Unit 11.1C · Term 1

Data Flow Diagrams (DFD)

A Data Flow Diagram is a visual tool that shows how data moves through a system — where it comes from, what processes transform it, and where it is stored. DFDs are essential in the analysis and design stages of the SDLC.

Learning Objectives

11.2.1.6 Draw and interpret Data Flow Diagrams

Lesson Presentation

11.1C-dfd.pdf · Slides for classroom use

View Download

Conceptual Anchor

The Plumbing Map Analogy

A DFD is like a plumbing diagram for data. The pipes (arrows) show where data flows. The taps and sinks (external entities) are where data enters and leaves. Valves and filters (processes) transform the data. Tanks (data stores) hold data for later use.

Rules & Theory

DFD Symbols

Symbol	Shape	Represents	Example
External Entity	Rectangle / Square	Source or destination of data (outside the system)	Customer, Bank, Teacher
Process	Rounded rectangle / Circle	An action that transforms data	Validate login, Calculate total
Data Store	Open-ended rectangle (two lines)	Where data is stored permanently	D1 Student Database
Data Flow	Arrow (→)	Direction data moves	Order details, Payment

DFD Levels

Level	Name	Detail
Level 0	Context Diagram	Shows the entire system as ONE process, with external entities
Level 1	Main DFD	Breaks the system into major processes, shows data stores
Level 2+	Detailed DFD	Further decomposition of Level 1 processes

Example: Library System

CONTEXT DIAGRAM (Level 0):

  ┌──────────┐                              ┌──────────┐
  │ Student  │──── Book Request ───────────→│          │
  │          │←─── Book / Rejection ────────│ LIBRARY  │
  └──────────┘                              │ SYSTEM   │
                                            │          │
  ┌──────────┐                              │          │
  │ Librarian│──── New Book Details ───────→│          │
  │          │←─── Reports ────────────────│          │
  └──────────┘                              └──────────┘


LEVEL 1 DFD:

  ┌──────────┐   Book Request    ┌─────────────┐
  │ Student  │─────────────────→│ 1. Search   │
  └──────────┘                   │    Catalogue │
       ↑                         └──────┬──────┘
       │                                │ Book ID
       │                                ↓
       │                         ┌─────────────┐    ══════════
       │    Book / Rejection     │ 2. Process  │←──║ D1 Books ║
       └─────────────────────────│    Loan     │    ══════════
                                 └──────┬──────┘
                                        │ Loan record
                                        ↓
                                  ══════════════
                                 ║ D2 Loans    ║
                                  ══════════════

DFD Rules

Rule	Description
No data flow between entities	External entities don't communicate directly — data must flow through a process
No data flow between stores	Data stores don't connect directly — a process must move data between them
Label all arrows	Every data flow must have a descriptive label
Process must have I/O	Every process must have at least one input and one output
Number processes	Use 1, 2, 3... for Level 1; 1.1, 1.2... for Level 2

DFD vs Flowchart

A DFD shows what data flows where. A flowchart shows the sequence of steps. DFDs don't show decisions (if/else) or loops — they focus purely on data movement.

Common Pitfalls

Connecting Entities Directly

Two external entities must NEVER have a data flow arrow between them. All data must flow through a process (the system must be involved).

Tasks

Remember

Draw and label the 4 DFD symbols.

Apply

Draw a Context Diagram for a school exam system (entities: Student, Teacher, Admin).

Apply

Draw a Level 1 DFD for an online pizza ordering system.

Self-Check Quiz

Q1: What are the 4 DFD symbols?

External Entity (rectangle), Process (rounded rect/circle), Data Store (open-ended rectangle), Data Flow (arrow).

Q2: What is a Context Diagram?

A Level 0 DFD that shows the entire system as a single process, with all external entities and their data flows.

Q3: Can two external entities be connected directly?

No — data must always flow through a process. External entities never communicate directly on a DFD.