W4 lecture 7&8 - requirements gathering

Lecture 7&8
Requirement
Engineering
CSC291 - Software Engineering
Concepts
(Fall 2019)

Outline
• Software Requirements
• Requirement Engineering
• Types of Requirements
• Stakeholders
26/3/2020 CSC291 - Software Engineering Concepts

Requirements
3
IEEE Standard 729 defines it as
• “A condition or capability needed by a user to solve a problem
or achieve an objective”
• “A condition or capability that must be met or possessed by a
system...to satisfy a contract, standard, specification, or other
formally imposed document”

Software Requirements and Requirement
Engineering
• The requirements for a system
• are the descriptions of the services provided by the system and its
operational constraints.
• These requirements reflect the
• needs of customers for a system that helps solve some problem
such as controlling a device, placing an order or finding
information.
• “The process of finding out, analysing, documenting and
checking these services and constraints is called
requirements engineering (RE).”

Why Requirements?
• System customers
• specify the requirements and read them to check if they
meet their needs
• Project managers
• Use the requirements document to plan a bid for system
and to plan the system development process
• System engineers
• Use the requirements to understand what system is to
be developed

Why Requirements? (Contd..)
• System test engineers
• Use the requirements to develop validation tests for the
system
• System maintenance engineers
• Use the requirements to help understand the system
and the relationships between its parts

Different Levels Of Abstraction
• User requirements
• are statements, in a natural language plus diagrams, of what
services the system is expected to provide and the constraints
under which it must operate.
• System requirements
• System requirements set out the system’s functions, services and
operational constraints in detail.
• The system requirements document (sometimes called a functional
specification) should be define exactly what is to be implemented.
• It may be part of the contract between the system buyer and the
software developers.
6/3/2020 CSC291 - Software Engineering Concepts 7

Example
• User requirement: The library system should provide a way to
allow a student to borrow a book from the library.
• System requirement: The library system should provide a
withdraw interaction that allows a student to withdraw a book
given the isbn and copy number of the book to be withdrawn.
• The interaction fails if: the book is already withdrawn, the book is
not in the library's collection, the student has already withdrawn 5
books, the student owes more than $5, the book is on hold by
someone else. Otherwise…(To be completed)

Types Of Requirements
Business Requirements
These are used to state the high-level business objectives of
the organization or customer requesting the system or product.
They are used to document main system features and
functionalities without going into their nitty-gritty details.
They are captured in a document describing the project vision
and scope.

Types Of Requirements
• Software system requirements are often classified as
• Functional requirements
• Non-functional requirements

Functional Requirements
• These are statements of services the system should
provide.
• They bring in the system’s view and define from the
system’s perspective
• Services the system should provide
• How the system should react to particular input
• How the system should behave in a particular situations
or
• What the system should not do

Non-functional Requirements (NFR’s)
• These are constraints on the services or functions offered
by the system.
• They include timing constraints, constraints on the
development process and constraints imposed by
standards.
• Non-functional requirements often apply to the system as
a whole. They do not usually just apply to individual
system features or services.

Types of NFR’s
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

Types of NFR’s (Contd..)
• Product requirements
• Requirements which specify that the delivered product
must behave in a particular way.
• e.g. execution speed, reliability, Usability, etc.
• Organizational requirements
• Requirements which are a consequence of organizational
policies and procedures in the customer’s and
developer's organization.
• External requirements
• Requirements which arise from factors which are external
to the system and its development process
• e.g. legislative requirements, etc.

Reliability Requirements:
Reliability requirements deal with the failure to provide
service.
Example :
The failure frequency of a heart-monitoring unit that will
operate in a hospital’s intensive care ward is required to be
less than one in 20 years. Its heart attack detection function
is required to have a failure rate of less than one per million
cases.

Efficiency Requirements:
• Deals with the hardware resources needed to perform the functions of
the software.
• The main hardware resources to be considered are the computer’s
processing capabilities (measured in MIPS – million instructions per
second, MHz or megahertz etc.), its data storage capability in terms
of memory and disk capacity (measured in MBs – megabytes, GBs –
gigabytes, etc.) and the data communication capability of the
communication lines (usually measured in KBPS – kilobits per
second., etc)
• The requirements may include the maximum values at which the
hardware resources will be applied in the developed software system

Portability Requirements:
Portability requirements tend to the adaptation of a
software system to other environments consisting of
different hardware, different operating systems, and so
forth.
Interoperability Requirements:
• Interoperability describes the extent to which systems and
devices can exchange data, and interpret that shared
data.
• For two systems to be interoperable, they must be able to
exchange data and present that data such that it can be
understood by a user

Integrity – deal with system security that prevent
unauthorized persons access.
Usability – deals with the scope of staff resources needed
to train new employees and to operate the software system.

Integrity

Usability

Example: A Word Processor
Let us now look at an example to understand the difference
between these different types of requirements.
Let us assume that we have a word-processing system that
does not have a spell checker. In order to be able to sell the
product, it is determined that it must have a spell checker

Contd..
• Business Requirement could be stated as:
user will be able to correct spelling errors in a document
efficiently. Hence, the Spell checker will be included as a
feature in the product.
• In the next step we need to describe what tasks must be
included to accomplish the above-mentioned business
requirement.
• The resulting User Requirement could be as follows:
Finding spelling errors in the document and deciding whether
to replace each misspelled word with one chosen from a list of
suggested words. It is important to note that this requirement is
written from a user’s perspective.

Contd..
• After documenting the user’s perspective in the form of user
requirements, we look at the system’s perspective:
• what is the functionality provided by the system and how will it
help the user to accomplish these tasks.
The functional requirement for the same user requirement
could be written as follows:
• The spell checker will find and highlight misspelled words.
• It will then display a dialog box with suggested replacements.
• The user will be allowed to select from the list of suggested
replacements.
• Upon selection it will replace the misspelled word with the selected
word.

Contd..
• Finally, a non-functional requirement of the system could
require that it must be integrated into the existing word-
processor that runs on windows platform

The Need For Comprehensive Software Non -Functional
Requirements

Contd..

Contd..
• Mostly software projects satisfactorily fulfilled the basic
requirements for correct calculations (correct inventory figures,
correct average class’s score, correct loan interest, etc.)
• Software projects suffered from poor performance in important
areas such as maintenance, reliability, software reuse, or
training.
Cause for the poor performance - lack of predefined
requirements

Specifying the Requirements
• Functional and nonfunctional requirements should be
specified in such a way that they are understandable by
system users who don’t have detailed technical
knowledge.
• These requirements are defined using natural language,
tables and diagrams as these can be understood by all
users.

Guidelines
• While specifying or writing the requirements, following
guidelines must be followed:
• Invent a standard format and use it for all requirements.
• Use language in a consistent way.
• Avoid the use of computer jargons.
• Separate functional and non-functional requirements.
• Distinguish requirements priorities
• Example: MoSCoW (Must, Shall, Could, Want/Will (no
TBD))
• Testable (write test cases

Example – Identify the problem
If sales for current month are below target sales, then
report is to be printed unless difference between target
sales and actual sales is less than half of difference
between target sales and actual sales in previous month, or
if difference between target sales and actual sales for the
current month is less than 5%.

Contd..
Problems:
• Difficult to read
• Ambiguity: sales and actual sales, 5% of what?
• Incomplete: what if sales are above target sales?

Stakeholders
Stakeholders are different people who would be
interested in the software.

Chapter Reading
• Chapter 4, Requirement Engineering,
Software Engineering by Ian Sommerville

W4 lecture 7&8 - requirements gathering

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