Data flow diagrams (DFDs) are a graphical representation of the system that shows how data moves through processes. DFDs were introduced by DeMarco, Gane, and Sarson and are an important tool for system analysts. A DFD shows what data the system will process, what transformations are done to the data, where data is stored, and where results flow. Common symbols in a DFD include processes, external entities, data stores, and data flows. As an example, a DFD of a mess management system was described to show students, vendors, the mess secretary, manager, and warden as external entities and requisitions as a data flow.

1. DATA FLOW DIAGRAMS

2. PRESENTATION BY –
 SAYEEDA HAJIRA
 ARSHIYA FARTHEEN .A.

3. WHAT ARE DATA FLOW
DIAGRAMS ????

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)………….

7. MESS
MANAGE
MENT
SYSTEM
VENDORSSTUDENTS
MESS
SECRETARY
CHIEF
WARDEN
MESS
MANAGER
Requisitions

8.  DFD’S ARE DIFFERENT
FROM FLOW CHARTS

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

12.  LEVELLING OF DFD’S………

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.

14. Logical and physical DFD’S…

15. PHYSICAL DFDs…
VENDOR
Checking
clerk
stores
clerk
Rejected note
inventory
Accepted
note
order

16. LOGICAL DFDs…
VENDOR
checking
delivery
Stores
clerk
Rejected note
inventory
order
Accepted
note