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Modelling information systems. Fundamentals of Information Technology Session 4. Models. Models are abstractions , which allow people to concentrate on the essentials of a system by omitting non-essential details (The tube map has no details of trains or staff, road or rivers).
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Modelling information systems Fundamentals of Information TechnologySession 4
Models • Models are abstractions, which allow people to concentrate on the essentials of a system by omitting non-essential details (The tube map has no details of trains or staff, road or rivers). • Models are simplifications: they help us to understand large, multi-faceted, complexsystems. • Models help us to solve problems without having to build concrete artefacts (e.g. we can build a model of a bridge before actually building the bridge – very helpful). • Models provide a common language of description (modelling language) so that we can communicate the complexity of systems to others.
Models • Models are used to represent problem situations. • A number of models may be used to provide distinct views of the same problem situation (here the London transport infrastructure).
Models and information systems • Information systems have several different aspects that need to be modelled. • Hardware • Software • Functionality (what the system does) • Data storage (what data the system holds) • User interface (how the user interacts with the system) • Network/communications • When designing IS, each aspect is modelled individually giving different views of a system. • Having a range of views helps to reduce complexity and help designers identify and remove potential problems at an early stage.
Static and dynamic models • Models need to represent both the static and dynamic elements of a system. • Static elements are those elements that do not change over time (e.g. hardware or data structures). • Dynamic elements are those elements which change as the system is used (e.g. a bank account balance). • Modelling dynamic elements is often referred to as process modelling. • Modelling data is often referred to as data modelling.
Modelling languages • Modelling languages are systems of notation that use one or more diagramming techniques. • A diagramming technique consists of a set of graphical symbols for representing objects, processes and relationships in information systems. • Modelling languages and diagramming techniques are designed to be: • Clear and expressive enough to represent complex reality • Unambiguous • Universal • Easy to produce • Supported by drawing tools
Modelling languages • There are a number of different modelling languages available to designers of information systems. Amongst the most commonly encountered are: • Unified Modelling Language (UML) • Structured Systems Analysis and Design Method (SSADM) • Business Process Modelling • Jackson Structured Programming
UML diagramming examples Use case diagram Activity diagram
SSADM diagramming examples Data flow diagram Entity relationship diagram
Objects, processes, relationships • Diagramming notations are generally made up of symbols for objects, processes and relationships. • Objects are things; they are referred to using nouns. • Processes are actions; they are referred to with verbs. • Processes have inputs and outputs and usually result in some form of transformation.
Objects, processes, relationships • Relationships exist between two or more objects. • In systems diagramming they are actions, and are referred to with verbs.
Modelling tools • A number of tools exist for creating models of information systems. • Some are specific to particular modelling languages (e.g. Rational Rose for UML). • Others are general and provide the notation for many different modelling languages and diagramming techniques (e.g. Microsoft Visio). • Simple diagrams can also be created using basic drawing tools like Word, although it is difficult to produce professional level drawings.
Modelling tools • One of the most popular general modelling tools on the market is Visio. • SCSIS has Visio installed in its labs. It can also be downloaded free from ELMS. • Visio has a huge range of diagramming notations including: • UML • SSADM • Business Process • Workflow • Hardware • Networks • Web • Databases • It creates diagrams that can be imported into other documents, such as Word.
Flowcharts • Flowcharts are a basic and much used diagramming technique that have their origin in Business Process Modelling. • They are mainly used to model simple business or organisational processes, but they can be used to model any kind of process (e.g. business, mathematical, etc.).
Basic flowchart symbols • A terminator indicates the beginning or end of a system or sub-system. • A process flow shows the direction of the process. • A process indicates an action within the system. • A decision shows a conditional point in the system. where two or more outcomes are possible. • A loop shows an action that can be repeated.
Designing a flowchart • Brainstorm the system you want to model. • Define scope of the system (e.g. start point and end point). • Define and logically order each process in the system. • Create the flow between each process. • Include any decision branches, loops, terminations. • Check the completed chart and look for logic problems, inaccuracies, etc.. • Redraw the flowchart until you are satisfied it is a correct model of the system.
Hands-on exercises • Now do • Activity 4 – Modelling information systems