Revision

Chapter 4
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.

1. May be the basis for a bid for a contract – therefore must be open to
interpretation.

2. May be the basis for the contract itself – therefore must be defined in detail.

3. Both these statements may be called requirements.

Types of requirements
User Requirements System Requirements

Statements in natural language plus diagrams A structured document setting out detailed
of the services the system provides and its descriptions of the system’s functions,
operational constraints. Written for services, and operational constraints. Defines
customers. what should be implemented so may be part
1. Client Managers. of a contract between client and contractor.
2. System End-Users. 1. System End-users.
3. Client Engineers. 2. Client Engineers.
4. Contractor Managers. 3. System Architect.
5. System Architecture. 4. Software developers.
Functional and non-functional requirements
1. Functional requirements
✓ Statements of services the system should provide, how the system should react to
inputs and how the system should behave situations.

✓ May state what the system should not do.

2. 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 rather than individual features or services.

3. Domain requirements
✓ Constraints on the system from the domain of operation.

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.

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.

✓ 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.

Requirements completeness and consistency

• In principle, requirements should be both complete and consistent.
1. Complete
✓ They should include descriptions of all facilities required.

2. 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.

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

several related functional requirements that define system services that are required.

✓ It may also generate requirements that restrict existing requirements.

Non-functional classifications
1. Product requirements
• Requirements which specify that the delivered product must behave in a particular way
e.g., execution speed, reliability, etc.

2. Organizational requirements
• Requirements which are a consequence of organizational policies and procedures e.g.,
process standards used, implementation requirements, etc.

3. External requirements
• Requirements which arise from factors which are external to the system and its
development process e.g., interoperability requirements, legislative requirements, etc.

Goals and requirements

• Non-functional requirements may be very difficult to state precisely, and imprecise
requirements may be difficult to verify.

1. Goal
✓ A general intention of the user such as ease of use.

2. 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)
Domain requirements
• The system’s operational domain imposes requirements on the system.
✓ For example, a train control system must consider 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.

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

1. Understandability
✓ Requirements are expressed in the language of the application domain.

✓ This is often not understood by software engineers developing the system.

2. Implicitness
✓ Domain specialists understand the area so well that they do not think of making the
domain requirements explicit.
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.

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.

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.

Requirements specification
• The process of writing down the user and system requirements in a requirements
document.
 • User requirements must 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.

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:

1. A system architecture may be designed to structure the requirements.

2. The system may inter-operate with other systems that generate design requirements.

3. 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.
Problems with natural language
1. Lack of clarity
✓ Precision is difficult without making the document difficult to read.

2. Requirements confusion
✓ Functional and non-functional requirements tend to be mixed-up.

3. Requirements amalgamation
✓ Several different requirements may be expressed together.

Structured specifications
• An approach to writing requirements where the freedom of the requirements writer is
limited, and requirements are written in a standard way.

• This works well for some types of requirements e.g., requirements for embedded control
system but is sometimes too rigid for writing business system requirements.

Form-based specifications
1. Definition of the function or entity.

2. Description of inputs and where they come from.

3. Description of outputs and where they go to.

4. Information about the information needed for the computation and other entities used.

5. Description of the action to be taken.

6. Pre and post conditions (if appropriate).

7. The side effects (if any) of the function.

Requirements engineering processes.
• The processes used for RE vary widely depending on the application domain, the people
involved and the organization developing the requirements.

• However, there are several generic activities common to all processes:

1. Requirements elicitation.
2. Requirements analysis.
3. Requirements validation.
4. 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 of requirements analysis

• Stakeholders don’t know what they really want.

• Stakeholders’ express requirements in their own terms.

• Different stakeholders may have conflicting requirements.

• Organizational 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.
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:
1. Requirements discovery,
2. Requirements classification and organization,
3. Requirements prioritization and negotiation,
4. Requirements specification.

Process activities
1. Requirements discovery
✓ Interacting with stakeholders to discover their requirements. Domain requirements
are also discovered at this stage.

2. Requirements classification and organization

✓ Groups related requirements and organized them into coherent clusters.

3. Prioritization and negotiation

✓ Prioritizing requirements and resolving requirements conflicts.

4. Requirements specification
✓ Requirements are documented and input into the next round of the spiral.

Problems of requirements elicitation

1. Stakeholders don’t know what they really want.

2. Stakeholders’ express requirements in their own terms.

3. Different stakeholders may have conflicting requirements.

 4. Organizational and political factors may influence the system requirements.

5. The requirements change during the analysis process. New stakeholders may
emerge and the business environment change.

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.

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

• Types of interviews
1. Closed interviews based on pre-determined list of questions.
2. 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.

Scenarios
• Scenarios are real-life examples of how a system can be used.

• They should include:

1. A description of the starting situation.
2. A description of the normal flow of events.
3. A description of what can go wrong.
4. Information about other concurrent activities.
5. A description of the state when the scenario finishes.

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.

• High-level graphical model supplemented by more detailed tabular description (see

Chapter 5).

• Sequence diagrams may be used to add detail to use-cases by showing the sequence of
event processing in the system.

Ethnography
• A social scientist spends a considerable time observing and analyzing how people
work.

• People do not have to explain or articulate their work.

 • Social and organizational factors of importance may be observed.

• Ethnographic studies have shown that work is usually richer and more complex than
suggested by simple system models.

Scope of ethnography
• Requirements that are derived from the way that people work rather than the way in
which process definitions suggest that they ought to work.

• Requirements that are derived from cooperation and awareness of other people’s
activities.

• Awareness of what other people is doing leads to changes in the ways in which we do
things.

• Ethnography is effective for understanding existing processes but cannot identify new
features that should be added to a system.

Focused ethnography
• Developed in a project studying the air traffic control process.

• Combines ethnography with prototyping.

• Prototype development results in unanswered questions that focus on the ethnographic

analysis.

• The problem with ethnography is that it studies existing practices that may have some
historical basis that is no longer relevant.

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.

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

Requirements validation techniques

1. Requirements reviews
✓ Systematic manual analysis of the requirements.

2. Prototyping
✓ Using an executable model of the system to check requirements.

3. Test-case generation
✓ Developing tests for requirements to check testability.

Review checks
1. Verifiability
✓ Is the requirement realistically testable?

2. Comprehensibility
✓ Is the requirement properly understood?

3. Traceability
✓ Is the origin of the requirement clearly stated?

4. Adaptability
✓ Can the requirement be changed without a large impact on other requirements?
Chapter Questions
1. https://quizlet.com/3231158/software-engineering-9th-ed-by-sommerville-chapter-
4-flash-cards/
2. https://quizlet.com/207492680/software-engineering-9th-ed-by-sommerville-
chapter-4-flash-cards/
3. https://quizlet.com/39441603/software-engineering-9th-ed-by-sommerville-
chapter-4-flash-cards/