1. SAD - DFD Context Diagrams
And Franchise Colleges
03 Data Flow Diagrams
- Context Diagrams
By MANSHA NAWAZ
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SAD - DFD Context Diagrams

2. SAD - DFD Context Diagrams
Learning Aims
Aim
to construct a context diagram for a proposed computerised information system
Objectives
to identify and represent the whole system
to identify and label external entities
to identify and label system in/outputs
to compile a consistent event list
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3. SAD - DFD Context Diagrams
Lecture Outline
the nature of models
basic notation
building a context diagram
building an event list
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4. SAD - DFD Context Diagrams
Data Flow Diagrams
What Is a Data Flow Diagram?
Data Flow Diagrams (DFDs) are a System Modelling CASE Tool
DFDs are graphical views used to represent the system, sub-systems and functional view (systems decomposition).
System Model is depicted by a series of DFD’s
Used to model a proposed system
DFD’s show how data travels within a system.
flows of data to and from external entities such as people, departments and organisations outside the system.
flows of data to and from a systems processes
show the flows to and from data stores within the system area.
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5. Types of Data Flow Diagram
Stage 1 – at the beginning of the systems analysis phase
– current physical DFDs to represent graphically the current system.
Stage 2 – as part of the systems analysis phase
– current logical DFDs to depict the current system, without reference to where things happen or who does what.
Stage 3 – also during the systems analysis phase
– required logical DFDs including the new requirements.
Stage 4 – as part of the systems design phase
– required physical DFDs showing how the new system will be implemented.
Many methods make use of DFDs for some, or all, of these purposes.
We’re going to look at how to use DFDs to model systems at a logical level using Yourdon notation for new systems (focus on Stage 3 & 4).
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6. Data Flow Diagrams - Yourdon’s Notation
Symbol
Symbol used if duplicated in diagrams
Description
Entity – has an input into the system or receives output from it. Does not exist within the system itself
Process – represents the processes which must take place inside the system
Data store – where information is held over time
Data flow – the direction of the arrow shows the direction in which the data flows within the system
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7. SAD - DFD Context Diagrams
Entity
An external entity (terminator or source) is something outside the system. It is shown as a square and is identified by a lower case letter and name. a
Customer b
Sales Dept c
Accounts
Package
Eg people, departments, organisations that provide information to the system (source) or receive information from the system (sink).
An external entity can be a source and a sink.
If external entities have to be duplicated, an
angled line is put in the corner of the box (all copies). a
Customer
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8. SAD - DFD Context Diagrams
Process
A process is an action on or transformation of data. It is shown as a circle and is identified by a number and name. The name is a description of the data processing taking place. 1
Check Credit Rating 2
Split Order 3
Adjust Stock
The description should be an imperative statement, consisting of an active verb and object.
The description does not have a subject, ie we don’t say who carries out the process or where.
This helps our logical view of the system.
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9. SAD - DFD Context Diagrams
Data Flow
Date
A data flow is shown as a named arrow.
Invoice
The name of the data flow is the name of the piece of data passing between the 2 connected symbols.
The direction of the arrow shows the direction in which the data is flowing.
Two way flows of data are best shown by 2 arrows,
but may be shown by one double-headed arrow.
Stock Adjustment
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10. SAD - DFD Context Diagrams
Data Store
The final symbol is the data store, represented by parallel lines, identified by a number and name. The name is usually in the plural.
1 Customers
2 Invoices
The data in a data store is data at rest (persistent data), ie a file or database.
If data stores have to be duplicated, a vertical line is added to the left (all copies).
When data is written to a data store, the data flow is shown going in.
When data is read from a data store, the data flow is shown coming out.
If data is being read and written, a double-headed arrow can be used.
1 Customers
1 Stock
Stock Level
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11. System modelling using DFD’s
There are many ways to go about drawing DFDs models for system representation.
We will focus on 3 level system views
Context Diagram (CD) lecture 03
System View
Top-Level Data Flow Diagram lecture 04 lecture 05
TL-DFD, DFD-0 or Level-0
A systems Sub-Systems View
Low-Level Data Flow Diagrams lecture 06 lecture 07
LL-DFD, DFD-1 or Level-1
A sub-systems a Function view
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12. Context Diagrams (CD) Basic Notation I
Representing the proposed system
whole system view
a process is shown as a circle
the system is represented as a single process
it is named
The XY
System
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Basic Notation II
Representing external entities
outside will be agencies that react with the system
who or what is going to communicate with our system
shown as squares
numbered a..z
named a
XXX
The XY
System b
YYY
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Basic Notation III
Representing inputs & outputs
data that flows between the system and the outside
shown as arrows
have direction
data in – information out
named
xdetail a
XXX
The XY
System b
YYY
yreport
Should Context Diagrams have datastores?
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15. SAD - DFD Context Diagrams
Context Diagram (CD)
Yourdon also proposes the notion of a CD
This is a very high level diagram consisting of one process that represents the whole system, and all the external entities that interact with it.
The purpose of such a diagram is to show clearly all a system’s interfaces with its environment.
No stores appear on this diagram.
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16. SAD - DFD Context Diagrams
The Event List
things happen outside of the system
those that the system must respond to are termed events, these are listed
each event on the list corresponds to an input on the context diagram
An XXX sends in an xdetail
the exception are temporal events
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17. Building Context Diagrams
Draw one process to represent the system
Name it appropriately
“Who or what does the system interact with”
Draw each of these entities onto the diagram
Identify the dataflow required by the system – Data in - Information out.
Do not expect to obtain all entities or dataflow on your first draft.
Expect a number of revisions – the more time you spend the more awareness you have on the system
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18. Development : Context Diagram
Context Diagram (CD)
Provide a supporting text description of the system view as a introduction to the CD
Provide the CD
Event List
List all events and make sure it is consistent with your context diagram
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19. Document : Analysis Specification
Terms Of Reference lecture 02
Context Diagram lecture 03
Context Diagram Support Text
Context Diagram
Event List
Top-Level Data Flow Diagram lecture 04 lecture 05
Low-Level Data Flow Diagrams lecture 06 lecture 07
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20. SAD - DFD Context Diagrams
Summary & Self Study
Summary
Structured methods arose to meet the problems of growth
Context Diagrams show system and its interaction with the outside world
Self Study
review the lecture
complete 3a & b from the course workbook
can you achieve the objectives set out in slide 2?
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SAD - DFD Context Diagrams