SoftEng Chap 4 - Lecture 1 (Requirements Engineering)

Requirements engineering

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

Requirements

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

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.

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.

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 user requirements

(FR) may be high-level statements of what the system should do.

Functional system requirements

(FR)should describe the system services in detail.

complete and consistent.

In principle, requirements should be both_ and _

Complete

They should include descriptions of all facilities required.

Consistent

There should be no conflicts or contradictions in the descriptions of the system facilities.

Process requirements

(NFR) may also be specified mandating a particular IDE, programming language or development method.

more critical

Non-functional requirements may be _ than functional requirements

overall architecture

Non-functional requirements may affect the _ of a system rather than the individual components.

- Product requirements
- Operational requirements
- External 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.

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.

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.

a number of related functional requirements

A single non-functional requirement, such as a security
requirement, may generate _ that define system services that
are required.

Properties:
Speed
Size
Ease of Use
Reliability
Robustness
Portability

Metrics for specifying nonfunctional requirements*

- Processed transactions/seconds
- User/event response time
-Sscreen refresh time

(Metrics) Speed*

- Mbytes
- Number of ROM chips

(Metrics) Size*

- Training time
- Number of help frames

(Metrics) Ease of use*

- Mean time to failure
- Probability of unavailability
- Rate of failure occurrence
- Availability

(Metrics) Reliability*

- Time to restart after failure
- Percentage of events causing failure
- Probability of data corruption on failure

(Metrics) Robustness*

- Percentage of target dependent statements
- Number of target systems

(Metrics) Portability*