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)

1. Process (Circle/Rounded Rect): Transforms data.
   • Format: Verb + Noun.
   • Example: Calculate Tax, Validate User.
2. External Entity (Square): Source or Sink of data (Outside scope).
   • Example: Customer, Bank API, Admin.
3. Data Store (Open Rectangle/Parallel Lines): Resting data.
   • Example: Customer DB, Orders File.
4. 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:

1. Process 1.0: Validate Order
   • Input: Order (from Customer)
   • Output: Valid Order
2. Process 2.0: Calculate Total
   • Input: Valid Order + Price (from Product DB)
   • Output: Total Amount
3. Process 3.0: Process Payment
   • Input: Total + Card Info
   • Output: Receipt

Methodology Rules

1. Conservation of Data: A process cannot create data from nothing (must have input). A process cannot consume data without output (black hole).
2. 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)
3. 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

1. Identify Entities: Who sends/receives data? (Customer, Manager, API).
2. Identify Processes: What logical actions happen? (Check inventory, Save user).
3. Identify Data Stores: Where is info kept? (Files, Tables).
4. Draw Level 0: Connect Entities to "The System".
5. Explode to Level 1: Break system into sub-processes.
6. 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).