Requirements Engineering: The KAOS GoalOriented Approach

1
Requirements Engineering The KAOS Goal-Oriented
Approach

• UFCE4S-10-3
• Lecture Four
• Stewart Green

2
Lecture Structure

• KAOS introduction
• KAOS inputs and outputs
• KAOS meta model
• KAOS process
• KAOS meeting scheduler example
• KAOS decomposition of schedule meeting system
  goal
• KAOS operationalising goals
• KAOS requirements model

3
KAOS References

• Goal-Oriented Requirements Engineering A Guided
  Tour, A. van Lamsweerde, 5th International
  Symposium on Requirements Engineering, IEEE
  Computer Society Press, 2001
• Goal-directed Requirements Acquisition, A.
  Dardenne, A. van Lamsweerde and S. Fickas,
  Science of Computer Programming, 20, pp 3 50,
  1993
• Goal-Oriented Approaches to Requirements
  Engineering, S. Green, (Lecture three handout-1)

4
Formalising Requirements

• Requirements expressed in English, perhaps
  augmented with diagrams, may be formalised. In
  other words, the goals may be re-expressed in a
  more formal language.

5
KAOS Knowledge Acquisition in Automated
Specification

• KAOS is one method for formalising goals into
  requirements
• KAOS is a research project, but it is well
  documented, and has tool support from a tool
  called GRAIL
• The aim of the KAOS approach is to derive a
  description of a systems behaviour and an
  initial analysis of its structure through
  acquiring and formalising functional and
  non-functional goals for a composite system

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KAOS Inputs and Outputs
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KAOS Meta Model 1
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KAOS Meta Model 2

• The meta model is like an ERA schema here it
  characterises composite systems.
• As such the meta model comprises
• Meta concepts e.g. goal, constraint, agent,
  action
• Meta relationships e.g. constraint-operationalise
  s-goal, agent-performs-action
• Meta attributes e.g. pre-condition is a meta
  attribute of the action meta concept
• The meta model is traversed from goals to acquire
  meta objects

9
KAOS Process 1
10
KAOS Process 2

• During the KAOS process, instances of the meta
  objects are acquired as a requirements model is
  created. The highest-level goals are identified
  from the requirements. Goals are system
  objectives that cannot be met by a single agent.
  These are reduced to constraints (leaf goals)
  which can be met by a single agent. Reduction is
  achieved by goal decomposition.

11
KAOS The Meeting Scheduler Problem

• See lecture handout for full details
• Meetings are typically arranged in the following
  way. A meeting initiator asks all the potential
  meeting attendees for the following information
• A set of dates on which they cannot attend the
  meeting
• A set of dates on which they would prefer the
  meeting to take place
• The initiator asks active participants to provide
  special equipment requirements
• A date conflict occurs when no satisfactory date
  can be found
• The meeting scheduler must handle several meeting
  requests in parallel

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KAOS Decomposition of Schedule Meeting System
Goal
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KAOS Stage One Process Outputs
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KAOS Operationalising Goals 1
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KAOS Operationalising Goals 2

• The AttendeesPreferencesKnown system goal is
  operationalised into two constraints
• A Hard constraint RequestForAttendeesPreferencesM
  ade (such a constraint must not be violated)
• This is satisfied by a MeetingScheduler agent
  with RequestAteendeesPreferences action
• A Soft constraint RequestForAttendeesPreferencesS
  atisfied
• This is satisfied by Attendee agents with the
  SubmitPreferences action

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KAOS Meeting Scheduler Requirements Model 1
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KAOS Meeting Scheduler Requirements Model 2

• The requirements model shows for example
• How one system goal has been reduced to
  constraints
• How these constraints are met by the actions of
  agents on objects
• The objects involved such as Preferences
  Request, Attendee, and AttendeePreferences