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).