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

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).

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Repository

danhvb/my-ba-skills

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Jan 1, 1970

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