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).
• 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:
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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:
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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
Guest
guest
Reservation
number
Confirm
reservation
Reservation details
reservations
makes
Confirmed
reservation
reservation
Room number
Allocate room
Unoccupied rooms
rooms
takes
Staff
Data flow diagram
Entity relationship
diagram
Business Process Modelling
diagramming Examples
Send
confirmation
No
Start
Receive
reservation
request
Check
occupancy
end
Hotel is full?
Yes
Send apology
and alternative
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.
Guest
Reservation
Room
• Processes are actions; they are referred to with verbs.
Confirm
reservation
Allocate room
receive
reservation
request
Check
occupancy
• Processes have inputs and outputs and usually result in
some form of transformation.
Reservation
Confirm
reservation
Confirmed
reservation
Objects, processes, relationships
• Relationships exist between two or more objects.
• In systems diagramming they are actions, and are
referred to with verbs.
makes
Guest
Reservation
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:
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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.).
Send
confirmation
Start
Receive
reservation
request
No
Check
occupancy
end
Hotel is full?
Yes
Send apology
and alternative
Basic flowchart symbols
Checkout?
yes
Start
terminator
Process
flow
Add product
to cart
Checkout?
process
decision
no
Add product
to cart
loop
• 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
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