/ paper jammed
do: det ermine locat ion
do: provide correct ivemsg.
do: int errupt making copies
Figure 7.6 Preliminary UML st at e diagram for a phot ocopier
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
15
Negotiating Requirements
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Identify the key stakeholders
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Determine each of the stakeholders “win conditions”
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These are the people who will be involved in the negotiation
Win conditions are not always obvious
Negotiate
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Work toward a set of requirements that lead to “win-win”
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
16
Reasons we need to negotiate
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Large software systems must improve the current
situation. It is hard to anticipate the effects that the new
system will have on the organization
Different users have different requirements and priorities.
There is a constantly shifting compromise in the
requirements
System end-users and organizations who pay for the
system have different requirements
Prototyping is often required to clarify requirements
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
17
Requirements Document
Structure
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Purpose
Overall Description
System Features (Functional Requirements/Use Cases)
External Interface Requirements
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User interface requirements or standards
Hardware interfaces to other systems
Software interfaces to other systems
Communication Interfaces
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Non-functional Requirements
Appendices
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See SRS Template on the CS421 project page
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(provided by http://www.processimpact.com/).
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
18
Requirements Rationale
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It is important to provide rationale with requirements (this will
be part of our description in the requirements specification
document).
This helps the developer understand the application domain
and why the requirement is stated in its current form
Particularly important when requirements have to be changed.
The availability of rationale reduces the chances that change
will have unexpected effects
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
19
Non-Functional Requirement
Types
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Product requirements
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Organizational requirements
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Requirements which specify that the delivered product must
behave in a particular way e.g. execution speed, reliability, etc.
Requirements which are a consequence of organisational
policies and procedures e.g. process standards used,
implementation requirements, etc.
External requirements
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Requirements which arise from factors which are external to the
system and its development process e.g. interoperability
requirements, legislative requirements, etc.
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
20
Non-Functional Requirement
Examples
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Product requirement
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Organizational requirement
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4.C.8 It shall be possible for all necessary communication between the
APSE and the user to be expressed in the standard Ada character set.
9.3.2 The system development process and deliverable documents
shall conform to the process and deliverables defined in XYZCo-SPSTAN-95.
External requirement
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7.6.5 The system shall provide facilities that allow any user to check if
personal data is maintained on the system. A procedure must be
defined and supported in the software that will allow users to inspect
personal data and to correct any errors in that data.
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
21
Requirements must be testable
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Requirements should be written so that they can be objectively
verified
The problem with this requirement is its use of vague terms
such as ‘errors shall be minimized”
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The system should be easy to use by experienced controllers
and should be organized in such a way that user errors are
minimized.
The error rate should be been quantified
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Experienced controllers should be able to use all the system
functions after a total of two hours training. After this training, the
average number of errors made by experienced users should
not exceed two per day.
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
22
Requirements must be testable
Property
Speed
Size
Ease of use
Reliability
Robustness
Portability
Measure
Processed transactions/second
User/Event response time
Screen refresh time
K Bytes
Number of RAM chips
Training time
Number of help frames
Mean time to failure
Probability of unavailability
Rate of failure occurrence
Availability
Time to restart after failure
Percentage of events causing failure
Probability of data corruption on failure
Percentage of target dependent statements
Number of target systems
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
23
System Level Requirements
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Some requirements place constraints on the system as a whole
rather than specific system functions
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Example
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The time required for training a system operator to be proficient
in the use of the system must not exceed 2 working days.
These may be emergent requirements which cannot be derived
from any single sub-set of the system requirements
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
24
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
Prototyping is an important technique of
requirements validation
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
25
Validating Requirements-I
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Is each requirement consistent with the overall objective for
the system/product? Work with all parties on this question!
Have all requirements been specified at the proper level of
abstraction? That is, do some requirements provide a level of
technical detail that is inappropriate at this stage?
Is the requirement really necessary or does it represent an
add-on feature that may not be essential to the objective of
the system?
Is each requirement bounded and unambiguous?
Does each requirement have attribution? That is, is a source
(generally, a specific individual) noted for each requirement?
Do any requirements conflict with other requirements?
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
26
Validating Requirements-II
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Is each requirement achievable in the technical environment that will house
the system or product?
Is each requirement testable, once implemented?
Does the requirements model properly reflect the information, function and
behavior of the system to be built.
Has the requirements model been “partitioned” in a way that exposes
progressively more detailed information about the system.
Have requirements patterns been used to simplify the requirements model.
Have all patterns been properly validated? Are all patterns consistent with
customer requirements?
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
27
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
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
28
Requirements Reviews Check
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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?
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
29
Quality Function Deployment
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Function deployment determines the “value” (as
perceived by the customer) of each function required of
the system
Information deployment identifies data objects and
events
Task deployment examines the behavior of the system
Value analysis determines the relative priority of
requirements
These courseware materials are to be used in conjunction with Software Engineering: A Practitioner’s Approach, 6/e and are provided
with permission by R.S. Pressman & Associates, Inc., copyright © 1996, 2001, 2005
30