4. DATA FLOW DIAGRAMS…
Abbreviated as DFD
Introduced by DE MARCO , GANE , and SARSON
Most important tool used by system analysts
Main merit of DFD is that it can provide an
overview of :-
• what data system would process
• what data transformations are done
• what data are stored and which stored data is used
• where the results flow
The graphical representation of the system makes
it a good communication tool between a user and an
analyst and also the analyst and a system designer
5. SYMBOLS USED IN DFDS :
Used to depict a
process
Shows the direction
of flow of data
Used to represent
external entities
Represents data store
6. DESCRIBING A SYSTEM WITH A DFD :
Let us consider an example of a mess
management system and describe it with the
help of a data flow diagram (DFD)………….
9. A GOOD DFD SHOULD
NOT HAVE……
Loops
Crossing lines
Data flow split into flows with
different names and meanings.
10. GOOD STYLE IN DRAWING DFD
ƒUse meaningful names for data flows, processes
and data stores.
ƒUse top down development starting from context
diagram and successively levelling DFD
ƒA process can only transfer input to output.It
cannot create new data
ƒData stores cannot create new data
11. RULES OF DATA FLOW
Data can flow from
external entity to process
process to external entity
process to store and back
process to process
Data cannot flow from
external entity to external entity
external entity to store
store to external entity
store to store
13. LEVELLING RULES
If a process numbered ‘p’ levelled into n
processes, then each new process is numbered
p1,p2,………pn respectively.
All data flows entering a process must also enter
its levelled DFD.
All data flows leaving a process must also leave
its levelled DFD.
No external entites may appear in lower level
DFD’S.
