Data Flow Diagrams

Symbols

Note that the letters along the arrows in the example diagrams represent the name of the information being transferred in each direction – you should use a description of what the information is, e.g. Patient name – these names should match across the different levels of your DFD. Likewise, each process should given a name, with the number only being used in the top half of the box. There is a sample DFD fragment overleaf.

Context Diagram

The context diagram just shows the system in the context of the external entities, and what information flows in and out of the system as a whole.

The system is then subsequently broken down into its component parts – which are themselves broken down, until each process represents a single step in your system.

Level 0 DFD

Level 1 DFDFor process 2

0

Processname

Data storeD1

External entity

0

Information System

Entity AEntity A Entity BEntity BX

Z

Y

Information flow

1

Process 1

2

Process 2

3

Process 3

Entity AEntity A Entity BEntity B

Data store ND1

X

YZ

N

N

V

W

2.1

Process2.1

2.2

Process2.2

2.3

Process2.3

Data store ND1

V

N

N

Y

W

G

H

J

Level 2 DFDFor process 2.2

Example DFD Fragment

Decomposition Diagram

Each process in your system should be linked together in the form of a tree diagram to show how they are related, as shown below.

2.2.1

Process2.2.1

2.2.2

Process2.2.2

2.2.3

Process2.2.3

Data store ND1

J

J1

J2

H1

H2

H

N N

K

L

2

Maintain appointments

AppointmentsD2PatientsD1

PatientPatientPatient name

Patient appointmentAppointment to change

Patient information

Possible appointmentsAppointment

AppointmentAvailable

Appointment

0The System

1Process 1

2Process 2

3Process 3

2.2Process

2.1Process

2.3Process

2.2.3Process

2.2.1Process

2.2.2Process

2.1.2Process

2.1.1Process

3.1Process

3.2Process

3.3Process

3.4Process

The number of processes, and the names and numbers inside the boxes, should match the processes described in the main part of the data flow diagram. Note that only the processes are shown – not the data stores or external entities.