Software engineering requirements help11

1
Chapter 5
 Lecture 5: Understanding Requirements
Slide Set to accompany
Software Engineering: A Practitioner’s Approach, 7/e
by Roger S. Pressman
Slides copyright © 1996, 2001, 2005, 2009 by Roger S. Pressman
For non-profit educational use only
May be reproduced ONLY for student use at the university level when used in conjunction
with Software Engineering: A Practitioner's Approach, 7/e. Any other reproduction or use is
prohibited without the express written permission of the author.
All copyright information MUST appear if these slides are posted on a website for student
use.

2
Requirements Engineering-I
 Inception—ask a set of questions that establish …
 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

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Requirements Engineering-II
 Specification—can be any one (or more) of the following:
 A written document
 A set of models
 A formal mathematical
 A collection of user scenarios (use-cases)
 A prototype
 Validation—a review mechanism that looks for
 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

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Inception
 Identify stakeholders
 “who else do you think I should talk to?”
 Recognize multiple points of view
 Work toward collaboration
 The first questions
 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?

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Eliciting Requirements
 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
 to identify the problem
 propose elements of the solution
 negotiate different approaches, and
 specify a preliminary set of solution requirements

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Elicitation Work Products
 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.

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Building the Analysis Model
 Elements of the analysis model
 Scenario-based elements
• Functional—processing narratives for software functions
• Use-case—descriptions of the interaction between an
“actor” and the system
 Class-based elements
• Implied by scenarios
 Behavioral elements
• State diagram
 Flow-oriented elements
• Data flow diagram

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Use-Cases
 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?

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Class Diagram
From the SafeHome system …

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State Diagram
Reading
Commands
System status = “ready”
Display msg = “enter cmd”
Display status = steady
Entry/subsystems ready
Do: poll user input panel
Do: read user input
Do: interpret user input
State name
State variables
State activities

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Negotiating Requirements
 Identify the key stakeholders
 These are the people who will be involved in the
negotiation
 Determine each of the stakeholders “win
conditions”
 Win conditions are not always obvious
 Negotiate
 Work toward a set of requirements that lead to “win-
win”

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Validating Requirements - I
 Is each requirement consistent with the overall objective for the
system/product?
 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?

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Validating Requirements - II
 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?

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