Data Flow Diagrams

Objectives:• To define what is meant by structured analysis• To define the symbols used in a Data Flow

Diagram (DFD)• To draw a context diagram (Level 0 DFD) for a

given scenario• To refine a DFD to a more detailed (lower-level)

view

Structured analysis

•A widely-used top-down method for defining system inputs, processes and outputs. •It shows how information flows through a system, using several diagrams showing progressively more and more detail at each level. •The primary tool of structured analysis is the Data Flow Diagram (DFD).

Data Flow DiagramsThe following four symbols are used in data flow diagrams: 

External entity   

Process   

Data store 

Data flow

Data flow diagram of a travel agent booking system

CustomerTravel-query

Booking confirmation

Available flights

Booking

flightsBook flight

Data flow diagram of part of an order processing system

Customer

Order

Invoice

Available stock stock

Process order

Out-of-stock noticeUnfilled order

backorders

Warehouse

Despatch note

Context diagramsa top-level DFD shows the least amount of detail and is known as a Level 0 DFD or context diagram.

Customer

Order

Invoice

Process orderOut-of-stock notice

Picking ListWarehouse

Order rejection notice

• Level 0 or context diagram shows a system as a single process with inputs and outputs flowing to or from external entities.

• Level 1 DFD will split up that single process into subsystems and show more detail about the data flows and data stores.

• Level 2 DFD may decompose a single subsystem even further.