1. 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

2. 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).

3. Data Flow Diagrams
The following four symbols are used in data flow diagrams:
External entity  
Process  
Data store 
Data flow

4. Data flow diagram of a travel agent booking system
Customer
Travel-query
Available flights
Book flight
flights
Booking
Booking confirmation

5. Data flow diagram of part of an order processing system
Customer
Order
Available stock
stock
Process order
Invoice
Unfilled order
Out-of-stock notice
backorders
Despatch note
Warehouse

6. Order rejection notice
Context diagrams
a top-level DFD shows the least amount of detail and is known as a Level 0 DFD or context diagram.
Order
Customer
Invoice
Warehouse
Picking List
Process order
Out-of-stock notice
Order rejection notice

7. 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.