Data Flow Diagrams (DFD) Skill
Purpose
Visualize how data moves through a system. DFDs show *what* the system does with data, unlike Flowcharts which show control flow (sequence).
When to Use
- Understanding legacy systems.
- Analyzing data privacy (where does PII go?).
- Designing integrations.
- Security auditing (identifying trust boundaries).
DFD Notation (Gane & Sarson or Yourdon)
- Process (Circle/Rounded Rect): Transforms data.
- Format: Verb + Noun. - Example: Calculate Tax, Validate User.
- External Entity (Square): Source or Sink of data (Outside scope).
- Example: Customer, Bank API, Admin.
- Data Store (Open Rectangle/Parallel Lines): Resting data.
- Example: Customer DB, Orders File.
- Data Flow (Arrow): Movement of data packets.
- Label: Order Details, Invoice.
DFD Levels
Level 0: Context Diagram
Scope: Entire System as ONE process. Shows: System boundary and interaction with external world.
Example:
graph LR
Customer[Customer] -->|Order| System((Order Mgmt System))
System -->|Invoice| Customer
System -->|Payment Info| Bank[Bank]
Bank -->|Confirmation| System
Warehouse[Warehouse] -->|Stock Levels| System
System -->|Shipping Request| Warehouse*Note: NO data stores are shown in Level 0 unless shared with external systems.*
Level 1: System Breakdown
Scope: Breaks the "One Process" into major sub-processes (3-7 bubbles). Shows: Data stores used by these processes.
Example:
- Process 1.0: Validate Order
- Input: Order (from Customer) - Output: Valid Order
- Process 2.0: Calculate Total
- Input: Valid Order + Price (from Product DB) - Output: Total Amount
- Process 3.0: Process Payment
- Input: Total + Card Info - Output: Receipt
Methodology Rules
- Conservation of Data: A process cannot create data from nothing (must have input). A process cannot consume data without output (black hole).
- No Direct Connections:
- Entity ↔ Entity (Out of scope) - Entity ↔ Data Store (Must go through a process) - Data Store ↔ Data Store (Must go through a process)
- Labeling: All arrows must be labeled with the *data* moving (not the action).
DFD vs. Flowchart
| DFD | Flowchart |
|---|---|
| Data Focus | Control/Sequence Focus |
| "Calculate Tax" | "If Tax > 0 Then..." |
| No Looping/Branching | Has Loops/Decisions |
| Parallel/Async implied | Sequential implied |
Creating a DFD in Steps
- Identify Entities: Who sends/receives data? (Customer, Manager, API).
- Identify Processes: What logical actions happen? (Check inventory, Save user).
- Identify Data Stores: Where is info kept? (Files, Tables).
- Draw Level 0: Connect Entities to "The System".
- Explode to Level 1: Break system into sub-processes.
- Verify: Check for data conservation and missing flows.
Best Practices
- Keep it logical: Don't worry about physical implementation (e.g., "Paper file" vs "SQL DB" is irrelevant for logical DFD).
- Number processes: 1.0, 2.0, 3.0. Sub-processes: 1.1, 1.2.
- Limit complexity: If a level has > 7 processes, decompose further (Level 2).
Tools
- Visio.
- Lucidchart.
- Draw.io.
- Mermaid (Graph style).