Data Flow Diagram (DFD)
Definition
A Data Flow Diagram (DFD) is a graphical representation of how data flows through a system — showing processes, data stores, external entities, and the data flows connecting them, WITHOUT showing control logic or timing.
DFD Symbols (Yourdon/DeMarco Notation)
| Symbol | Meaning |
|---|---|
| Circle / Rounded Rectangle | Process — transforms input data into output data |
| Rectangle | External Entity — source or destination of data outside the system (user, other system) |
| Open-ended Rectangle / Two parallel lines | Data Store — where data is held (file, database table) |
| Arrow | Data Flow — direction of data movement, labeled with the data name |
Levels of DFD
Context Diagram (Level 0)
- Shows the entire system as a single process
- Shows external entities and the data flows in/out of the system
- No internal detail
Level 1 DFD
- Decomposes the single process into major sub-processes
- Shows internal data stores and flows between sub-processes
Level 2+ DFD
- Further decomposes any Level 1 process into more detail, as needed
- Decomposition continues until processes are simple enough to specify directly
Example: Simple Order System (Level 1)
[Customer] ──order──▶ (1.0 Process Order) ──order details──▶ [Order Data Store]
│
confirmation
▼
[Customer]
Rules for DFDs
- Every process must have at least one input and one output
- Data cannot flow directly between two external entities (must go through a process)
- Data cannot flow directly between two data stores (must go through a process)
- Each process should be numbered consistently across levels (e.g., Process 1 decomposes into 1.1, 1.2, ...)
Advantages
- Simple, intuitive graphical notation
- Good for communicating with non-technical stakeholders
- Basis for Structured Design (DFD → Structure Chart via Transform/Transaction Analysis)
Limitations
- Doesn't show timing, sequencing, or control logic
- Doesn't scale well visually for very complex systems without careful leveling
Exam tip: Practice drawing a Context Diagram (Level 0) and a Level 1 DFD for a simple system (Library, ATM, Order Processing) — a near-guaranteed diagram question. Remember the "no direct entity-to-entity or store-to-store flow" rule.