ANU comp2110 Software Design lecture 4 COMP2110 Software Design in 2003 lecture 4 Requirements Specifications lecture 2 of 3 1.The process of creating requirements specifications 2.A Req Spec is an information model: meaning? 3.Examples of models 4.object models 5.gathering, analysing and organising requirements

ANU comp2110 Software Design lecture 4 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 scenarios that express the relationships between the model and the real world of the problem other explanatory models: interfaces, states, decisions

ANU comp2110 Software Design lecture 4 Quality SRS - recap A good SRS is 1. unambiguous 2. complete 3. verifiable 4. consistent 5. modifiable 6. traceable

ANU comp2110 Software Design lecture 4 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 abstractions: they remove irrelevant and confusing detail

ANU comp2110 Software Design lecture 4 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 mass when 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

ANU comp2110 Software Design lecture 4 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

ANU comp2110 Software Design lecture 4 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 Murrumbidgee R. 627

ANU comp2110 Software Design lecture 4 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)

ANU comp2110 Software Design lecture 4 Information models – finite state model from Braude SRS for Encounter case study

ANU comp2110 Software Design lecture 4 Information models – ctd (2) data flow - networks of processes and stores related by information flows between them greet & seat take order prepare servings customer party kitchen orders orders by table order seated party spike order prepare bill order serve to table food servings request for bill collect payment bill detailed analysis of taking orders

ANU comp2110 Software Design lecture 4 Information models – ctd (2) data transformation expressing a relationship between the format and values of data that is input, to the format and values of the outputs e.g. lprogr’g language compiler as translator object centred viewing 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

ANU comp2110 Software Design lecture 4 Object oriented models (1) read Braude SD chapter 2 carefully 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)

ANU comp2110 Software Design lecture 4 Object oriented models (2) example of a library loans system person 2 kinds: borrowers and administrators maybe 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

ANU comp2110 Software Design lecture 4 Object oriented models (3) example of another kind of person: genealogy (family tree records) person name, date of birth, date of death partnerships, children partnerships (marriages) 2 persons, start and end dates

ANU comp2110 Software Design lecture 4 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

ANU comp2110 Software Design lecture 4 Information models – use cases