Software Engineering: A Practitioner’s Approach, 6/e
Chapter 7
Requirements Engineering
Modified to include
some Agile Concepts
copyright © 1996, 2001, 2005
R.S. Pressman & Associates, Inc.
NOTE: Some slides referenced from: Ian Sommerville Slides for Software Engineering.
Coming up: What is a requirement?
1
What is a requirement?
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Requirements are used to describe all aspects of a
system
They may range from a high-level abstract
statement of a service or of a system constraint to a
detailed mathematical functional specification
They serve many roles
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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
Are always used to communicate what you intend to build
Coming up: Requirements come in many forms
2
Requirements come in many forms
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Requirements Statements – ex: “The system shall ….”
UML Use-case diagrams
eXtreme Programming “User Stories”
Scrum Product Backlog
Any other documents that communicate what you intend
to build
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Prototypes can be used as requirements
Existing systems can serve as requirements “Build this system,
but use Java instead of Fortran”
UML Sequence diagrams, State charts, activity diagrams…
Coming up: All Requirements
3
All Requirements
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Should specify external behavior of the system
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Includes functional and non-functional requirements
 Functional requirements are statements of the
services that the system must provide
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What must the system do? (Start with the phrase “The
system shall…”)
Non-functional requirements are constraints on
the services and functions offered by the system
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How must it do it? or a constraint on the system
Coming up: Typical Requirements Statements
4
Typical Requirements Statements
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Functional: The system shall display the heart rate,
blood pressure and temperature of a patient connected
to the patient monitor.
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Non-Functional: "Display of the patient's vital signs must
respond to a change in the patient's status within 2
seconds.”
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‘ilities’ - Performance, Scalability, Capacity, Availability
Reliability, Recoverability, Maintainability, Serviceability,
Security, Regulatory,Manageability
What are some functional requirements on an iPod? Non-functional?
Coming up: Why do we care?
5
Why do we care?
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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
Requirements help by giving you the best
understanding you can have at the beginning
I’ll destroy you and your
little software project
to!
Coming up: Prescriptive versus Agile approach
6
Prescriptive versus Agile approach
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Prescriptive – approach is to minimize/prevent change
by understanding system thoroughly at the beginning
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Typically heard: “A change during implementation costs 10 times
more than a change during requirements phase”
Agile – approach is to manage change by being flexible
(i.e. Agile)
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Typically heard: Requirements planned for 6 months from now
are most likely completely inaccurate. Don’t waste resources
planning far ahead. Plan short iterations and adjust the plan as
you go.
Coming up: Typical Prescriptive
Requirements Engineering Process
7
Typical Prescriptive
Requirements Engineering Process
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Inception—ask a set of questions that establish …
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Elicitation—elicit requirements from all stakeholders
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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
to identify the problem
propose elements of the solution
negotiate different approaches, and
specify a preliminary set of solution requirements
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
Coming up: Requirements Engineering-II
8
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
Coming up: Typical Prescriptive Requirements Document Structure
9
Typical Prescriptive 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 - analysis model diagrams
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See SRS Template on CS421 Project Page
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(provided by http://www.processimpact.com/ … but essentially from IEEE).
Coming up: Inception
10
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.
Coming up: Analysis Model
14
Analysis Model
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Included to provide more clarification about
textual requirements (where needed)
Elements of the analysis model
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Scenario-based elements
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Coming up: Use-Cases
Functional—processing narratives for software functions
Use-case—descriptions of the interaction between an
“actor” and the system
Class-based elements – e.g. class diagram
Behavioral elements – e.g. state diagram, sequence
diagram, activity diagram, etc..
Flow-oriented elements – e.g. data flow diagram
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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:
 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?
Coming up: Use-Case Diagram
16
Use-Case Diagram
Each stick figure is an
actor in your system
Arms/ disarms
syst em
Accesses syst em
via Int ernet
homeow ner
Responds t o
alarm event
sensors
The block is an
external system that
interacts with your
system. Any system
you aren’t
developing is
external
Encount ers an
error condit ion
syst em
administ rat or
Coming up: Use Case Description
Reconf igures sensors
and relat ed
syst em f eat ures
Each oval is a
separate use case that
has a description
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Use Case Description
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ID - A unique to reference this use case U01, U02, etc…)
Objective - What is the ultimate objective of the use-case. What is it trying to
achieve? What was the source of the use-case requirement?
Priority – The overall priority of this use-case (Low, Medium, High)
Source – Who is the main source of this use case. Who cares most about this
functionality? This should be the one person you would ask if there is a
question about this use-case. (Make up a name and cite their: John Smith
(End-user) here.)
Actors - Who is involved in the use-case? Which actors/stakeholders?
Flow of Events
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Basic Flow - flow of events normally executed in the use-case
Alternative Flow(s) - a secondary flow of events due to infrequent conditions
Exception Flow(s) - Exceptions that may happen during the execution of the use case
Includes - other use case IDs that are referenced in steps in the flow of
events.
 Preconditions - Any condition that must be satisfied before the use case
begins. If the condition is “User is logged in”, then the first step of the use
case is NOT “User logs in”. They are already logged in if that is a precondition!
 Post conditions - The conditions that will be satisfied after the use case
successfully
completes
(typically same as objective)
Coming
up: eXtreme Programming
(XP): User Stories
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eXtreme Programming (XP): User
Stories
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Serve as requirements within the XP process
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Written by customers
Time Estimated by developers
Replaces large requirements documents
Represents anything that is “progress” to the customer
Examples:
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Coming up: eXtreme Programming (XP): User Stories
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eXtreme Programming (XP): User
Stories
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Short – 1 or 2 sentences. Usually supposed to fit
on a 3x5 notecard
Easy to estimate (project velocity)
Allows requirements to change frequently with
less impact
Without accompanying validation tests may lead
to misinterpretation
Difficult to scale to very large projects
Require close customer contact to ensure correct
implementation
Coming up: Format for User Stories
(CS421 only… not standard!)
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Format for User Stories
(CS421 only… not standard!)
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Unique Identifier
Source – who should developer contact with questions
about this?
Story – short (1-3 sentence) high-level description of
story. When the developer plans to implement they
should discuss with the customer the details.
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As a [user role], I want to [goal], so I can [reason].
Time estimate – how long in “ideal development time” –
time without meetings or any interruptions
Client’s Priority – how important is the story to the client?
(Low, Medium, High)
Coming up: The web has MANY references about User Stories and Use Cases.
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The web has MANY references about
User Stories and Use Cases.
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User Stories: http://www.agile-softwaredevelopment.com/search/label/user%20stories
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Use Cases:
http://alistair.cockburn.us/Use+case+fundamentals
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Which is better? Many arguments – here is one:
http://alistair.cockburn.us/Why+I+still+use+use+cases
Coming up: Agile and Traditional Requirements share more than they differ
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Agile and Traditional Requirements
share more than they differ
Coming up: Negotiating Requirements
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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”
Coming up: Reasons we need to negotiate
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Reasons we need to negotiate
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Discuss Large software systems must improve the
current situation. It is hard to anticipate the effects that
the new system will have on the organization
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Discuss Prototyping is often required to clarify
requirements
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Negotiate Different users have different requirements and
priorities. There is a constantly shifting compromise in the
requirements
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Negotiate System end-users and organizations who pay
for the system have different requirements
Coming up: Requirements Rationale
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Requirements Rationale
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It is important to provide rationale with requirements
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This helps the developer understand the application domain
and why the requirement is stated in its current form
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Particularly important when requirements have to be changed.
The availability of rationale reduces the chances that change
will have unexpected effects
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In this class we’ll provide the requirement’s source and assume
they can contact the source to get a rationale if needed instead
of an explicit “rationale”
Coming up: Non-Functional Requirement Types
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Non-Functional Requirement
Non Functional =
Types Remember:
How the system
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Product requirements
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Requirements which specify that the delivered product must
behave in a particular way e.g. execution speed, reliability, etc.
Organizational requirements
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should do something
Requirements which are a consequence of organizational
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.
Coming up: Non-Functional Requirement Examples
27
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.
constraint: interoperability
9.3.2 The system development process and deliverable documents
shall conform to the process and deliverables defined in XYZCo-SPSTAN-95.
constraint: quality
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.
security, maintainability
Coming up: Requirements must be testable
Agile stories?
How?
28
Requirements must be testable
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Traditional requirements must be written so that they can be
objectively verified.
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XP requirements have explicit acceptance test cases written by
the customers to solve this problem.
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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.
Coming up: Requirements: testable metrics
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Requirements: testable metrics
Property
Speed
Size
Ease of use
Reliability
Robustness
Portability
Coming up: System Level Requirements
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
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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
Coming up: Requirements Validation
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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
Coming up: Validating Requirements-I
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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?
Coming up: Validating Requirements-II
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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?
Coming up: Requirements Reviews
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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
Coming up: Requirements Reviews Check
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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?
Coming up: Requirements final thoughts…
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Requirements final thoughts…
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Requirements should specify one and only one thing
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Requirements must be testable
Requirements must specify what YOUR system must do,
not what other (external) systems do
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Requirements and User Stories can only pass or fail, they can’t
partially pass. If you want to partially pass a requirement split
into two
To make this easier, in this class all functional requirements
must begin with “The system shall…”
Must have a source (make it up for CS421)
Coming up: Requirements final thoughts…
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Requirements final thoughts…
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Should not be a design choice (this is hard to get right).
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The system shall store user information including name, DOB, address
and SSN. <-- Good!
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The system shall store user information in an Oracle database including
name, DOB, address, SSN. <-- bad
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Is Oracle really REQUIRED? Hard to say… maybe, but probably not. This is
a decision you would make at implementation design time.
Question: Does the customer care that you use Oracle? MySQL? Etc..
Maybe someone found some other MUCH BETTER approach storing the
data on moon rocks.
Again: This is hard to avoid… and I’m not to concerned with it on the SRS,
but I want you to be very aware of when you are making design choices
instead of required features.
Coming up: Requirements must have a unique ID
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Requirements must have a unique ID
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Must have a unique ID.
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When testing you need to reference REQ-1 or REQ-287.
Multiple things cannot be labeled REQ-1.
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Later our test cases will say: This test case validates
requirements REQ-1, REQ-27, and REQ-56.
Coming up: Bad Example Requirements Statements
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Bad Example Requirements
Statements
Bad requirements examples:
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The system shall validate and accept credit cards and cashier’s
checks. High priority.
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The system shall process all mouse clicks very fast to ensure
user’s do not have to wait.
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The user must have Adobe Acrobat installed.
These don’t have a source or unique ID, but what else is wrong?
Coming up: Bad Examples
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Bad Examples
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The system shall validate and accept credit cards and cashier’s
checks. High priority.
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The system shall process all mouse clicks very fast to ensure
user’s do not have to wait.
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Problem: two requirements instead of one.
If the credit card processing works, but the cashier’s check
validation does not… is this requirement pass or fail? Has to be fail,
but that is misleading.
Maybe only credit cards are high priority and cashier’s checks are
low priority.
Problem: This is not testable. Quantify how fast is acceptable?
The user must have Adobe Acrobat installed.
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Problem: This is not something our system must do. It could be in
the constraints/assumptions or maybe operating environment
sections, but is not a functional requirement of our system
Coming up: The Result of Good Requirements
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The Result of Good Requirements
Your wicked
problems melt into
small manageable
problems ….
End of presentation
I’m melting!
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