Data Flow Diagrams

Definition

Data Flow Diagrams (DFDs) model how data moves through a system by showing the processes that transform it, the data stores that hold it, and the external entities that produce or consume it. DFDs focus exclusively on data movement and transformation — they do not model control logic, decisions, or sequences. They are used at multiple levels of detail, from high-level context diagrams to granular process decompositions.

Inputs

• Defined system scope and boundaries
• Identified external entities (people, systems, or organizations)
• Business processes and data stores within scope
• Requirements and interface documentation

Outputs

• Context diagram (Level 0) showing the system boundary and external entities
• Level 1 and Level 2 DFDs decomposing major processes
• Identified data stores, flows, and transformation points
• Inputs to data modelling, integration design, and requirements analysis

When to Use

• Understanding and communicating the data requirements of a system
• Identifying processes, data stores, and external dependencies before system design
• As a starting point for data modelling or interface analysis
• Legacy system documentation and modernization planning

When Not to Use

• When control flow or process sequencing is the primary concern — use process models or sequence diagrams instead
• When the system boundary is unclear and scope is still being defined
• When stakeholders need to see decision logic — DFDs do not model decisions