software Requirements functional and non-functional

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 1
Software Requirements
(utvalgte foiler fra Kap 6 og 7
i Sommerville)

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
1. The softw
are must pr
ovide a means of repr
esenting and
1. accessing e
xternal files cr
eated b
y other tools
.
1.1 The user should be pr
ovided with facilities to define the type of
1.2 external files
.
1.2 Each external file type ma
y have an associa
ted tool w
hich may be
1.2 applied to the file
.
1.3 Each external file type ma
y be represented as a specific icon on
1.2 the user’
s display
.
1.4 Facilities should be pr
ovided f
or the icon r
epr
esenting an
1.2 external file type to be defined b
y the user
.
1.5 When a user selects an icon r
epresenting an e
xternal file
, the
1.2 effect of that selection is to apply the tool associated with the type of
1.2 the external file to the file represented by the selected icon.
User requir
ement definition
System requirements specification

Requirements readers
Client managers
System end-users
Client eng
ineers
Contractor managers
System ar
chitects
System end-users
Client eng
ineers
System ar
chitects
Software developers
Client eng
ineers (perha
ps)
System ar
chitects
Software developers
User
requir
ements
System
requir
ements
Software design
specifica
tion

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.

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 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
Organisational
requirements
External
requirements
Non-functional
requirements

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

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.

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.

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 Š 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

Sequence diagram of ATM withdrawal
ATM Database
Card
Card number
Card OK
PIN request
PIN
Option menu
<<exception>>
invalid card
Withdraw request
Amount request
Amount
Balance request
Balance
<<exception>>
insufficient cash
Debit (amount)
Debit response
Card
Card removed
Cash
Cash removed
Receipt
Validate card
Handle request
Complete
transaction

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.

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
Use the requirements to
develop validation tests for
the system
Use the requirements
document to plan a bid for
the system and to plan the
system development process
Use the requirements to
understand what system is to
be developed
System test
eng
ineers
Managers
System
eng
ineers
Specify the requirements and
read them to check that they
meet their needs. T
hey
specify changes to the
requirements
System
customers
Use the requirements to help
understand the system and
the relationships between its
parts
System
maintenance
engineers

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.

The requirements engineering process
Feasibility
stud
y
Requir
ements
elicitation and
analysis
Requir
ements
specification
Requir
ements
validation
Feasibility
repor
t
System
models
User and system
requirements
Requir
ements
document

Requirements engineering
Requirements
specification
Requirements
validation
Requirements
elicitation
System requirements
specification and
modeling
System
requirements
elicitation
User requirements
specification
User
requirements
elicitation
Business requirements
specification
Prototyping
Feasibility
study
Reviews
System requirements
document

Problems of requirements analysis

Stakeholders don’t know what they really want.

Stakeholders express requirements in their own terms.

Different stakeholders may have conflicting requirements.

Organisational and political factors may influence the
system requirements.

The requirements change during the analysis process.
New stakeholders may emerge and the business
environment change.

The requirements spiral
Requirements
classification and
organisation
Requirements
prioritization and
negotiation
Requirements
documentation
Requirements
discovery

ATM stakeholders

Bank customers

Representatives of other banks

Bank managers

Counter staff

Database administrators

Security managers

Marketing department

Hardware and software maintenance engineers

Banking regulators

Viewpoints

Viewpoints are a way of structuring the
requirements to represent the perspectives of
different stakeholders. Stakeholders may be
classified under different viewpoints.

This multi-perspective analysis is important as
there is no single correct way to analyse system
requirements.

Types of viewpoint

Interactor viewpoints
• People or other systems that interact directly with the system.
In an ATM, the customer’s and the account database are
interactor VPs.

Indirect viewpoints
• Stakeholders who do not use the system themselves but who
influence the requirements. In an ATM, management and
security staff are indirect viewpoints.

Domain viewpoints
• Domain characteristics and constraints that influence the
requirements. In an ATM, an example would be standards for
inter-bank communications.

LIBSYS viewpoint hierarchy
Article
providers
Finance
Library
manager
Library
staff
Users
Interactor
Indirect
All VPs
Classification
system
UI
standards
Domain
External
Staff
Students Cataloguers
System
managers

Scenarios

Scenarios are real-life examples of how a system
can be used.

They should include
• A description of the starting situation;
• A description of the normal flow of events;
• A description of what can go wrong;
• Information about other concurrent activities;
• A description of the state when the scenario finishes.

Requirements checking

Validity. Does the system provide the functions which
best support the customer’s needs?

Consistency. Are there any requirements conflicts?

Completeness. Are all functions required by the customer
included?

Realism. Can the requirements be implemented given
available budget and technology

Verifiability. Can the requirements be checked?

software Requirements functional and non-functional

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