Capturing RequirementsCapturing Requirements

Questions to Ask about RequirementsQuestions to Ask about Requirements

1) Are the requirements correct?2) Consistent?3) Unambiguous?4) Complete?5) Feasible?6) Relevant?7) Testable?8) Traceable (in documentation)?

Requirement ScopeRequirement Scope

1) Expand to include what was overlooked (think in most general terms);

2) Contract to remove everything that is not needed / non-critical.

Requirement ModelingRequirement Modeling

Helps form a visual image of what goes into the project and produce detailed description of the process.

Entity-Relationship Diagram (ERD)Entity-Relationship Diagram (ERD)

* Entities are the "things" about which we seek information. * Attributes are the data we collect about the entities. * Relationships provide the structure needed to draw information from multiple entities.

Message Sequence Chart Message Sequence Chart

* Top Square: an entity* Horizontal Arrows: messages connecting entities in time; dashed indicates entity creation* Square Boxes on Lines: actions taken in respond to messages* Hexagonal Boxes: conditions / states* Solid Black Box: designates the end of definition, cross – entity destruction (end of execution)

time

Petri NetsPetri Nets

State transition notation used to model concurrent events.Circles – places, activities, conditionsBars – transitionsArrows (arcs) – connect transitions with their input and output placesTokens – each arc can have a weight that specifies how many tokens are removed from arc’s input place and moved to its output place. Transition is enabled if its input places have enough tokens to accommodate arc weight.

Petri Net ExamplePetri Net Example

Transitions: Borrow, Return, Withdraw Return Request, Withdraw Loan Request

Tokens: Loan Request place has to have a token for Borrow and Withdraw Loan Request transitions to be enabled.

Initiating a Loan Request transition moves a token from Loan Request place to OnLoan place provided that Avail place also has a token!

Data-Flow DiagramsData-Flow Diagrams

Parallel Lines: persistent data storeRectangles: actors providing data / receiving the resultsEllipse: abstract / nested data flow diagram

Requirements DocumentRequirements Document

1) Scope: project purpose & scope including relevant benefits, objectives and goals.

2) Background: background & rationale for development (i.e. before we were filling out the forms on paper and now we do it on computers, which saves time and boost accuracy).

3) Essential Solution: prioritized core functionality.4) Environment: hardware & software environment

intended for the solution.5) Implementation Outline (if any).6) Operational Assumptions: i.e. what makes system

fail and what environmental changes cause requirement reevaluation.

Technical SpecificationTechnical Specification

Spec is equivalent to the requirement document except that it reflects developer’s point of view and states the requirements in technical terms.

• Data Formats• Defines Interfaces: inputs and outputs of

all modules.• Restates Functionality in terms of

interface methods and their parameters, entity-relationship diagrams, state machines, UML, etc.

• Testable Quality requirements.

Requirement ValidationRequirement Validation

We must make sure that the requirements that we have written down accurately reflect stake holder's needs. How?

We present the requirements to all the stake holders and cross-reference the responses to find out inconsistencies, ambiguities or omissions.

Requirement Validation MeansRequirement Validation Means

• Walkthroughs• Requirement reading• Presentations• Review meetings• System models• Business scenarios• Simulation & prototypes

Requirement MeetingsRequirement Meetings

1) Review stated goals and objectives2) Compare written requirements with goals &

objectives3) Review the environment and interfaced

among modules4) Review information flow, impose data

constraints5) Evaluate risks and discuss alternatives6) Discuss testing planMost faults arise from requirements errors!

Requirement VerificationRequirement Verification

We need to ensure that our technical specification corresponds to requirements document. How?

We need to trace all entities and relationships that we modeled back to the requirement document. Tedious, but a must.

For critical systems we must prove that the spec realized each and every function and satisfies all the imposed constraints. Thus we must computer-aided model checking tools to uncover conditions that would break the model, unreachable conditions and assumptions about the environment that when incorrect (or out of range) can result in faults.

Individual HomeworkIndividual Homework

1) Produce requirement document for your part of the project.

2) Produce technical specification.3) Choose appropriate modeling

notation and produce requirement model (use UML or similar tools).

Read Chapter 4.