What is Requirement Engineering? - Requirement Engineering Lessons and Short Note

- What is Requirement Engineering?
- What is the Requirement?
- Types of Requirements
- Functional & Non-functional Requirements
- Requirement Engineering Process

1. What is Requirement 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 the 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.

3. Types of Requirements

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.



4. 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

4.1 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.

Requirement Imprecision

  • Problems arise when requirements are not precisely stated.
  • Ambiguous requirements may be interpreted in different ways by developers and users.

Requirements completeness and consistency

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.

4.2 Non-functional requirements

  • They 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 Non-functional requirements



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.

Non-functional requirements implementation

Non-functional requirements may affect the overall architecture of a system rather than the individual 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. 

5. Software Requirement Specification

  • 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 WHAT the system should do rather than HOW it should do it.
  • Information in requirements document depends on the 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.
The structure of the requirement document is as follows.
  • Preface
  • Introduction
  • Glossary
  • User Requirements
  • System Architecture
  • System Requirement Specification
  • System Models
  • System Evolution
  • Appendices
  • Index

6. 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.

7. Users of a requirements document 


8. Requirements specification

  • The process of writing 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.
  • Requirements can be specified by using different methods.

9. Requirements & 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.
    • System architecture may be designed to structure the requirements.
    • The system may inter-operate with other systems that generate design requirements.

10. Requirement Engineering Process

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

However, there are a number of generic activities common to all processes.
  • Requirements elicitation
  • Requirements analysis
  • Requirements validation
  • Requirements management

10.1 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. They are called stakeholders.
  • 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.

11. Requirement 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.

Consider the following Patients Management System.

There are several stakeholders,
  • Patients
  • Doctors
  • Nurses
  • Medical Receptionists
  • IT Staff
  • Medical ethics manager 
  • Health care managers
  • Medical records staff

12. Interviewing

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

Types of interview

Closed interviews based on a 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 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.

13. 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.

4. 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. 

15. Ethnography

A social scientist spends considerable time observing and analyzing how people actually 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.

16. Requirements Validation

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.

There are several factors that are considered in requirements validation.
  • Validity
  • Consistency
  • Completeness
  • Realism
  • Verifiability
Several techniques can be used in requirement validation.

Requirements reviews

  • Systematic manual analysis of the requirements.
  • Regular reviews should be held while the requirements definition is being formulated.
  • Both client and contractor staff should be involved in reviews.
  • May be formal or informal.

Prototyping

  • Using an executable model of the system to check requirements. 

Test-case generation

  • Developing tests for requirements to check testability.

17. 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.

Requirements evolution can be represented as follows.


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