DFD diagrams are widely used graphic tools for describing the movement of the data within or outside the system. Or DFD is a graphical technique that describes information flow and the transformation that is applied as data move from input to output.
               DFD is very useful in understanding a system.

Purpose of DFD:-

  • To provide an indication of how data is transformed/change as they moved via the system.
  • To make a report of the function that transforms the data flow.

Characteristics of DFD are:-

  • DFD shows the flow of the data via the system.
  • They focus on the processes that transform incoming data (input) into outgoing data (output).
  • The processes that perform this transformation normally create as well as use data.
  • External entities supply data from the system.
  • Support a top-down approach to analyze.

CONSIDERATION  INVOLVED  IN DEVELOPING  DFD's

    1.  The symbol used for constructing DFD's



A DFD uses a limited number of symbols to represent the functions performed by a system and the data flow among these functions. These symbols are:

  • External entity symbol

The external entity symbol represents the source of data to the system. A rectangle represents an external entity. External entities are named with the appropriate names. The external entities are physical entities.
 EXAMPLE:-
External entity CUSTOMER
                                                        

  • Data flow symbol

The arrow represents data flow. It represents the path over which data travels in the system.
The data flow can take place between:-
         1.   Two processes.
         2.   File and process.
         3.   An external entity and processing the direction of the data flow arrow.
Data flow cannot take place between files or external entities. Data flow is always named and depicts the direction of flow.  One data flow can contain any data element.
 EXAMPLE:-
                                     

  • Function/process symbol

A function is represented graphically as a circle. Some system analyses use an oval or a rectangle with rounded edges to represent a function. This symbol is also called a bubble or a transformation.
Processes have identification numbers also from top to bottom and left to right.
EXAMPLE:-
                                           

  • Datastore symbols

An open rectangle is a data store. A datastore is like a warehouse for data. Datastore indicates the data file or registers where it is saved.
EXAMPLE:-Customer master file, Employee register, etc are data store.
            

2.  Guidelines for developing DFD


There is no standard method that is used to develop a DFD for a given problem.
The following steps are taken to develop a DFD:-

  • Draw a context-level digram.
  • Decompose the context level diagram into a second level DFD.
  • Decompose the second level DFD to an elementary level DFD. This level of diagram explains in detail the actual processes.

3.  Context diagram

A diagram consisting of only one process which represents the entire system is called a context diagram.
It is a top-level data flow diagram.
EXAMPLE:- A publisher's ordering system.

4.  Detailed-level DFD

A detailed level DFD is that where the context level DFD is decomposed to elementary level using DFD symbols.

Data flow diagram of a publisher's ordering system is given below:-