Software requirements

Software Requirements

Loganathan R

Objectives
• To introduce the concepts of user requirements
and system requirements
• To describe functional and non-functional
requirements
• To explain how software requirements may be
organised in a requirements document

Prof. Loganathan R., CSE, HKBKCE 2

Requirements engineering
• The process of finding out, analysing,
documenting, and checking the services that the
customer requires from a system and its
operational constraints is called RE.
• Requirement may range from a high-level abstract
statement of a service or of a system constraint to
a detailed mathematical functional specification.


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 organisation’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.”

Types of requirement
• User requirements (high level abstract requirements)
– Statements in natural language plus diagrams of what
services the system provides and its operational
constraints. Written for customers.
• System requirements (description of what system should do)
– A structured document(also called functional specification)
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.


Definitions and specifications
User requirement Definition

1. LIBSYS shall keep track of all data required by copyright licensing
1. agencies in INDIA and elsewhere

System Requirements Specification

1.1 On making a request for a document from LIBSYS, the requestor shall be
presented with a form that records details of user and the request made
.
1.2 LIBSYS request forms shall be stored on the system for 5 years from the
date of the request.
1.3 All LIBSYS request forms must be indexed by user, by the name of the
.
material requested and by the supplier of the request
.
1.4 LIBSYS shall maintain a log of all requests that have been made to the
system.
1.5 For material where authors lending rights apply, loan details shall be sent
monthly to copyright licensing agencies that have registered with LIBSYS.


Requirements readers

Client Managers
System End-Users
User
Client Engineers
Requirements
Contractor Managers
System Architect

System End-Users
System Client Engineers
Requirements System Architect
Software Developers


Functional and non-functional requirements
• Software system requirements are classified as:
• 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(and sometimes what it
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. Apply to the system as whole.
• Domain requirements
– Requirements that come from the application domain of the
system and that reflect characteristics of that domain.

Functional requirements
• Describe what the system should do.
• 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, its i/o, exceptions
and so on.

Example: The LIBSYS system
• A library system that provides a single interface to
a number of databases of articles in different
libraries.
• Users can search for, download and print these
articles for personal study.


Examples of functional requirements of LIBSYS

• 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.

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 ‘appropriate viewers’
– User intention - special purpose viewer for each
different document type;
– Developer interpretation - Provide a text viewer that
shows the contents of the document.


Requirements completeness and consistency
• In principle, requirements should be both
complete and consistent.
• Completeness
– All services required by the user should be defined.
• Consistent
– Requirements should NOT have conflicts or
contradictions in the descriptions.
• In practice, it is impossible to produce a complete and consistent
requirements document.


Non-functional requirements
• Related to emergent properties and constraints .
Emergent properties are reliability, response time,
storage requirements and so on. Constraints are
I/O device capability, data 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.


Non-functional requirement types
• Product requirements
– Specify product behaviour. E.g. execution speed, memory
required, reliability (failure rate), portability requirements,
usability requirements, etc.
• Organisational requirements
– Derived from customer and developer organisational policies
and procedures. e.g. process standards used, implementation
requirements, delivery requirements, etc.
• External requirements
– Derived from factors which are external to the system and its
development process e.g. interoperability requirements,
legislative requirements, ethical requirements etc.

Non-functional requirement types
Non-functional
Requirements

Product Organisational External
Requirements Requirements Requirements

Efficiency Reliability Portability Interoperability Ethical
Requirements Requirements Requirements Requirements Requirements

Usability Delivery Implementation Standards Legislative
Requirements Requirements Requirements Requirements Requirements

Performance Space Privacy Safety
Requirements Requirements Requirements Requirements

LIBSYS Non-functional requirements
• Product requirement
8.1 The user interface for LIBSYS shall be implemented as simple HTML without
frames or Java applets.
• 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.


Problems in non-functional 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, recovery
from failure, etc.
• 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.


Examples
• A system goal
– The system should be easy to use by experienced controllers and should be
organised in such a way that user errors are minimised.
• A verifiable non-functional requirement
– Experienced controllers shall be able to use all the system functions after a
total of two hours training. After this training, the average number of errors
made by experienced users shall not exceed two per day.


Requirements Measures
Property Measure
Processed transactions/second
Speed User/Event response time
Screen refresh time
M Bytes
Size
Number of ROM chips
Training time
Ease of use
Number of help frames
Mean time to failure
Probability of unavailability
Reliability
Rate of failure occurrence
Availability
Time to restart after failure
Robustness Percentage of events causing failure
Probability of data corruption on failure
Percentage of target dependent statements
Portability
Number of target systems


Requirements interaction
• Conflicts between different non-functional
requirements are common in complex systems.
• Example :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?

Domain requirements
• Derived from the application domain and describe
system characteristics and features that reflect the
domain.
• 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.


Example of LIBSYS domain requirements
• There shall be a standard user interface to all
databases which shall be based on the Z39.50
standard.
• Because of copyright restrictions, some
documents must be deleted immediately on
arrival. Depending on the user’s requirements,
these documents will either be printed locally on
the system server for manually forwarding to the
user or routed to a network printer.


Example : Train protection system
• Computation of deceleration to stop the train:
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.
• Problem: Since it is written in application domain
language its difficult to understand by s/w
engineers.

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.


User requirements
• Should describe functional and non-functional
requirements in such a way that they are
understandable by system users who don’t have
detailed technical knowledge.
• User requirements are defined using natural
language, tables and diagrams as these can be
understood by all users.


Problems with natural language(UR)
• 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.


A user requirement for accounting
system in LIBSYS

4..5 LIBSYS shall provide a financial accounting
system that maintains records of all payments
made by users of the system. System managers
may configure this system so that regular users
may receive discounted rates.


A user requirement for Editor grid in LIBSYS

2.6 Grid facilities To assist in the positioning of entities on a
diagram, the user may turn on a grid in either centimetres or
inches, via an option on the control panel. Initially, the grid is off.
The grid may be turned on and off at any time during an editing
session and can be toggled between inches and centimetres at
any time. A grid option will be provided on the reduce-to-fit view
but the number of grid lines shown will be reduced to avoid
filling the smaller diagram with grid lines.


Requirement problems
• Database requirements includes both conceptual and
detailed information
– Describes the concept of a financial accounting system that is to
be included in LIBSYS;
– However, it also includes the detail that managers can configure
this system - this is unnecessary at this level.
• Grid requirement mixes three different kinds of
requirement
– Conceptual functional requirement is the need for a grid. It
presents rationale for this
– Non-functional requirement of grid units (inches/cm)
– Non-functional UI requirement grid switching(on/off)


Definition of an editor grid facility
• Grid requirements are rewritten to focus on the
essential system feature
2.6.1 Grid facilities
The editor shall provide a grid facility where a matrix of horizontal
and vertical lines provide a background to the editor window. This
grid shall be a passive grid where the alignment of entities is the user's
responsibility.
Rationale: A grid helps the user to create a tidy diagram with well-
spaced entities. Although an active grid, where entities 'snap-to'
grid lines can be useful, the positioning is imprecise. The user is the
best person to decide where entities should be positioned.
Specification: ECLIPSE/WS/Tools/DE/FS Section 5.6
Source: Ray Wilson, Glasgow Office

Guidelines for writing User Requirements
• Invent a standard format and use it for all
requirements.
• Use language in a consistent way. Use shall for
mandatory requirements, should for desirable
requirements.
• Use text highlighting(Bold, Italic or Colour) to
identify key parts of the requirement.
• Avoid the use of computer jargon.


System requirements
• More detailed specifications of system functions,
services and constraints than user requirements.
• They are intended to be a basis for designing the
system.
• They may be incorporated into the system
contract.
• Natural Language(NL) is often used to write
system requirements specification as well as user
requirements.


Inseparable : Requirements and design
• In principle, requirements should state what the
system should do and the design should describe
how it does this.
• In practice, requirements and design are
inseparable
– A system architecture may be designed to structure the
requirements;
– The system may inter-operate with other systems that
generate design requirements;
– The use of a specific design may be a domain
requirement.

Problems with NL specification(Sys Req.)
• Ambiguity
– The readers and writers of the requirement must
interpret the same words in the same way. NL is
naturally ambiguous so this is very difficult.
• Over-flexibility
– The same thing may be said in a number of different
ways in the specification.
• Lack of modularisation
– NL structures are inadequate to structure system
requirements.


Notations for requirements specification
• These can be used as alternatives to NL specification
Notation Description
Structured natural This approach depends on defining standard forms or templates to express the
language requirements specification.
Design description This approach uses a language like a programming language but with more
languages abstract features to specify the requirements by defining an operational model
of the system. This approach is not now widely used although it can be useful
for interface specifications.
Graphical notations A graphical language, supplemented by text annotations is used to define the
functional requirements for the system. An early example of such a graphical
language was SADT. Now, use-case descriptions and sequence diagrams are
commonly used .
Mathematical These are notations based on mathematical concepts such as finite-state
specifications machines or sets. These unambiguous specifications reduce the arguments
between customer and contractor about system functionality. However, most
customers don’t understand formal specifications and are reluctant to accept it
as a system contract.

Structured Language Specifications
• The freedom of the requirements writer is limited
by a predefined template for requirements.
• All requirements are written in a standard way.
• The terminology used in the description may be
limited.
• The advantage is that the most of the
expressiveness of natural language is maintained
but a degree of uniformity is imposed on the
specification.


Form-based Approach
• One or more standard forms/templates are
designed to express the requirements. It should
include the following information.
– Description of the function or entity.
– Description of inputs and where they come from.
– Description of outputs and where they go to.
– Indication of what other entities required.
– Description of action to be taken.
– Pre and post conditions what must be true before & after function call(if
appropriate).
– Description the side effects (if any) of the function.


Form-based specification Example
Insulin Pump/Control Software/SRS/3.3.2
Function Compute insulin dose: Safe sugar level
Description Computes the dose of insulin to be delivered when the current measured sugar level is
in the safe zone between 3 and 7 units
Inputs Current sugar reading (r2), the previous two readings (r0 and r1)
Source Current sugar reading from sensor. Other readings from memory.
Outputs CompDose – the dose in insulin to be delivered
Destination Main control loop
Action CompDose is zero if the sugar level is stable or falling or if the level is increasing but the
rate of increase is decreasing. If the level is increasing and the rate of increase is
increasing, then CompDose is computed by dividing the difference between the current
sugar level and the previous level by 4 and rounding the result. If the result, is rounded
to zero then CompDose is set to the minimum dose that can be delivered.
Requires Two previous readings so that the rate of change of sugar level can be computed.
Pre-condition The insulin reservoir contains at least the maximum allowed single dose of insulin
Post-condition r0 is replaced by r1 then r1 is replaced by r2
Side-effects None

Tabular specification
• Used to supplement natural language.
• Particularly useful when you have to define a number of possible
alternative courses of action.
• Example : Tabular specification of computation
Condition Action
Sugar level falling (r2 < r1) CompDose = 0
Sugar level stable (r2 = r1) CompDose = 0
Sugar level increasing and rate of increase
CompDose = 0
decreasing ((r2-r1)<(r1-r0))
CompDose = round ((r2-r1)/4)
Sugar level increasing and rate of increase
If rounded result = 0 then
stable or increasing. ((r2-r1) ≥ (r1-r0))
CompDose = MinimumDose


Graphical models
• Graphical models are most useful when you need to show
how state changes or where you need to describe a
sequence of actions.
• Example Sequence diagrams
• These show the sequence of events that take place during
some user interaction with a system.
• Read them from top to bottom to see the order of the
actions that take place.
• Cash withdrawal from an ATM
– Validate card : By checking the card number and user’s PIN
– Handle request : user requests are handled. Query database for withdrawal
– Complete transaction: return the card and deliver cash & receipt.

Sequence diagram of ATM withdrawal
ATM Database

Card
Card number
Card OK
PIN request
PIN
Option menu Validate card
<<exception>>
invalid card

Withdraw request Balance request
Balance
Amount request
Amount Handle request
Debit (amount)

<<exception>>
Debit response
Insufficient cash

Card

Card removed
Complete
Cash transaction
Cash removed
Receipt
42
Prof. Loganathan R., CSE, HKBKCE

Interface specification
• Most systems must operate with other systems
and the operating interfaces must be specified as
part of the requirements.
• Three types of interface may have to be defined
– Procedural interfaces : APIs;
– Data structures : Passed from one subsystem to another
– Data representations (ordering of bits) : for existing
subsystem
• Formal notations(Program Design Language - PDL)
are an effective technique for interface
specification.

PDL interface description
interface PrintServer {
// defines an abstract printer server
// requires: interface Printer, interface PrintDoc
// provides: initialize, print, displayPrintQueue, cancelPrintJob, switchPrinter
void initialize ( Printer p ) ;
void print ( Printer p, PrintDoc d ) ;
void displayPrintQueue ( Printer p ) ;
void cancelPrintJob (Printer p, PrintDoc d) ;
void switchPrinter (Printer p1, Printer p2, PrintDoc d) ;
} //PrintServer


The Software Requirements Document
• The requirements document is the official
statement of what is required of the system
developers.
• Should include both a definition of user
requirements and a specification of the system
requirements.
• It is NOT a design document. As far as possible, it
should set of WHAT the system should do rather
than HOW it should do it


Users of a requirements document
Specify the requirements and
read them to check that they
System meet their needs. They
Customers specify changes to the
requirements

Use the requirements
document to plan a bid for
Managers the system and to plan the
system development process

Use the requirements to
System understand what system is to
Engineers be developed

Use the requirements to
System Test develop validation tests for
Engineers the system

Use the requirements to help
System
understand the system and
Maintenance
the relationships between its
Engineers parts

46
Prof. Loganathan R., CSE, HKBKCE

IEEE requirements standard
• Defines a structure for a requirements document.
1. Introduction.
1.1 Purpose of the requirements document
1.2 Scope of the product
1.3 Definition, acronyms & abbreviations
1.4 References
1.5 Overview of the remainder of the document
2. General description.
2.1 Product perspective
2.2 Product functions
2.3 User characteristics
2.4 General constraints
2.5 Assumptions & dependencies
3. Specific requirements.(covers functional, non-functional & interface requirements)
4. Appendices.
5. Index.


Requirements document structure
• Organisation for a requirements document based on IEEE standard
– Preface : Defines expected readership, version history & changes in each version
– Introduction : Describes the needs of the system
– Glossary : Defines technical terms used in the document
– User requirements definition : Describes services provided & non-
functional requirements
– System architecture : Presents high level overview of system
– System requirements specification : Describes functional & non-
functional requirements in detail
– System models : Provides 1 or more system models
– System evolution : Describes anticipated changes
– Appendices
– Index

Software requirements

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