UML Data Flow Diagram in Software Engineering

A Data Flow Diagram (DFD) is just a picture (diagram) that shows how data moves inside a system:
Where data comes from (input)


Where it goes (output)


How it gets processed in between


Where it gets stored


It helps us understand a system clearly without writing long text.

1. UML Data Flow Diagram
Prepared by:
Asst. Prof. Nikul Zinzuvadiya

2. What is a DFD?
A Data Flow Diagram (DFD) is just a picture (diagram) that shows how data moves inside a system:
●Where data comes from (input)

●Where it goes (output)

●How it gets processed in between

●Where it gets stored

It helps us understand a system clearly without writing long text.

3. Characteristics of DFD
Graphical – It uses circles, arrows, rectangles, etc. instead of text. So it’s easy to read.

Problem Analysis – Helps in understanding how a system works and where improvements are needed.

Abstraction – Shows the flow of data, not the technical coding details.

Hierarchy – We can make DFDs at different levels:

●Level 0 (Context Diagram) – Big picture (overview of the system).

●Level 1, 2, … – More detailed breakdown of each process.

4. DFD = A map that shows how information travels in a system.

5. Levels of DFD
A DFD has levels, just like zooming in on a map:
●Level 0 → Big picture (overview)

●Level 1 → Breaks big picture into smaller parts

●Level 2, 3… → Goes deeper step by step

6. 0-Level DFD (Context Diagram)
Shows the whole system as one single process.
Only shows:
●Inputs (what comes in)
●Outputs (what goes out)
●External entities (users, other systems, etc.)
No details of inside working.
?????? Example: ATM = one big process → “Withdraw Cash System” with arrows for input (Card + PIN) and
output (Cash).

7. 1-Level DFD
Breaks the big process into sub-processes.

Shows how the system actually works inside.

Still not too detailed, but clearer than Level 0.
?????? Example: ATM → processes like Verify PIN, Check Balance, Dispense Cash.

8. 2-Level DFD
Goes even deeper.

Each sub-process from Level 1 is further divided.

Helps in understanding specific requirements.
?????? Example: Verify PIN → sub-processes like Read Card Info, Compare PIN with Database.

9. 3-Level DFD
Most detailed DFD.
Shows every small step, data store, and data flow.
Useful for complex systems where we need full clarity.
?????? Example: Dispense Cash → subprocesses like Send Signal to Cash Dispenser, Update Transaction
Record, Print Receipt.
In short:
●Level 0 → Big picture (just system + inputs/outputs).

●Level 1 → Break system into main processes.

●Level 2 → Break processes into smaller steps.

●Level 3 → Very detailed view of everything.

10. Main Components of a DFD
1.Process (Circle / Oval / Rounded Rectangle)
●Represents the work or function happening in the system.

●Converts input data → output data.

●Example: Verify PIN, Process Payment, Generate Report.

11. 2.Data Flow (Arrow →)
●Shows how data moves between processes, data stores, and external entities.

●Arrow direction shows where the data goes.

●Example: Customer Info → Order System.

3.Data Store (Two Parallel Lines or Open Rectangle)
●Represents where data is stored for later use.

●Can be a database, file, or even a manual folder.

●Example: Bank Database, Order Records.

●If data comes out → reading, if data goes in → storing/updating.

12. 4.External Entity (Square / Oval / Stick Figure)
●Things outside the system that interact with it.

●Can be people, organizations, or other systems.

●Example: Customer, Bank, Supplier.

In Short:
●Process → Work happening inside.

●Data Flow → Movement of information.

●Data Store → Where info is kept.

●External Entity → Outside actors interacting with the system.

14. Data CAN Flow:
1.External Entity → Process
(Example: Customer → Place Order)

2.Process → External Entity
(Example: Generate Bill → Customer)

3.Process → Data Store
(Example: Save Transaction → Bank Database)

4.Data Store → Process
(Example: Bank Database → Verify PIN)

5.Process → Process
(Example: Verify PIN → Check Balance)

15. Data CANNOT Flow:
1.External Entity → External Entity
(Customer cannot directly talk to Bank inside system without a process)

2.External Entity → Data Store
(Customer cannot directly write into Bank Database)

3.Data Store → External Entity
(Bank Database cannot directly give data to Customer without a process)

4.Data Store → Data Store
(One database cannot directly send data to another without a process)

16. Example of 0-level DFD

17. Shows the system as a single process (Railway Reservation System).

External entities interact with it:

●Passenger → sends request (book ticket).

●Train Schedule → provides train timings/details.

●Reservation → system gives confirmation/output.

Focus is only on inputs and outputs, not on internal details.

18. Example of 1-level DFD



What Level 1 DFD Shows?
●It takes the single process from Level 0 (Railway Reservation System)

●Breaks it into smaller processes (sub-processes)

●Shows data flow between processes, entities, and data stores

20. Processes in Level 1 DFD (Railway Reservation System)
1.Enter Passenger Detail – Passenger provides details to start the process.

2.Make Reservation – Reservation request is handled here.

3.Cancel Reservation – Used if a passenger wants to cancel.

4.Check Availability – Looks into train schedule for seat availability.

5.Update Reservation – Updates records after booking or cancellation.

21. External Entities
●Passenger – Provides details, requests booking/cancellation.

●Reservation Clerk – Helps in reservation and cancellation.

●Train Time Table – Provides train schedule.

Data Stores
●Passenger Data – Stores passenger information.

●Reservation Data – Stores reservation records.

●Train Schedule – Stores timetable and availability info.

22. Example of 2-level DFD

●It takes each process from Level 1 and breaks it into smaller sub-processes.

●Shows more details about data movement inside the system.

●Useful when we need to analyze specific parts of the system in depth.

23. "Make Reservation" Process (from Level 1 DFD)
In Level 2, this can be expanded into:
1.Collect Passenger Details
○Get name, age, ID proof, journey date, etc.
○Data goes into Passenger Data store.
2.Check Train Availability
○Cross-check with Train Schedule.
○If seats available → proceed, else reject.
3.Process Payment
○Passenger pays via cash, card, or online.
○Payment request sent to Payment System.
4.Generate Ticket
○Create a ticket with reservation number.
○Save into Reservation Data Store.
○Send confirmation to Passenger.

24. Data Stores in Level 2
●Passenger Data (details of travelers)

●Reservation Data (bookings/cancellations)

●Train Schedule (train timings and seat availability)

●Payment Records (transaction details)

25. Advantages of DFD
1.Helps Understand the System – Shows how information flows step by step.

2.Simple Visual Representation – Easy to understand for both technical and non-technical people.

3.Breaks Down Complex Systems – Divides into smaller processes for clarity.

4.Good for Documentation – Useful for recording system design for current and future use.

26. Disadvantages of DFD
1.Time-Consuming – Making detailed DFDs for big systems takes a lot of effort.

2.Limited Scope – Focuses only on data flow, doesn’t show security, performance, or UI.

3.Updating Issues – If the system changes often, keeping DFDs updated is hard.

4.Needs Expertise – Easy to read, but hard to create correctly without proper knowledge.