INTRODUCTION to software engineering

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Software Requirements

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What is a requirement?
 Requirements are a specification of what should be implemented. They
are descriptions of how the system should behave, or of a system
property or attribute. They may be a constraint on the development
process of the system.
 The requirements are the descriptions of the system services and
constraints that are generated during the requirements engineering
process.
 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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5.  Ways to represent user requirements include use cases, user stories, and
event-response tables. User requirements describe what the user will be
able to do with the system. An example of a use case is “Check in for a
flight” using an airline’s website or a kiosk at the airport. Written as a
user story, the same user requirement might read: “As a passenger, I
want to check in for a flight so I can board my airplane.”.
 The business analyst (BA)1 documents functional requirements in a
software requirements specification (SRS), which describes as fully as
necessary the expected behavior of the software system
 System requirements A good example of a “system” is the cashier’s
workstation in a supermarket. There’s a bar code scanner integrated with
a scale, as well as a hand-held bar code scanner. The cashier has a
keyboard, a display, and a cash drawer. You’ll see a card reader and PIN
pad for your loyalty card and credit or debit card, and perhaps a change
dispenser. You might see up to three printers for your purchase
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6.  Domain requirements
– Requirements that come from the application domain of the system and that reflect
characteristics of that domain
contract:
 A legally binding document agreed upon by the customer and supplier. This includes the
technical and organizational requirements, cost, and schedule for a product. A contract
may also contain informal but useful information such as the commitments or expectations
of the parties involved.
customer:
 The person, or persons, who pay for the product and usually (but not necessarily) decide
the requirements. In the context of this recommended practice the customer and the
supplier may be members of the same organization.
supplier:
 The person, or persons, who produce a product for a customer. In the context of this
recommended practice, the customer and the supplier may be members of the same
organization.
user:
 The person, or persons, who operate or interact directly with the product. The user(s) and
the customer(s) are often not the same person(s). 6

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Requirements Analysis & Specification Definitions
 Requirements Analysis
– The process of studying and analyzing the customer and the user needs to arrive at a
definition of software requirements.1
 Requirements Specification
A document that clearly and precisely describes, each of the essential requirements
(functions, performance, design constraint, and quality attributes) of the software and
the external interfaces. Each requirement being defined in such a way that its
achievement is capable of being objectively verified by a prescribed method; for
example inspection, demonstration, analysis, or test.
It is also detailed software description which can serve as a basis for a design or
implementation. Written for developers
Requirements engineering is the process of establishing
 the services that the customer requires from a system
 the constraints under which it operates and is developed
The descriptions of the system services and constraint that are generated during the
requirements engineering process
Requirements engineering is divided into requirements development and requirements
management regardless of what development life cycle your project is following.

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Requirements Engineering
Requirements Elicitation Requirements Analysis
Requirements Specification Requirements Verification
Requirements Management
Requirements Engineering

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Requirements readers
Client managers
System end-users
Client engineers
Contractor managers
System architects
System end-users
Client engineers
System architects
Software developers
Client engineers (perhaps)
System architects
Software developers
User requirements
System requirements
Software design
specification

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Functional Requirements
 Functional requirements specify the functions of the system, how it records, computes,
transforms, and transmits data. Each interface has its own functions. Typically, the user
interface is the most important interface, since most of the data are recorded and shown
through it.
 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 BUT functional system requirements should describe the system services in detail
Examples of functional requirements
 The user shall be able to search either all of the initial set of databases or select a subset
from it.
 The system shall provide appropriate viewers for the user to read documents in the
document store.
 Every order shall be allocated a unique identifier (ORDER_ID) which the user shall be
able to copy to the account’s permanent storage area.

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Non-functional requirements
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 CASE system, programming
language or development method
 Non-functional requirements may be more critical
than functional requirements. If these are not met, the
system is useless

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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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Non-functional requirement types
Performance
requirements
Space
requirements
Usability
requirements
Efficiency
requirements
Reliability
requirements
Portability
requirements
Interoperability
requirements
Ethical
requirements
Legislative
requirements
Implementation
requirements
Standards
requirements
Delivery
requirements
Safety
requirements
Privacy
requirements
Product
requirements
Organizational
requirements
External
requirements
Non-functional
requirements

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Non-functional requirements examples
 Product requirement
– 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
 Organisational requirement
– 9.3.2 The system development process and deliverable documents
shall conform to the process and deliverables defined in XYZCo-
SP-STAN-95
 External requirement
– 7.6.5 The system shall not disclose any personal information about
customers apart from their name and reference number to the
operators of the system

15.  Product vs. project requirements
 Product requirements are the ones that describe properties of a software system to be built.
Projects certainly do have other expectations and deliverables that are not a part of the
software the team implements, but that are necessary to the successful completion of the
project as a whole. These are project requirements but not product requirements.
 An SRS houses the product requirements, but it should not include design or
implementation details (other than known constraints), project plans, test plans, or similar
information. Project requirements include:
– Physical resources the development team needs, such as workstations, special
hardware devices, testing labs, testing tools and equipment, team rooms, and
videoconferencing equipment.
– Staff training needs.
– User documentation, including training materials, tutorials, reference manuals, and
release notes.
– Support documentation, such as help desk resources and field maintenance and service
information for hardware devices.
– Infrastructure changes needed in the operating environment.
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16. – Requirements and procedures for releasing the product, installing it in the
operating environment, configuring it, and testing the installation.
– Requirements and procedures for transitioning from an old system to a new one,
such as data migration and conversion requirements, security setup, production
cutover, and training to close skills gaps; these are sometimes called transition
requirements (IIBA 2009).
– Product certification and compliance requirements.
– Revised policies, processes, organizational structures, and similar documents.
– Sourcing, acquisition, and licensing of third-party software and hardware
components.
– Beta testing, manufacturing, packaging, marketing, and distribution requirements.
– Customer service-level agreements.
– Requirements for obtaining legal protection (patents, trademarks, or copyrights)
for intellectual property related to the software.
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Requirements measures
Property Measure
Speed Processed transactions/second
User/Event response time
Screen refresh time
Size K Bytes
Number of RAM 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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Requirements interaction
 Conflicts between different non-functional requirements are
common in complex systems.
 Spacecraft system
– To minimise weight, the number of separate chips in the system
should be minimised
– To minimise power consumption, lower power chips should be
used
– However, using low power chips may mean that more chips have
to be used.
Which is the most critical requirement?

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Domain requirements
 Derived from the application domain and describe system characteristics and features
that reflect the domain
 May be new functional requirements, constraints on existing requirements or define
specific computations
 If domain requirements are not satisfied, the system may be unworkable
Domain requirements problems
 Understandability
• Requirements are expressed in the language of the application domain
• Need domain-specific knowledge terminology or technology
• Usually these domain-specific terminology is difficult for SW Engineer
• This is often not understood by software engineers developing the system
● Implicitness
• Need domain expert to leave the information from a requirement
• Domain specialists(experts) understand the area so well that they do not think of making
the domain requirements explicit
• But the developer may implement the system unsatisfactory because they are not know
about the domain-specific knowledge

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User requirements
 Should describe functional and non-functional requirements so
that they are understandable by system users who don’t have detailed
technical knowledge
 User requirements are defined using natural language, tables and
diagrams
Problems with natural language
 Lack of clarity
– Precision is difficult without making the document difficult to read
 Requirements confusion
– Functional and non-functional requirements tend to be mixed-up
 Requirements amalgamation
– Several different requirements may be expressed together

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IEEE requirements standard
 Introduction
 General description
 Specific requirements
 Appendices
 Index
 This is a generic structure that must be instantiated for
specific systems

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Requirements document structure
 Introduction
 Glossary
 User requirements definition
 System architecture
 System requirements specification
 System models
 System evolution
 Appendices
 Index

24. Some of the most common requirements risks are described in the following sections.
 Insufficient user involvement
Customers often don’t understand why it is so essential to work hard on eliciting
requirements and assuring their quality. Developers might not emphasize user involvement,
perhaps because they think they already understand what the users need. Insufficient user
involvement leads to late-breaking requirements that generate rework and delay completion.
 Inaccurate planning
Vague, poorly understood requirements lead to overly optimistic estimates, which come back
to haunt you when the inevitable overruns occur. The top contributors to poor software cost
estimation are frequent requirements changes, missing requirements, insufficient
communication with users, poor specification of requirements, and insufficient requirements
analysis (Davis 1995). Estimating project effort and duration based on requirements means
that you need to know something about the size of your requirements and the development
team’s productivity.
Creeping user requirements
 As requirements evolve during development, projects often exceed their planned
schedules and budgets (which are nearly always too optimistic). To manage scope creep,
begin with a clear statement of the project’s business objectives, strategic vision, scope,
limitations, and success criteria. Evaluate all proposed new features or requirements
changes against this reference. Requirements will change and grow.

25.  Ambiguous requirements
One symptom of ambiguity in requirements is that a reader can interpret a requirement
statement in several ways (Lawrence 1996). Another sign is that multiple readers of a
requirement arrive at different understandings of what it means.
 Gold plating
Gold plating takes place when a developer adds functionality that wasn’t in the
requirements specification (or was deemed out of scope) but which the developer believes
“the users are just going to love.” If users don’t care about this functionality, the time spent
implementing it is wasted. Rather than simply inserting new features, developers and BAs
should present stakeholders with creative ideas for their consideration. Developers should
strive for leanness and simplicity, not going beyond what stakeholders request without
their approval.
 Overlooked stakeholders
Most products have several groups of users who might use different subsets of features,
have different frequencies of use, or have varying levels of experience. If you don’t
identify the important user classes for your product early on, some user needs won’t be
met. After identifying all user classes, make sure that each has a voice,. You might have
stakeholders who don’t even know the project exists, such as government agencies that
mandate standards that affect your system, yet you need to know about them and their
influence on the project.

26. Benefits from a high-quality requirements process
Investing in good requirements will virtually always return more than it costs.
 Sound requirements processes emphasize a collaborative approach to product
development that involves stakeholders in a partnership throughout the project. The
potential payoff includes:
 Fewer defects in requirements and in the delivered product.
 Reduced development rework.
 Faster development and delivery.
 Fewer unnecessary and unused features.
 Lower enhancement costs.
 Fewer miscommunications.
 Reduced scope creep.
 Reduced project chaos.
 Higher customer and team member satisfaction.
 Products that do what they’re supposed to do.
 Even if you can’t quantify all of these benefits, they are real.
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27. SRS should provide several specific benefits, such as the following:
 Establish the basis for agreement between the customers and the
suppliers on what the software product is to do. The complete
description of the functions to be performed by the software specified in
the SRS will assist the potential users to determine if the software
specified meets their needs or how the software must be modified to meet
their needs.
 Reduce the development effort. The preparation of the SRS forces the
various concerned groups in the customer’s organization to consider
rigorously all of the requirements before design begins and reduces later
redesign, recoding, and retesting. Careful review of the requirements in
the SRS can reveal omissions, misunderstandings, and inconsistencies
early in the development cycle when these problems are easier to correct.
 Provide a basis for estimating costs and schedules. The description of
the product to be developed as given in the SRS is a realistic basis for
estimating project costs and can be used to obtain approval for bids or
price estimates.

28.  Provide a baseline for validation and verification. Organizations can
develop their validation and verification plans much more productively
from a good SRS. As a part of the development contract, the SRS
provides a baseline against which compliance can be measured.
 Facilitate transfer. The SRS makes it easier to transfer the software
product to new users or new machines. Customers thus find it easier to
transfer the software to other parts of their organization, and suppliers
find it easier to transfer it to new customers
 Serve as a basis for enhancement. Because the SRS discusses the
product but not the project that developed it, the SRS serves as a basis for
later enhancement of the finished product. The SRS may need to be
altered, but it does provide a foundation for continued production
evaluation.
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Key points
 Requirements set out what the system should do and define constraints on its
operation and implementation
 Functional requirements set out services the system should provide
 Non-functional requirements constrain the system being developed or the
development process
 User requirements are high-level statements of what the system should do
 User requirements should be written in natural language, tables and diagrams
 System requirements are intended to communicate the functions that the system
should provide
 System requirements may be written in structured natural language, a PDL or in a
formal language
 A software requirements document is an agreed statement of the system requirements