11

CS 426Senior Projects in Computer

Science

Chapter 3: The Requirements Workflow

[Arlow & Neustadt, 2005]

February 11, 2014

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OutlineOutline

The requirements workflow Metamodel for software requirements Requirements workflow details The importance of requirements Defining requirements

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The Requirements Workflow.The Requirements Workflow. Fig. 3.2 [Arlow & Neustadt 2005] shows that most of the work in requirements workflow occurs in Inception and Elaboration phases

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.The Requirements .The Requirements WorkflowWorkflow

The purpose of the requirements workflow is to reach an agreement on what the system should do, expressed in a way accessible to the users of the system

Requirements engineering involves: elicitation, negotiation, conflict resolution, prioritization, documentation, and maintenance of requirements

Various stakeholders are involved in establishing the set of requirements for the system

UML uses cases describe functional requirements, but non-functional requirements need different description

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Metamodel for Software Metamodel for Software RequirementsRequirements

Arlow & Neustadt’s approach for requirements engineering is shown in

Fig. 3.3 [Arlow 2002]. Details can be found in Section 3.3

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Requirements Workflow Requirements Workflow Detail.Detail.

Specific tasks for UP (Unified Process) requirements workflowFig. 3.4 [Arlow & Neustadt 2005]

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.Requirements Workflow Detail.Requirements Workflow DetailArlow and Neustadt extend slightly the UP requirements

workflow withthe addition of new tasks: find functional requirements, find non-functional requirements, prioritize requirements, & trace

requirementsto use cases. As such, non-functional requirements can be

addressed as well, along with the traditional UP/UML treatment of functionalrequirements via use cases. Fig. 3.5 [Arlow & Neustadt 2005]

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The Importance of The Importance of RequirementsRequirements

Requirements engineering is about establishing what the stakeholders need from the system

Requirements engineering encompasses elicitation, negotiation, conflict resolution, prioritization, documentation, and maintenance of requirements

Poor requirements engineering is the major cause of software project failure

The second major cause of software project failure is lack of user participation

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Defining Requirements.…Defining Requirements.…

Requirement: “a specification of what should be implemented”

There are two types of requirements: Functional requirements: describe the desired

behaviour of the system Non-functional requirements: specify particular

properties of or constraints on the system In theory, the set of requirements should be

only about “what” the system should do, but in practice it is not possible to avoid “how” aspects of the system

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.Defining Requirements...

The SRS (Systems Requirements Specification) is the document that contains the set of requirements expected to be satisfied by the system, both functional and non-functional

There are many ways to write an SRS (“company dependent” ways)

The SRS provides the input for the analysis and design phases of the development process

The bottom line regarding the SRS is: “does it help me to understand what the system should do or not?”

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..Defining Requirements....Defining Requirements.. Simple format recommended for well-formed requirements:

<id> The <system> shall <function>

Examples of functional requirements (what the system should do):

1 The ATM shall check the validity of the ATM card inserted.2 The ATM shall validate the PIN number entered by the client.2 The ATM shall validate the PIN number entered by the client.3 The ATM shall dispense no more than $500 against any ATM card 3 The ATM shall dispense no more than $500 against any ATM card in any 24-hour periodin any 24-hour period

Examples of non-functional requirements (constraints on or properties of the system):

1 The ATM shall be written in C++.2 The ATM shall validate the PIN in three seconds or less.

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……Defining Requirements.Defining Requirements. Organizing requirements: a more complex

taxonomy can be used when there are many requirements, e.g. Functional requirementsFunctional requirements

CustomersCustomersProductsProductsOrdersOrdersSales channelsSales channelsPaymentsPayments

Non-functional requirements: Non-functional requirements: PerformancePerformanceCapacityCapacityAvailability Availability Compliance with standardsCompliance with standardsSecuritySecurity

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…….Defining Requirements.Defining Requirements Requirements may have attributes, e.g.

Must haveMust have Should haveShould have Could haveCould have Want to have [the Want to have [the MoSCoWMoSCoW system] system]

Requirement Requirement attributesattributes in RUP: in RUP: Status (proposed, approved, rejected, Status (proposed, approved, rejected,

incorporated)incorporated) Benefit (critical, important, useful)Benefit (critical, important, useful) Effort (measured in person*day or function points)Effort (measured in person*day or function points) Risk (high, medium, low)Risk (high, medium, low) Stability (high, medium, low)Stability (high, medium, low) TargetRelease (product version when TargetRelease (product version when

implemented)implemented)

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Finding Requirements..Finding Requirements.. Requirements come from the context of the

system:

Direct usersDirect users Other stakeholders (e.g., managers, maintainers, Other stakeholders (e.g., managers, maintainers,

installers)installers) Other systems that interact with the systemOther systems that interact with the system Hardware devices attached to the systemHardware devices attached to the system Legal and regulatory constraintsLegal and regulatory constraints Technical constraintsTechnical constraints Business goals Business goals

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.Finding Requirements.Finding Requirements “The map is not the territory” (that is, a model is not

the real thing). When modeling, we apply three cognitive filters that simplify our effort [Chomsky, 1975]: Deletion (information is filtered out) Distortion (information is modified) Generalization (information is abstracted into rules,

principles, etc) In requirements specification we need to identify the

application of the above filters and find “challenges” for them to recover information

In particular, universal quantifiers such as all, everyone, always, never, nobody, none should be inspected closely for accuracy

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..Finding Requirements..Finding Requirements Interviews:

Don’t imagine a solution Don’t mind read Ask context-free questions Listen Have patience!

Questionnaire: they can supplement interviews. Good at getting answers to closed questions

Requirements workshop Participants: facilitator, requirements engineer,

stakeholders, domain experts Procedure: 1 Brainstorm (accept all ideas) 2 Identify key

requirements 3 Iterate over, refine, and prioritize requirements