Requirements

Reference: Chapters 5, 6, & 8

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Objectives

• To introduce the concepts of user and system requirements

• To explain functional and non-functional requirements• To present guidelines for writing system requirements• To introduce the concept of use cases for describing

functional requirements• To discuss prototyping as a means for requirements

elicitation

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Requirements Engineering

• The process of establishing– the services that the required of the system,– and the constraints under which it operates and

is developed

• The requirements themselves are the descriptions of the system services and constraints.

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Who’s Who?• Client – the person(s) paying for the development

and will become the owner of the product• Customer – the person who will buy the product

off the shelf (mass marketing), or who has the final say as to whether the product is acceptable (in-house development). May be the same as the client

• Stakeholder – Anyone who should have some direct or indirect influence on the system requirements

Reference: Mastering the Requirements Process, Robertson and Robertson

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Types of Requirements• User requirements

– Should describe requirements so that they are understandable by those who do not have detailed technical knowledge. Written mainly for customers (end users)

• System requirements– A structured document setting out detailed descriptions

of the system services and constraints. Written as a contract between client and contractor

• Software specification– A detailed software description that can serve as a basis

for a more detailed design. Adds further detail to the systems requirements. Written for developers

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Requirements Readers

Client managersSystem end-usersClient engineersContractor managersSystem architects

System end-usersClient engineersSystem architectsSoftware developers

Client engineers (perhaps)System architectsSoftware developers

User requirements

System requirements

Software designspecification

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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

• Non-functional requirements– constraints on the services or functions offered by

the system such as timing constraints, constraints on the development process, standards, etc.

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Functional Requirements

• Describe functionality or system services• These services depend on

– the type of software being developed– the expected users of the software

• Functional user requirements may be high-level statements of what the system should do, but functional system requirements should describe the system services in detail.

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Examples of Functional Requirements(written as user requirements)

• The user shall be able to search either all of the initial set of databases or select a subset from it.

• The system shall provide appropriate viewers for the user to read documents in the document store.

• Every order shall be allocated a unique identifier which the user shall be able to copy to the account’s permanent storage area.

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Requirements Imprecision

• Problems arise when requirements are not precisely stated

• Ambiguous requirements may be interpreted in different ways by developers and clients

• Consider the term ‘appropriate viewers’– Client intention - special purpose viewer for each different

document type

– Developer interpretation - Provide a text viewer that shows the contents of the document

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Requirements Completeness and Consistency

• In principle, requirements should be both complete and consistent

• Complete– They should include descriptions of all required

functionality

• Consistent– There should be no conflicts or contradictions in the

descriptions of the system functions

• In practice, it is impossible to produce a complete and consistent requirements document

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Non-functional Requirements

• Define system properties and constraints• Examples:

– response time

– storage requirements

– process requirements (e.g., must use a particular CASE system, programming language, or development method)

• Non-functional requirements may be more critical than functional requirements. If one is not met, the system may be useless.

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Non-functional Requirements Examples

• Product requirement– “It shall be possible for all necessary communication between the

APSE and the user to be expressed in the standard Ada character set.”

• Organizational requirement– “The system development process and deliverable documents

shall conform to the process and deliverables defined in XYZCo-SP-STAN-95”

• External requirement– “The system shall not disclose any personal information about

customers apart from their name and reference number to the operators of the system”

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Goals and Requirements• Non-functional requirements may be very difficult to

state precisely. Imprecise requirements are difficult to verify.

• 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

• Goals may be helpful to developers as they convey the intentions of the system users. But all requirements, functional or non-functional, MUST be verifiable.

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Example

• A system goal– “The system should be easy to use by experienced

controllers and should be organized in such a way that user errors are minimized.”

• A verifiable non-functional requirement– “Experienced controllers shall be able to use all the

system functions after a total of two hours training. After this training, the average number of errors made by experienced users shall not exceed two per day.”

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Some Requirements Measures

Property MeasureSpeed Processed transactions/second

User/Event response timeScreen refresh time

Size K BytesNumber of RAM chips

Ease of use Training timeNumber of help frames

Reliability Mean time to failureProbability of unavailabilityRate of failure occurrenceAvailability

Robustness Time to restart after failurePercentage of events causing failureProbability of data corruption on failure

Portability Percentage of target dependent statementsNumber of target systems

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Writing Requirements

• Requirements may be written using a natural language (common for user requirements)– Lack of clarity, possibly ambiguous– Requirements confusion

• Functional and non-functional requirements tend to be mixed-up

– Requirements amalgamation• Several different requirements may be expressed

together

• Tables and diagrams may help

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Example:Editor Grid Requirement

2.6 Grid facilities To assist in the positioning of entities on a diagram, the user may turn on a grid in either centimetres or inches, via an option on the control panel. Initially, the grid is off. The grid may be turned on and off at any time during an editing session and can be toggled between inches and centimetres at any time. A grid option will be provided on the reduce-to-fit view but the number of grid lines shown will be reduced to avoid filling the smaller diagram with grid lines.

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Problems!

• Difficult to read

• Mixes three different requirements:– Functional requirement (the need for a grid)– Non-functional requirement (grid units)– Non-functional UI requirement (grid switching)

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Alternatives to NL SpecificationNotation DescriptionStructurednaturallanguage

This approach depends on defining standard forms ortemplates to express the requirements specification.

Designdescriptionlanguages

This approach uses a language like a programming languagebut with more abstract features to specify the requirementsby defining an operational model of the system.

Graphicalnotations

A graphical language, supplemented by text annotations isused to define the functional requirements for the system.An early example of such a graphical language was SADT(Ross, 1977; Schoman and Ross, 1977). More recently, use-case descriptions (Jacobsen, Christerson et al., 1993) havebeen used. I discuss these in the following chapter.

Mathematicalspecifications

These are notations based on mathematical concepts suchas finite-state machines or sets. These unambiguousspecifications reduce the arguments between customer andcontractor about system functionality. However, mostcustomers don’t understand formal specifications and arereluctant to accept it as a system contract. I discuss formalspecification in Chapter 9.

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Some Guidelines for Writing Requirements

• Invent or find a standard format and use it for all requirements.

• Use language in a consistent way. • Use “shall” or “will” for mandatory requirements,

“should” for desirable requirements.• Use text highlighting (e.g., italics) to identify key parts

of the requirement.• Do not use vague phrases (e.g., “around a month,”

“have basic knowledge of”)• Every requirement must be verifiable.• Every requirement should be numbered for traceability.

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Use CasesA way of describing a system’s functional requirements• Describes the system’s behavior under various

conditions as the system responds to a request from one of the stakeholders called the primary actor.

• The primary actor initiates some interaction with the system to accomplish some goal.

• The system responds, protecting the interests of the stakeholders.

• Different sequences of behaviors (scenarios) can unfold, depending on the request and the conditions surrounding the request.

• The use case gathers these scenarios together.

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Use Case Format and Example (for this class)

Use Case N

Use Case: <the name as a short, active verb phrase>

Context of Use: <a longer statement of the goal, if needed>

Scope: <which system is under discussion>

Priority: <scale of 1 (high) to 5 (low)>

Level: <one of: summary, user goal, subfunction>

Primary Actor: <a role name for the primary actor>

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Use Case N

Use Case: Print letters

Context of Use: User wants to print one or more of the same letter

Scope: The Perfect Form

Priority: 5

Level: User goal

Primary Actor: Faculty member

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Stakeholders and Interests: <list of stakeholders and key interests in the use case>

Precondition: <what must be true for the use case to proceed>

Minimal Guarantees: <how the interests are protected under all exits – worst case situation>

Success Guarantees: <what must be true after the user case runs>

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Stakeholders and Interests: Faculty member – wants to print letter(s)

Precondition: User has opened a form letter

Minimal Guarantees: No letters will print and a message stating why will be displayed

Success Guarantees: All of the letters will be printed

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Trigger: <what starts the use case>

Main Success Scenario: <steps from trigger to goal delivery in which nothing goes wrong>

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Trigger: Menu selection of “print”

Main Success Scenario:

1) User selects “print”

2) A print dialog box appears

3) User changes prints settings

4) User presses “print”

5) The letters print

6) The print dialog box closes

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Extensions: <what can happen differently during the scenario>

Related Information: <any necessary additional information>

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Extensions: 2a) User has not filled in the custom tags, the

date tag, the sender tag, or the return address tag.

2a.1) The user will be informed of which tags to specify replacements for.

2a.2) The print dialog box will not open.Related Information: None