requirement engineering chapter 4 .ppt slide

Chapter 4 – Requirements Engineering
Lecture 1
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Chapter 4 Requirements engineering

Topics covered
 Functional and non-functional requirements
 The software requirements document
 Requirements specification
 Requirements engineering processes
 Requirements elicitation and analysis
 Requirements validation
 Requirements management
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Requirements engineering
 The process of establishing the services that the
customer requires from a system and the constraints
under which it operates and is developed.
 The requirements themselves are the descriptions of the
system services and constraints that are generated
during the requirements engineering process.
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What is a requirement?
 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
 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.
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Requirements abstraction (Davis)
“If a company wishes to let a contract for a large software development
project, it must define its needs in a sufficiently abstract way that a
solution is not pre-defined. The requirements must be written so that
several contractors can bid for the contract, offering, perhaps, different
ways of meeting the client organization’s needs. Once a contract has
been awarded, the contractor must write a system definition for the
client in more detail so that the client understands and can validate what
the software will do. Both of these documents may be called the
requirements document for the system.”
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Types of requirement
 User requirements
 Statements in natural language plus diagrams of the services the
system provides and its operational constraints. Written for
customers.
 System requirements
 A structured document setting out detailed descriptions of the
system’s functions, services and operational constraints. Defines
what should be implemented so may be part of a contract
between client and contractor.
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User and system requirements
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Readers of different types of requirements
specification
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Functional and non-functional requirements
 Functional requirements
 Statements of services the system should provide, how the
system should react to particular inputs and how the system
should behave in particular situations.
 May state what the system should not do.
 Non-functional requirements
 Constraints on the services or functions offered by the system
such as timing constraints, constraints on the development
process, standards, etc.
 Often apply to the system as a whole rather than individual
features or services.
 Domain requirements
 Constraints on the system from the domain of operation
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Functional requirements
 Describe functionality or system services.
 Depend on the type of software, expected users and the
type of system where the software is used.
 Functional user requirements may be high-level
statements of what the system should do.
 Functional system requirements should describe the
system services in detail.
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Functional requirements for the MHC-PMS
 A user shall be able to search the appointments lists for
all clinics.
 The system shall generate each day, for each clinic, a
list of patients who are expected to attend appointments
that day.
 Each staff member using the system shall be uniquely
identified by his or her 8-digit employee number.
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Requirements imprecision
 Problems arise when requirements are not precisely
stated.
 Ambiguous requirements may be interpreted in different
ways by developers and users.
 Consider the term ‘search’ in requirement 1
 User intention – search for a patient name across all
appointments in all clinics;
 Developer interpretation – search for a patient name in an
individual clinic. User chooses clinic then search.
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Requirements completeness and consistency
 In principle, requirements should be both complete and
consistent.
 Complete
 They should include descriptions of all facilities required.
 Consistent
 There should be no conflicts or contradictions in the descriptions
of the system facilities.
 In practice, it is impossible to produce a complete and
consistent requirements document.
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Non-functional requirements
 These define system properties and constraints e.g.
reliability, response time and storage requirements.
Constraints are I/O device capability, system
representations, etc.
 Process requirements may also be specified mandating
a particular IDE, programming language or development
method.
 Non-functional requirements may be more critical than
functional requirements. If these are not met, the system
may be useless.
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Types of nonfunctional requirement
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Non-functional requirements implementation
 Non-functional requirements may affect the overall
architecture of a system rather than the individual
components.
 For example, to ensure that performance requirements are met,
you may have to organize the system to minimize
communications between components.
 A single non-functional requirement, such as a security
requirement, may generate a number of related
functional requirements that define system services that
are required.
 It may also generate requirements that restrict existing
requirements.
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Non-functional classifications
 Product requirements
 Requirements which specify that the delivered product must
behave in a particular way e.g. execution speed, reliability, etc.
 Organisational requirements
 Requirements which are a consequence of organisational
policies and procedures e.g. process standards used,
implementation requirements, etc.
 External requirements
 Requirements which arise from factors which are external to the
system and its development process e.g. interoperability
requirements, legislative requirements, etc.
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Examples of nonfunctional requirements in the
MHC-PMS
Product requirement
The MHC-PMS shall be available to all clinics during normal working
hours (Mon–Fri, 0830–17.30). Downtime within normal working hours
shall not exceed five seconds in any one day.
Organizational requirement
Users of the MHC-PMS system shall authenticate themselves using
their health authority identity card.
External requirement
The system shall implement patient privacy provisions as set out in
HStan-03-2006-priv.
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Goals and requirements
 Non-functional requirements may be very difficult to state
precisely and imprecise requirements may be difficult to
verify.
 Goal
 A general intention of the user such as ease of use.
 Verifiable non-functional requirement
 A statement using some measure that can be objectively tested.
 Goals are helpful to developers as they convey the
intentions of the system users.
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Usability requirements
 The system should be easy to use by medical staff and
should be organized in such a way that user errors are
minimized. (Goal)
 Medical staff shall be able to use all the system functions
after four hours of training. After this training, the
average number of errors made by experienced users
shall not exceed two per hour of system use. (Testable
non-functional requirement)
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Metrics for specifying nonfunctional
requirements
Property Measure
Speed Processed transactions/second
User/event response time
Screen refresh time
Size Mbytes
Number of ROM chips
Ease of use Training time
Number of help frames
Reliability Mean time to failure
Probability of unavailability
Rate of failure occurrence
Availability
Robustness Time to restart after failure
Percentage of events causing failure
Probability of data corruption on failure
Portability Percentage of target dependent statements
Number of target systems
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Domain requirements
 The system’s operational domain imposes requirements
on the system.
 For example, a train control system has to take into account the
braking characteristics in different weather conditions.
 Domain requirements be new functional requirements,
constraints on existing requirements or define specific
computations.
 If domain requirements are not satisfied, the system may
be unworkable.
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Train protection system
 This is a domain requirement for a train protection
system:
 The deceleration of the train shall be computed as:
 Dtrain = Dcontrol + Dgradient
 where Dgradient is 9.81ms2 * compensated gradient/alpha and
where the values of 9.81ms2 /alpha are known for different types
of train.
 It is difficult for a non-specialist to understand the
implications of this and how it interacts with other
requirements.
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Domain requirements problems
 Understandability
 Requirements are expressed in the language of the application
domain;
 This is often not understood by software engineers developing
the system.
 Implicitness
 Domain specialists understand the area so well that they do not
think of making the domain requirements explicit.
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Key points
 Requirements for a software system set out what the
system should do and define constraints on its operation
and implementation.
 Functional requirements are statements of the services
that the system must provide or are descriptions of how
some computations must be carried out.
 Non-functional requirements often constrain the system
being developed and the development process being
used.
 They often relate to the emergent properties of the
system and therefore apply to the system as a whole.
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requirement engineering chapter 4 .ppt slide

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