REQUIREMENTS

ENGINEERING
FUNCTIONAL AND NON FUNCTIONAL
Outline

 Functional and non-functional requirements

 The software requirements specification document (SRS)
 Requirements specification
 Requirements engineering processes
 Requirements elicitation and analysis
 Requirements validation
 Requirements management
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;
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.
User and system requirements
Readers of different types of requirements
specification
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.
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.
Functional requirements for the MHC-
PMS

 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.
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 requirement 1
 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.
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 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.
Types of nonfunctional requirement
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.
Non-functional classifications

 Product requirements
 Requirements which specify that the delivered product must behave in a particular way
e.g. execution speed, reliability, etc.
 Organizational requirements
 Requirements which are a consequence of organizational 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.
Examples:

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.
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.
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)
Metrics for non-functional requirements
Domain requirements

 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 can be new functional requirements, constraints on
existing requirements, or define specific computations.
 If domain requirements are not satisfied, the system may be unworkable
Train protection system

 This is a domain requirement for a 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.
 It is difficult for a non-specialist to understand the implications of this and how it
interacts with other requirements.
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.
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
The software requirements document

 The software requirements document is the official statement of what is required

of the system developers.
 Can 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
Requirements specification

 The process of writing down 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
Ways of writing a system requirements
specification
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.
Natural language specification

 Requirements are written as natural language sentences supplemented by

diagrams and tables.
 Used for writing requirements because it is expressive, intuitive and universal.
This means that the requirements can be understood by users and customers.
Guidelines for writing requirements

 Create 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.
 Include an explanation (rationale) of why a requirement is necessary.
Example requirements for the insulin
pump software system

3.2 The system shall measure the blood sugar and deliver insulin, if required, every 10
minutes. (Changes in blood sugar are relatively slow so more frequent measurement is
unnecessary; less frequent measurement could lead to unnecessarily high sugar levels.)

3.6 The system shall run a self-test routine every minute with the conditions to be tested
and the associated actions defined in Table 1. (A self-test routine can discover hardware and
software problems and alert the user to the fact the normal operation may be impossible.)
Requirements engineering processes

 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.
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.
Problems with the requirements
gathering

 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 may change.
The requirements elicitation

 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
The requirements elicitation
Methods for requirements
gathering/discovery

 Interview
 Use case
 Observations
 Literature study
Interview

 Formal or informal interviews with stakeholders are part of most RE processes.

 Types of interview
 Closed interviews based on pre-determined list of questions
 Open interviews where various issues are explored with stakeholders.
 Effective interviewing
 Be open-minded, avoid pre-conceived ideas about the requirements and are willing to
listen to stakeholders.
 Prompt the interviewee to get discussions going using a springboard question, a
requirements proposal, or by working together on a prototype system.
Interviews in practice

 Normally a mix of closed and open-ended interviewing.

 Interviews are good for getting an overall understanding of what stakeholders do
and how they might interact with the system.
 Interviews are not good for understanding domain requirements
 Requirements engineers cannot understand specific domain terminology;
 Some domain knowledge is so familiar that people find it hard to articulate or think that
it isn’t worth articulating.
Use cases

 Use-cases are a scenario based technique in the UML which identify the actors in
an interaction and which describe the interaction itself.
 A set of use cases should describe all possible interactions with the system.
 Sequence diagrams may be used to add detail to use-cases by showing the
sequence of event processing in the system.
Use cases for the MHC-PMS
Ethnography

 A social scientist spends a considerable time observing and analysing how

people actually work.
 People do not have to explain or articulate their work.
 Social and organisational factors of importance may be observed.
 Ethnographic studies have shown that work is usually richer and more complex
than suggested by simple system models.
Requirements analysis

 Requirements classification and organization

 Groups related requirements and organizes them into coherent clusters.
 Prioritization and negotiation
 Prioritizing requirements and resolving requirements conflicts.
 Requirements specification
 Requirements are documented and input into the next round of the spiral.
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.
How to validate?

 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?
Properties of a good requirement

 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?
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.
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 goals.
 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.
Overview of SRS document