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, complex systems.

• 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

Confirm reservation

Allocate room

Reservation numberguest reservationsReservation details

Confirmed reservation

roomsUnoccupied rooms

Room number

Guest

reservation

Staff

makes

takes

Data flow diagram

Entity relationship diagram

Business Process Modelling diagramming Examples

StartReceive

reservation request

Check occupancy

Hotel is full?

Yes

No

Send apology and alternative

Send confirmation

end

Flowchart

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.

Guest Reservation Room

Confirm reservation

Allocate room

receive reservation

request

Check occupancy

Confirm reservation

ReservationConfirmed reservation

Objects, processes, relationships

• Relationships exist between two or more objects.

• In systems diagramming they are actions, and are referred to with verbs.

Guest Reservationmakes

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.).

StartReceive

reservation request

Check occupancy

Hotel is full?

Yes

No

Send apology and alternative

Send confirmation

end

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.

Start

terminator

Add product to cart Checkout?

Process flow process decision

Checkout?

yes

noAdd product

to cart

loop

Designing a flowchart

1. Brainstorm the system you want to model.2. Define scope of the system (e.g. start point and

end point).3. Define and logically order each process in the

system.4. Create the flow between each process.5. Include any decision branches, loops, terminations.6. Check the completed chart and look for logic

problems, inaccuracies, etc.. 7. 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