COMP2110 Software Design in 2003 lecture 4 Requirements Specifications lecture 2 of 3

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COMP2110 Software Design in 2003 lecture 4
Requirements Specifications lecture 2 of
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• The process of creating requirements
  specifications
• A Req Spec is an information model meaning?
• Examples of models
• object models
• gathering, analysing and organising requirements

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The process of creating SRS

• The product of the Analysis phase is
• a well defined information model
• a set of labelled, organised requirements
  statements functional consumer requirements and
  developer requirements, system requirements,
  performance requirements
• a set of use cases or scenariosthat express the
  relationships between the model and the real
  world of the problem
• other explanatory models interfaces, states,
  decisions

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Quality SRS - recap

• A good SRS is
• 1. unambiguous
• 2. complete
• 3. verifiable
• 4. consistent
• 5. modifiable
• 6. traceable

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Information models

• A requirements specification contains an
  information model of the real world
• covers those parts of the world that are needed
  to describe the problem
• enough detail to be useful for designing a
  program to solve the problem
• all models are abstractionsthey remove
  irrelevant and confusing detail

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Information models other models (1)

• Example mathematical models
• many mathematical models are used in engineering
  or physics
• usually very abstract
• for example a person who is the passenger in a
  lift may be abstracted to just their masswhen
  analysing the load in the lift for lift
  acceleration, braking, motor power, building
  structural strength

• rolling inertia or friction may be ignored when
  modelling objects on a slopes as blocks sliding
  on inclined planes.
• this model will be inadequate to predict speed
  and time if friction prevents slipping and the
  object has a large moment of inertia and
  therefore will roll more slowly

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Information models other models (2)

• economics models
• price versus demand curve
• many assumptions to simplify the market time,
  season, fashion, whether the product is an
  unsubstitutable necessity or a choice

demand
price
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Information models other models (3)

• geographical models
• a map is a model that represents some abstracted
  aspect of the real world in a diagram borders of
  land and sea, political borders, routes of roads
  (and their intended traffic carrying capacity),
  heights and vegetation, geology, temperatures

MonaVale
One Tree Hill
. 627
Murrumbidgee R
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Information models- kinds of info model(1)

• Information models also make abstract views of
  the real world,but typically have more detail
  than mathematical models, particularly about
  relationships between things
• Important different kinds
• entity-relationship models (naming and
  classifying the associations between distinct
  classes each of very similar things)
• finite state models (expressing control
  relationships between the externally visible
  input values and output values, and some hidden 
  state changes)

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Information models finite state model
from Braude SRS for Encounter case study
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Information models ctd (2)

• data flow - networks of processes and stores
  related by information flows between them

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Information models ctd (2)

• data transformationexpressing a relationship
  between the format and values of  data that is
  input, to the format and values of the outputs
  e.g. lprogrg language compiler as translator
• object centredviewing the world as objects and
  classes of objects, with associations between
  them, actions and methods on them, and internal
  states
• formal (mathematical) models, usually in the
  domain of set theory, functions and
  relations(e.g. Z comp2600)
• All kinds may be used together for some purposes

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Object oriented models (1)

• read Braude SD chapter 2carefully and critically
• this is not only about object oriented
  programming languages but about the nature of an
  OO view of the world (an OO information model)

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Object oriented models (2)

• example of a library loans system
• person
• 2 kinds borrowers and administratorsmaybe have
  nothing in common in this info model
• borrower limit, card number, address, books out,
  long term loan record, books requested, overdue
• book identifying tag (ISBN copy no.)
• loan record book, person, date

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Object oriented models (3)

• example of another kind of person genealogy
  (family tree records)
• person
• name, date of birth, date of deathpartnerships,
  children
• partnerships (marriages)
• 2 persons, start and end dates

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Information models use cases

• the way in which the objects act and interact in
  the model is important
• often more important than the contents or
  fields of the objects as described before
• can be gathered by writing use cases
• can be verified by checking use cases against
  requirements
• create domain classes

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Information models use cases