INTRODUCTION A Data Flow Diagram is a graphical representation of flow of data through information system. DFD can be used to visualize a data processing. DFD shows what kind of data will be input to the system and what data is received as the output. Also it tells where the data will come from and go to and where it is stored in the process. DFDs are drawn with the help of Mac Draw.
COMPONENTS OF A DFD The Process The Flow The Store The Terminator
THE PROCESS Also known as bubble, function or transformation. Shows the part of the system that transforms inputs to the outputs. Also represents who is actually carrying out the operation.
THE FLOW Represented graphically by an arrow. Shows the movement of information packets i.e., data in motion. A flow carries only one type of packet and is represented by the flow name.
THE FLOW (CONTD.) Data flows can diverge or converge in DFDs.
THE STORE Models a collection of data packets that are at rest. Acts as a storage area for two processes that occur at different times or simultaneously. Flow from the store is interpreted as access to read the information from the store. Flow to a store is interpreted as write update or delete.
THE TERMINATOR Represents an external entity with which the system communicates. Can be a person, or a group outside organisation, department within the organisation or some other computer system . Source – Entity that supplies data to the system. Sink – Entity that receives data from the system
PHYSICAL DFD A physical data flow diagram is a graphical representation of a system showing the system’s internal and external entities, and the flows of data into and out of these entities. A physical DFD specifies who is actually carrying out the process. A physical DFD does not tell us what is being accomplished.
LOGICAL DFD It specifies the logical processes performed on the data. A logical DFD portrays system’s activities, whereas a physical DFD depicts a system’s infrastructure.
SYSTEM DESCRIPTION WITH A DFD An entire system is depicted by a single data flow diagram which gives the complete system overview. This is called context diagram. Successive expansion of processes is done to show the operations in details. This is levelling of DFD. In the process of levelling the DFD we still do not specify how the processing is being done, only the data flow is specified.
GOOD CONVENTIONS IN PREPARING THEDFDS Accurately level the components of the DFD. Less number of processes. Number the processes. Avoid complex DFDs. Logically consistent DFDs should be prepared.