chapter 4 requirements engineering. requirements engineering it helps software engineers to better...
TRANSCRIPT
Chapter 4
Requirements Engineering
Requirements Engineering It helps software engineers to better
understand the problem they will work to solve
It encompasses the set of tasks that lead to an understanding of what the business impact of the software will be, what the customer wants?
Types of Requirements Functional Requirements
These are statements of services the system should provide, how the system should react to particular inputs and how the system should behave in particular situations
Non-Functional Requirements These are constraints on the services or
functions offered by the system. They include timing constraints, constraints on the development process and standards
Requirements Engineering Tasks Requirements engineering provides the appropriate
mechanism for understanding what the customer wants, analyzing need, assessing feasibility, managing requirements as they are transformed into an operational system and so on
Requirements engineering is accomplished through seven functions: Inception Elicitation Elaboration Negotiation Specification Validation Management
Requirements Engineering Tasks - Inception 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
Requirements Engineering Tasks - Elicitation Ask the customer, the user, and others
What the objectives for the system? What is to be accomplished? How the system fits into the needs of the
business? How the system is to be used on a day-to-day
basis?
Requirements Engineering Tasks - Elicitation A number of problems that makes the
requirements elicitation as a difficult task: Problems of scope
The boundary of the system is ill-defined Problems of understanding
The customers are not completely sure of what is needed?
Problems of volatility The requirements change over time
Requirements Engineering Tasks - Elaboration Elaboration is an analysis modeling action
that is composed of a number of modeling and refinement tasks
The end-result of elaboration is an analysis model that defines the informational, functional, and behavioral domain of the problem
Requirements Engineering Tasks - Negotiation It is common for different customers to propose
conflicting requirements The requirements engineer must reconcile these conflicts
through a process of negotiation Customers, users, and other stakeholders are asked to
rank requirements and discuss conflicts in priority Risks associated with each requirement are identified and
analyzed Rough “guestimates” of development effort are made and
used to assess the impact of each requirement on project cost and delivery time
Requirements Engineering Tasks - Specification A specification can be any on of the following:
A written document A set of graphical model A formal mathematical model A collection of user scenarios A prototype
The specification is the final work product produced by the requirements engineer
It serves as the foundation for subsequent software engineering activities
Requirements Engineering Tasks - Validation Requirements validation examines the specification to
ensure that all requirements have been stated unambiguously
The review team that validates requirements includes software engineers, customers, users, other stakeholders
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 Engineering Tasks – Requirements Management Requirement management is a set of
activities that help the project team identify, control, and track requirements and changes to requirements at any time as the project proceeds
Requirements management begins with identification. Once requirements are identified, traceability tables are developed
Requirements Engineering Tasks – Requirements Management Features traceability table
shows how requirements relate to important customer observable system features
Source traceability table identifies the source of each requirement
Dependency traceability table indicates how requirements are related to one another
Subsystem traceability table categorizes requirements by the subsystem that they govern
Interface traceability table shows how requirements relate to both internal and external
system interfaces
Initiating the Requirements Engineering Process - 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?
Eliciting Requirements - Elicitation Collaborative Requirements Gathering
In order to encourage a collaborative, team-oriented approach to requirements gathering, a team of stakeholders and developers work together to identify the problem, propose elements of the solution, negotiate different approaches, and specify a preliminary set of solution required
Eliciting Requirements - Elicitation Guidelines for collaborative requirements gathering
meeting: Meetings are conducted and attended by both software engineer
and customers Rules for preparation and participation are established An agenda is suggested that is formal enough to cover all
important points but informal enough to encourage the free flow of ideas
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
Eliciting Requirements - Elicitation Quality Function Deployment (QFD)
Quality Function Deployment (QFD) is a technique that translates the needs of the customer into technical requirements for software
QFD concentrates on maximizing customer satisfaction from the software engineering process
QFD emphasizes an understanding of what is valuable to the customer and then deploys these values throughout the engineering process
Eliciting Requirements - Elicitation Quality Function Deployment Many QFD concepts are
applicable to requirements elicitation activity. They are: Function deployment
is used to determine the value of each function that is required for the system
Information deployment identifies both the data objects and events that the system must
consume and produce Task deployment
examines the behavior of the system within the context of its environment
Value analysis is conducted to determine the relative priority of requirements
determined during each of the three deployments
Elicitation Work Products The work products produced as a consequence of
requirements elicitation will vary depending on the size of the system.
The work product include: A statement of need and feasibility A bounded statement of scope for the system A list of customers, users, and other stakeholders A description of the system’s technical environment A list of requirements (organized by function) A set of usage scenarios that provide insight into the
use of the system Any prototype developed to better define requirements
Developing Use-Cases The first step in writing a use-case is to define the
set of “actors” that will be involved in the system Actors are different people (or devices) that use
the system or product within the context of the function and behavior that is to be described
Actors represent the roles that people play as the system operates
An actor is anything that communicates with the system or product and that is external to the system
Developing Use-Cases Once actors have been defined, use-cases
can be developed The following questions should be
answered by a use-case: Who is the primary actor, secondary actor? What are the actor’s goals? What main tasks are performed by the actor? What information does the actor desire from
the system?
Building the Analysis Model The analysis model is to provide a description of
the required informational, functional, and behavioral domains for a computer-based system.
The model changes dynamically as software engineers learn more about the system to be built, and stakeholders understand more about what they really require
Building the Analysis Model The elements of the analysis model
Scenario-based elements The system is described from the user’s point of
view using scenario based approach Use-cases, use-case diagram, activity diagram
Class-based elements Each usage scenario implies a set of “objects” that
are manipulated as an actor interacts with the system These objects are categorized into classes Class diagram, collaboration diagram
Building the Analysis Model Elements of analysis model (cond…)
Behavioral elements The analysis model must provide modeling
elements that depict behavior State diagram, sequence diagram
Flow-oriented elements Information is transformed as it flows through a
computer-based system Data-flow diagram
Use-Case Diagram
homeowner
Arms/ disarms system
Accesses system via Internet
Reconfigures sensors and related
system features
Responds toalarm event
Encounters anerror condition
system administrator
sensors
State Diagram
Figure 7.6 Preliminary UML state diagram for a photocopier
Initialization
system status=“not ready” display msg = “please wait” display status = blinking
entry/ switch machine on do: run diagnostics do: initiate all subsystems
turn copier “on“
subsystems ready 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
Readingcommands
system status=“Copying” display msg= “copy count =” display message=#copies display status= steady
entry/ start copies do: manage copying do: monitor paper tray do: monitor paper flow
Making copies
start copies
system status=“Jammed” display msg= “paper jam” display message=location display status= blinking
entry/ paper jammed do: determine location do: provide corrective msg. do: interrupt making copies
problem diagnosis
paper jammed
system status=“load paper” display msg= “load paper” display status= blinking
entry/ paper empty do: lower paper tray do: monitor fill switch do: raise paper tray
load paper
paper tray empty
not jammed
paper full
turn copier “off”
not jammed
copies complete
Negotiating Requirements The customer and the developer enter into a
process of negotiation, where the customer may be asked to balance functionality, performance, and other system characteristics against cost and time
The best negotiations strive for a “win-win” result. The customer wins by getting the system that satisfies the majority of the customer’s needs, and software team wins by working to realistic and achievable budgets and deadlines
Negotiating Requirements The negotiating activities carried at the beginning
of each software process iteration are: Identification of the system or subsystem’s key
stakeholders These are the people who will be involved in the negotiation
Determination of the stakeholders’ “win conditions” Win conditions are not always obvious
Negotiate of the stakeholders’ win conditions to reconcile them into a set of win-win conditions for all concerned Work toward a set of requirements that lead to “win-win”
Validating Requirements The requirements represented by the model are
prioritized by the customer and grouped within requirements packages that will be implemented as software increments and delivered to the customer
A review of the analysis model addresses the following questions: Is each requirement consistent with the overall
objective for the system? Have all requirements been specified at the proper
level of abstraction? Is each requirement bounded and unambiguous?
Validating Requirements A review of the analysis model addresses
the following questions: (cond…) Do any requirements conflicts with other
requirements? Is each requirement testable, once
implemented? Does the requirements model properly reflect
the information, function, and behavior of the system to be built?