1. Conceptions

The process of establishing the services that a customer requires from a system and the constraints under which it operates and is developed.

The system requirements are the descriptions of the system services and constraints that are generated during the requirements engineering process.

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.

1.1 Types of requirements

(1) User Requirements: Statements in natural language plus diagrams of the services the system provides and its operational constraints. Written for customers.

(2) 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.

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.

User Requirement:

• client managers
• system end-users
• client engineers
• contractor managers
• system architects

System Requirements:

• system end-users
• client engineers
• system architects
• software developers

1.2 System Stakeholders

Any person or organization who is affected by the system in some way and so who has a legitimate interest. The stakeholder types:

• end users
• system managers
• system owners
• external stakeholders

1.3 Agile Methods and Requirements

(1) Many agile methods argue that producing detailed system requirements is a waste of time as requirements change so quickly.

(2) The requirements document is therefore always out of date.

(3) Agile methods usually use incremental requirements engineering and may express requirements as ‘user stories’.

(4) This is practical for business systems but problematic for systems that require pre-delivery analysis (e.g. critical systems) or systems developed by several teams.

2. Functional and non-functional requirements

2.1 Functional and non-functional requirements

(1) 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.

The 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.

(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 as a whole rather than individual features or services.

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.

(3) Domain Requirements

Constraints on the system from the domain of operation

2.2 Requirements Imprecision

(1) Problems arise when functional requirements are not precisely stated.

(2) Ambiguous requirements may be interpreted in different ways by developers and users.

2.3 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.

Because of system and environmental complexity, it is impossible to produce a complete and consistent requirements document.

2.4 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.

Non-functional classifications

• Product Requirements: Requirements which specify that the delivered product must behave in a particular way, execution speed, reliability, etc.
• Organisational Requirements: Requirements which are a consequence of organisational policies and procedures. process standard used, implementation requirements.
• External Requirements: Requirements which arise from factors which are external to the system and its development process e.g. interoperability requirements, legislative requirements.

2.5 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.

2.6 Metrics for Specifying non-functional requirements

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;

3. Requirements engineering processes

3.1 Requirements Engineering Processes

The processes used for RE vary widely depending on the application domain, the people involved and the organisation developing the requirements.

Generic Activities:

• Requirements Elicitation.
• Requirements Analysis.
• Requirements Validation.
• Requirements Management.

In practice, RE is an iterative activity in which these processes are interleaved.

3.1.1 Requirements Elicitation and Analysis

(1) Sometimes called requirements elicitation or requirements discovery.

(2) 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.

(3) May involve end-users, managers, engineers involved in maintenance, domain experts, trade unions, etc. These are called stakeholders.

A. 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

Processing Stages:

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

Problems:

(1) Stakeholders dont know what they really want.

(2) Stakeholders express requirements in their own terms.

(3) Different stakeholders may have conflicting requirements.

(4) Organisational and political factors may influence the system requirements.

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

B. Requirements Discovery

The process of :

• gathering information about the required and existing systems
• 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.

C. Interviewing

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 stakeholder.
• 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

• 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.
• Interviewers need to be open-minded without pre-conceived ideas of what the system should do.
• prompt the use to talk about the system by suggesting requirements rather than simply asking them what they want.

Problems

• Application specialists may use language to describe their work that isn’t easy for the requirements engineer to understand.
• Interviews are not good for understanding domain requirements

D. Stories and Scenarios

Scenarios and user stories are real-life examples of how a system can be used.

Stories and scenarios are a description of how a system may be used for a particular task.

Because they are based on a practical situation, stakeholders can relate to them and can comment on their situation with respect to the story.

(1) Scenarios

A structured form of user story. Scenarios 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.

4. Requirements Specification

4.1 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

Ways of writing a system requirements specification:

• Natural language
• Structured natural language
• Design description languages
• Graphics notations
• Mathematical specifications

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.
• Include an explanation (rationale) of why a requirement is necessary.

4.2 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.

4.3 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.
• Information about the information needed for the computation and other entities used.
• Description of the action to be taken.
• Pre and post conditions (if appropriate).
• The side effects (if any) of the function.

4.4 Tabular Specifications

• used to supplement natural language
• particularly useful when you have to define a number of possible alternative courses of action.

4.5 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.

5. Requirements Validation

5.1 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

5.2 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 functiosn required by the customer?

Realism: Can the requirements be implemented given available budget and technology?

Verifiability: Can the requirements be checked?

5.3 Requirements validation techniques

(1) 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.

(2) Prototyping

(3) Test-case Generation

5.4 Review Checks

• Verifiability
• Comprehensibility
• Traceability
• Adaptability

6. Requirements Change

6.1 Changing Requirements

Reasons:

• The business and technical environment of the system always changes after installation.
• The people who pay for a system and the users of that system are rarely the same people.
• Large systems usually have a diverse user community, with many users having different requirements and priorities that may be conflicting or contradictory.

6.2 Requirements Management

Requirements management is the process of managing changing requirements during the requirements engineering process and system development.

Reason: New requirements emerge as a system is being developed and after it has gone into use.