Enhancing Requirements Engineering by Quality Modelling: a structured Framework Paolo Donzelli Dept of Informatics Office of the Prime Minister Rome, Italy [e-mail] (Cranfield University, RMCS

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Enhancing Requirements Engineeringby Quality
Modellinga structured Framework Paolo
Donzelli Dept of InformaticsOffice of the Prime
MinisterRome, Italyp.donzelli_at_governo.it(Cran
field University, RMCS Shrivenham UK)
15-16 novembre 2001

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Outline

• Problem description
• The Requirements Engineering Framework
• The Framework at work a case study
• From RE to system implementation

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Problem description
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Introducing an IT system into an organisation

• a successful system implementation relies on
• a firm understanding of the application domain
• our ability of envisioning its impact

• but the application domain is far from being
  stable
• drives the IT system development
• but must evolve to exploit the system capabilities

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Envisioning the to-be company (a difficult task)

• The system and its application domain form a
  larger social-technical system whose overall
  needs are the ones to be fulfilled.

• Organisations are made of people the goals, the
  expectations, and the needs of all the
  stakeholders, have to be explicitly addressed

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Advances in Requirements Engineering (1)

• Process modelling-based techniques have been
  suggested to support the Early RE Phases
  (requirements discovery, validation and
  verification), by providing a collaborative
  environment between analysts and stakeholders to
  
• understand the application domain
• translate application domain needs into system
  requirements
• design the to-be company (by reasoning through
  feasible alternatives)
• (i)

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Advances in Requirements Engineering (2)

• the WHY
• as
• driving factor
• of the requirements
• discovery/modelling
• process
• (KAOS I)

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Genesis of the proposed RE Framework (1)

• adopt new RE techniques (i.e. process
  modelling-based and why-oriented approaches)

• encompass Quality Modelling approaches, to better
  deal with stakeholders quality issues

• move from slow, reactive, adversarial, separated
  software engineering and systems engineering
  processes to unified, concurrent processes,
  which will also include business process
  reengineering.

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Genesis of the proposed RE Framework (2)

• Reasoning about the Why, the What and the How

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Main Objectives

• Support discovery, and early verification and
  validation of both user-oriented and
  organisation-oriented requirements
• Facilitate envisioning and designing the to-be
  company
• Force reasoning about non-functional requirements
  since the outset of the project, while dealing
  with functional ones
• Soft goals play a central role, providing a
  systematic and organized way of handling
  non-functional requirements ? constraints in
  operational terms, or fit criteria for assessment
  purposes!

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The Requirements EngineeringFramework
Main characteristics, Notation, Process
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The RE Framework main characteristics
case study
Process
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Organisation Modelling (1)
deals with the application domain structure
basic elements
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Organisation Modelling (2)

• simple
• agent

• complex
• agent

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Soft Goal Modelling
produces operational definitions of the soft
goals, decomposing them into sub hard goals and
constraints
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Hard Goal Modelling
determines how to achieve hard goals, by
decomposing them into sub hard goals and tasks
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Combining soft and hard goal models
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Applying the Framework the process

• Development Flow
• from top organisational level, down to simple
  agents level, the models feed each others, in a
  continuous loop. The complete organisation model
  will result into a flat network of interrelated
  goals, tasks, and resources, with agents acting
  as a modularization mechanism
• Elicitation and Validation Flow
• where interaction with the stakeholders occurs
• Verification Flow
• to guarantee consistency between models

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A Case Study
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The case study

• Synthetic Environments are distributed
  interactive simulations of real-world systems,
  used to support operational, political and
  economic decisions

• Strongly interrelated with the application
  domain, highly complex and expensive, they
  require a RE process strongly focused on the
  early verification and validation

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A ground-based simulator
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The Organisation Model
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Soft goal upon the Project Leader
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Hard goals upon the Project Leader
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From high level goals to system requirements

• there is not one single set of global goals that
  has to be achieved (from which the others are
  refined in a top-down way)
• an organisation consists of numerous processes
  occurring simultaneously, involving various
  agents, following different paths, and
  intersecting each other
• an agent may operate as goal generator, as well
  as a goal transformer
• goals propagate through the organisation in
  complex patterns and may even conflict

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Clashing Requirements detecting and resolving
them through soft goals

• Agents may have different opinions, and their
  goals collide. How can Soft Goals help?

• force us to clarify, very early in the project,
  concepts that are usually left blurred until it
  is too late (or too expensive) for any corrective
  intervention

• encourage interactions and a common terminology

• provide a way to reason about trade-offs, to
  freeze temporary solutions, and to formalise
  final decisions

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Realism soft goal for the Flight Test Crew
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Realism soft goal for the Avionics System Expert
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Resolved Realism soft goal
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Deliverables of the RE Framework
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Have the objectives been achieved?

• The RE Framework improves the early-stage
  verification and validation of the requirements
  of a IT system
• Verification is improved by reconciling the
  different models
• Validation is improved because of the visibility
  of the decisions made by stakeholders.
• Application results suggest that the Framework
  offers benefits to accreditation and maintenance
  activities, mainly thanks to the two-way goal lt-gt
  requirement link
• Accreditation should be made easier by tracing
  the IT system properties to application domain
  needs
• Maintainability is improved because changes in
  the application domain may be easily linked to
  system changes.

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From RE to System Implementation
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Deriving the System Requirements Architecture

• the RE framework allows us to model the
  application domain at a social-technical level,
  by providing a systematic approach to deal with
  agents, soft goals, hard goals and their
  incremental refinement

• requirements are expressed as a collection of
  hard goals and constraints placed upon the system
  by other agents

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An Extract from the Organisation Model
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The RE Framework outcomea set of Hard Goals and
Constraints
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RE Framework as forerunner for UML

• the front-end activity of any UML-based approach
  is the use-case modelling, i.e. capturing the
  ways in which the users intend to interact with
  it.
• use-case analysis can benefit from supporting
  methods (Business Process Modelling and Black Box
  approaches) that help to identify both the
  actors, and their goals

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Use-cases as a Goal-refinement Strategy

• a hard goal can lead to one or more use-cases
• constraints will become non-functional
  requirements

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Avionics System Expert Use-case Diagram
goals
constraints
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The Use-case Diagram
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Building the conceptual model the class diagram