Software requirement and specification

Software Requirement and
Specification
1

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.
2

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.
3

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 organization’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.”
4

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.
5

User and system requirements
6

Readers of different types of
requirements specification
7

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.
– May state what the system 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.
– Often apply to the system as a whole rather than
individual features or services.
• Domain requirements
– Constraints on the system from the domain of operation
8

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.
• Functional system requirements should describe the system
services in detail.
9

Functional requirements: An Example
• A user shall be able to search the appointments lists for all
clinics.
• The system shall generate each day, for each clinic, a list of
patients who are expected to attend appointments that day.
• Each staff member using the system shall be uniquely
identified by his or her 8-digit employee number.
10

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 ‘search’ in previous slide requirement
– User intention – search for a patient name across all
appointments in all clinics;
– Developer interpretation – search for a patient name
in an individual clinic. User chooses clinic then search.
11

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.
12

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 IDE, programming language or development
method.
• Non-functional requirements may be more critical than
functional requirements.
– If these are not met, the system may be useless.
13

Types of nonfunctional requirement
14

Non-functional requirements
implementation
• Non-functional requirements may affect the overall
architecture of a system rather than the individual
components.
– For example, to ensure that performance requirements are
met, you may have to organize the system to minimize
communications between components.
• A single non-functional requirement, such as a security
requirement, may generate a number of related
functional requirements that define system services
that are required.
– It may also generate requirements that restrict existing
requirements.
15

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.
16

Examples of nonfunctional requirements
Product requirement
The MHC-PMS shall be available to all clinics during normal working
hours (Mon–Fri, 0830–17.30). Downtime within normal working hours
shall not exceed five seconds in any one day.
Organizational requirement
Users of the MHC-PMS system shall authenticate themselves using
their health authority identity card.
External requirement
The system shall implement patient privacy provisions as set out in
HStan-03-2006-priv.
17

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.
18

Usability requirements
• The system should be easy to use by medical staff and should
be organized in such a way that user errors are minimized.
(Goal)
• Medical staff shall be able to use all the system functions after
four hours of training. After this training, the average number
of errors made by experienced users shall not exceed two per
hour of system use. (Testable non-functional requirement)
19

Metrics for specifying nonfunctional
requirements
Property Measure
Speed Processed transactions/second
User/event response time
Screen refresh time
Size Mbytes
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
20

Domain Requirement
• The system’s operational domain imposes
requirements on the system.
– For example, a train control system has to take into
account the braking characteristics in different
weather conditions.
• 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.
21

The software requirements document
• The software 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.
23

Agile methods and requirements
• Many agile methods argue that producing a
requirements document is a waste of time as
requirements change so quickly.
• The document is therefore always out of date.
• Methods such as XP use incremental requirements
engineering and express requirements as ‘user stories’
• This is practical for business systems but problematic for
systems that require a lot of pre-delivery analysis (e.g.
critical systems) or systems developed by several teams.
24

Users of a requirements document
25

Requirements document variability
• Information in requirements document depends on type
of system and the approach to development used.
• Systems developed incrementally will, typically, have less
detail in the requirements document.
• Requirements documents standards have been designed
e.g. IEEE standard.
• These are mostly applicable to the requirements for large
systems engineering projects.
26

The structure of a requirements
document
Chapter Description
Preface This should define the expected readership of the document and describe
its version history, including a rationale for the creation of a new version
and a summary of the changes made in each version.
Introduction This should describe the need for the system. It should briefly describe the
system’s functions and explain how it will work with other systems. It
should also describe how the system fits into the overall business or
strategic objectives of the organization commissioning the software.
Glossary This should define the technical terms used in the document. You should
not make assumptions about the experience or expertise of the reader.
User requirements
definition
Here, you describe the services provided for the user. The nonfunctional
system requirements should also be described in this section. This
description may use natural language, diagrams, or other notations that are
understandable to customers. Product and process standards that must be
followed should be specified.
System architecture This chapter should present a high-level overview of the anticipated system
architecture, showing the distribution of functions across system modules.
Architectural components that are reused should be highlighted.
27

The structure of a requirements document
Chapter Description
System
requirements
specification
This should describe the functional and nonfunctional requirements in more detail.
If necessary, further detail may also be added to the nonfunctional requirements.
Interfaces to other systems may be defined.
System models This might include graphical system models showing the relationships between
the system components and the system and its environment. Examples of
possible models are object models, data-flow models, or semantic data models.
System evolution This should describe the fundamental assumptions on which the system is based,
and any anticipated changes due to hardware evolution, changing user needs,
and so on. This section is useful for system designers as it may help them avoid
design decisions that would constrain likely future changes to the system.
Appendices These should provide detailed, specific information that is related to the
application being developed; for example, hardware and database descriptions.
Hardware requirements define the minimal and optimal configurations for the
system. Database requirements define the logical organization of the data used
by the system and the relationships between data.
Index Several indexes to the document may be included. As well as a normal alphabetic
index, there may be an index of diagrams, an index of functions, and so on.
28

Requirements specification
• The process of writing don the user and system requirements
in a requirements document.
• User requirements have to be understandable by end-users
and customers who do not have a technical background.
• System requirements are more detailed requirements and
may include more technical information.
• The requirements may be part of a contract for the system
development
– It is therefore important that these are as complete as
possible
29

Ways of writing a system requirements
specification
Notation Description
Natural language The requirements are written using numbered sentences in natural language.
Each sentence should express one requirement.
Structured natural
language
The requirements are written in natural language on a standard form or
template. Each field provides information about an aspect of the
requirement.
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 now rarely used although it can be
useful for interface specifications.
Graphical notations Graphical models, supplemented by text annotations, are used to define the
functional requirements for the system; UML use case and sequence
diagrams are commonly used.
Mathematical
specifications
These notations are based on mathematical concepts such as finite-state
machines or sets. Although these unambiguous specifications can reduce
the ambiguity in a requirements document, most customers don’t understand
a formal specification. They cannot check that it represents what they want
and are reluctant to accept it as a system contract
30

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 architecture to satisfy non-functional
requirements may be a domain requirement.
– This may be the consequence of a regulatory requirement.
31

Requirement Engineering Process
• The processes used for RE vary widely depending on the
application domain, the people involved and the
organisation developing the requirements.
• However, there are a number of generic activities
common to all processes
– Requirements elicitation;
– Requirements analysis;
– Requirements validation;
– Requirements management.
• In practice, RE is an iterative activity in which these
processes are interleaved.
32

The Requirements Engineering Process
33

Feasibility Study
• A feasibility study decides whether or not the proposed
system is worthwhile.
• A short focused study that checks
– If the system contributes to organisational objectives;
– If the system can be engineered using current technology
and within budget;
– If the system can be integrated with other systems that are
used.
34

Feasibility Study Implementation
• Based on information assessment (what is required),
information collection and report writing.
• Questions for people in the organisation
– What if the system wasn’t implemented?
– What are current process problems?
– How will the proposed system help?
– What will be the integration problems?
– Is new technology needed? What skills?
– What facilities must be supported by the proposed system?
35

Elicitation and Analysis
• Sometimes called requirements elicitation or requirements
discovery.
• Involves technical staff working with customers to find out
about the application domain, the services that the system
should provide and the system’s operational constraints.
• May involve end-users, managers, engineers involved in
maintenance, domain experts, trade unions, etc. These are
called stakeholders.
36

Problems of Requirement 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.
37

Requirement Validation
• Concerned with demonstrating that the requirements define
the system that the customer really wants.
• Requirements error costs are high so validation is very
important
– Fixing a requirements error after delivery may cost up to
100 times the cost of fixing an implementation error.
38

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?
39

Requirement Validation Technique
• Requirements reviews
– Systematic manual analysis of the requirements.
• Prototyping
– Using an executable model of the system to check
requirements.
• Test-case generation
– Developing tests for requirements to check testability.
40

Spiral View of Requirement Engineering
Process
41

Requirements elicitation and analysis
• Sometimes called requirements elicitation or requirements
discovery.
• Involves technical staff working with customers to find out
about the application domain, the services that the system
should provide and the system’s operational constraints.
• May involve end-users, managers, engineers involved in
maintenance, domain experts, trade unions, etc. These are
called stakeholders.
42

Problems of requirements analysis
• 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 may change.
43

Requirements elicitation and analysis
• Software engineers work with a range of system stakeholders
to find out about the application domain, the services that the
system should provide, the required system performance,
hardware constraints, other systems, etc.
• Stages include:
– Requirements discovery,
– Requirements classification and organization,
– Requirements prioritization and negotiation,
– Requirements specification.
44

The requirements elicitation and analysis
process
45

Process activities
• Requirements discovery
– Interacting with stakeholders to discover their requirements. Domain
requirements are also discovered at this stage.
• Requirements classification and organisation
– Groups related requirements and organises them into coherent
clusters.
• Prioritisation and negotiation
– Prioritising requirements and resolving requirements conflicts.
• Requirements specification
– Requirements are documented and input into the next round of the
spiral.
46

Problems of requirements elicitation
• 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.
47

Requirements Discovery
• The process of gathering information about the required and
existing systems and distilling the user and system
requirements from this information.
• Interaction is with system stakeholders from managers to
external regulators.
• Systems normally have a range of stakeholders.
49

Requirements validation
• Concerned with demonstrating that the requirements define
the system that the customer really wants.
• Requirements error costs are high so validation is very
important
– Fixing a requirements error after delivery may cost up to
100 times the cost of fixing an implementation error.
50

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?
51

Requirements Validation Techniques
• Requirements reviews
– Systematic manual analysis of the requirements.
• Prototyping
– Using an executable model of the system to check
requirements.
• Test-case generation
– Developing tests for requirements to check testability.
52

Requirements reviews
• Regular reviews should be held while the requirements
definition is being formulated.
• Both client and contractor staff should be involved in reviews.
• Reviews may be formal (with completed documents) or
informal. Good communications between developers,
customers and users can resolve problems at an early stage.
53

Review checks
• Verifiability
– Is the requirement realistically testable?
• Comprehensibility
– Is the requirement properly understood?
• Traceability
– Is the origin of the requirement clearly stated?
• Adaptability
– Can the requirement be changed without a large impact
on other requirements?
54

Requirements Management
• Requirements management is the process of managing
changing requirements during the requirements engineering
process and system development.
• New requirements emerge as a system is being developed
and after it has gone into use.
• You need to keep track of individual requirements and
maintain links between dependent requirements so that you
can assess the impact of requirements changes.
• You need to establish a formal process for making change
proposals and linking these to system requirements.
55

Changing Requirements
• The business and technical environment of the system always changes
after installation.
– New hardware may be introduced, it may be necessary to interface
the system with other systems, business priorities may change (with
consequent changes in the system support required), and new
legislation and regulations may be introduced that the system must
necessarily abide by.
• The people who pay for a system and the users of that system are rarely
the same people.
– System customers impose requirements because of organizational and
budgetary constraints. These may conflict with end-user requirements
and, after delivery, new features may have to be added for user
support if the system is to meet its
56

Changing Requirements
• Large systems usually have a diverse user community, with
many users having different requirements and priorities that
may be conflicting or contradictory.
– The final system requirements are inevitably a compromise
between them and, with experience, it is often discovered
that the balance of support given to different users has to
be changed.
57

Requirements Management Planning
• Establishes the level of requirements management detail that is
required.
• Requirements management decisions:
– Requirements identification Each requirement must be uniquely
identified so that it can be cross-referenced with other requirements.
– A change management process This is the set of activities that assess
the impact and cost of changes. I discuss this process in more detail in
the following section.
– Traceability policies These policies define the relationships between
each requirement and between the requirements and the system
design that should be recorded.
– Tool support Tools that may be used range from specialist
requirements management systems to spreadsheets and simple
database systems.
59

Requirements Change Management
• Deciding if a requirements change should be accepted
– Problem analysis and change specification
• During this stage, the problem or the change proposal is analyzed
to check that it is valid. This analysis is fed back to the change
requestor who may respond with a more specific requirements
change proposal, or decide to withdraw the request.
– Change analysis and costing
• The effect of the proposed change is assessed using traceability
information and general knowledge of the system requirements.
Once this analysis is completed, a decision is made whether or not
to proceed with the requirements change.
– Change implementation
• The requirements document and, where necessary, the system
design and implementation, are modified. Ideally, the document
should be organized so that changes can be easily implemented.
60

Software requirement and specification

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Software requirement and specification