Software Engineering - Ch6

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 1
Software Requirements

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

Topics covered
● Functional and non-functional requirements
● User requirements
● System requirements
● Interface specification
● The software requirements document

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.

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.

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
• 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.

Definitions and specifications

Requirements readers

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.
● Non-functional requirements
• constraints on the services or functions offered by the system
such as timing constraints, constraints on the development
process, standards, etc.
● Domain requirements
• Requirements that come from the application domain of the
system and that reflect characteristics of that domain.

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 but
functional system requirements should describe
the system services in detail.

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
● 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.
● 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.

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 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 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.

Non-functional requirement types

Non-functional requirements examples
● 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.

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.

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
Speed Processed transactions/second
User/Event response time
Screen refresh time
Size M Bytes
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

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?

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.

Library system 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.

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.

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
● 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.

LIBSYS requirement
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.

Editor grid requirement
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 (the need for a grid);
• Non-functional requirement (grid units);
• Non-functional UI requirement (grid switching).

Structured presentation
2.6.1 Grid facilities
The editor shall provide a grid facility where a m atrix 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 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 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.
● System requirements may be defined or
illustrated using system models discussed in
Chapter 8.

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
● 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.

Alternatives to NL specification
Notation Description
Structured natural
language
This approach depends on defining standard forms or templates to express the
requirements specification.
Design
description
languages
This approach uses a language like a programming language but with more 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
specifications
These are notations based on mathematical concepts such as finite-state 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 specifications
● Definition of the function or entity.
● Description of inputs and where they come from.
● Description of outputs and where they go to.
● Indication of other entities required.
● Pre and post conditions (if appropriate).
● The side effects (if any) of the function.

Form-based node specification
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.
OutputsCompDose – 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.

Tabular specification
Condition Action
Sugar level falling (r2 < r1) CompDose = 0
Sugar level stable (r2 = r1) CompDose = 0
Sugar level increasing and rate of
increase decreasing ((r2-r1)<(r1-r0))
CompDose = 0
Sugar level increasing and rate of
increase stable or increasing. ((r2-r1) ≥
(r1-r0))
CompDose = round ((r2-r1)/4)
If rounded result = 0 then
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.
● Different graphical models are explained in
Chapter 8.

Sequence diagrams
● These show the sequence of events that take
place during some user interaction with a
system.
● You read them from top to bottom to see the
order of the actions that take place.
● Cash withdrawal from an ATM
• Validate card;
• Handle request;
• Complete transaction.

Sequence diagram of ATM withdrawal

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;
• Data structures that are exchanged;
• Data representations.
● Formal notations 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 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

IEEE requirements standard
● Defines a generic structure for a requirements
document that must be instantiated for each
specific system.
• Introduction.
• General description.
• Specific requirements.
• Appendices.
• Index.

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

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
using natural language, tables and diagrams.

Key points
● System requirements are intended to
communicate the functions that the system
should provide.
● A software requirements document is an agreed
statement of the system requirements.
● The IEEE standard is a useful starting point for
defining more detailed specific requirements
standards.

Software Engineering - Ch6

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Software Engineering - Ch6