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Software Requirements

COMP201 Lectures

User Requirements

These are statements in natural language plus diagrams of services the system provides and its operational constraints.

This is written for customers.

System Requirements

A structured document setting out detailed descriptions of system services.

This can be used as a written contract between client and contractor.

Software Specification

A detailed software description which can serve as a basis for a design or implementation.

This is written for the developers.

Example

User requirement

  • We need to be able to spell check documents

System requirement

  • The system needs to be able to spell check documents and provide auto-correct facilities. Their will be support for the following languages, English, French and German will have plug in support for other languages.

Software specification

  • CheckResult spellCheck(String word, Dictionary dictionary)
    • Word is defined in UNICODE formatted string
    • The Dictionary structure is defined in S.1.2
    • The CheckResult is defined in S.1.3 and contains a flag if the word has been found or not, plus a Vector object containing a list of possible other word suggestions depending if the word has been found or not
    • spellCheck will ideally use Hashing tables to improve code efficiency
    • ……

Types of 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.

Non-functional requirements - Constraints of the services or functions offered by the system such as timing constraints and standards to be used:

These are usually defined on the system as a whole.

Domain Requirements - Come from the application domain of the system and reflect characteristics of that domain.

Product Requirements - Specify how the product must behave.

Organisational Requirements - A consequence of organisational policies and procedures.

External Requirements - Arise from factors external to the system such as legislation and interoperability requirements.

Completeness & Consistency

Requirements should be both complete and consistent:

  • Complete - Should include descriptions of all facilities required.
  • Consistent - There should be no conflicts in the descriptions of the system facilities.

Goals vs Requirements

A goal is a general intention of the user such as ease of use.

Goals are helpful to developers as they convey the intentions of the system users.

Requirement Measures

Verifiable non-functional requirements are a pass/fail test for a particular goal.

Example

Property Measure
Speeed Processed transactions/second, User/event response time; screen refresh time.