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.