The four DFD symbols at a glance
Every data flow diagram, from a simple context diagram to a detailed Level 2 DFD, uses the same four building blocks. The table shows what each one means and how it's drawn in the two common notations.
| Symbol | Meaning | Yourdon–DeMarco | Gane–Sarson |
|---|---|---|---|
| External entity | Source or destination of data outside the system | Rectangle | Rectangle (often with a shadow) |
| Process | Transforms incoming data into outgoing data | Circle (bubble) | Rounded rectangle with a number strip |
| Data store | Data at rest: a table, file or register | Two parallel lines | Open-ended box with an ID section |
| Data flow | Data moving between the other symbols | Labelled arrow | Labelled arrow |
1. External entity
An external entity (also called a terminator, source or sink) is a person, organisation or other system that sits outside the system you're modelling but sends data to it or receives data from it.
- Examples: Customer, Student, Supplier, Bank, Payment Gateway, Admin, Government Portal.
- Name it with a noun: the role, not a specific person (write “Customer”, not “Rahul”).
- Entities can only exchange data with processes, never directly with each other or with a data store.
2. Process
A process is work the system does that changes data. It takes one or more input flows and produces one or more output flows.
- Name it with a verb and a noun: “Validate Login”, “Calculate Fees”, “Generate Invoice”.
- Number it: 0 on a context diagram, then 1.0, 2.0, 3.0 on Level 1, and 1.1, 1.2 on Level 2. DFD Maker numbers processes for you.
- Every process needs at least one input and one output. A process with inputs but no output is a black hole, and one with outputs but no input is a miracle.
3. Data store
A data store is where data is kept for later use: a database table, a file, a register or even a paper folder. It shows data at rest.
- Name it with a plural noun: “Orders”, “Students”, “Payments”. Give it an ID like D1, D2.
- Only processes can read from or write to a data store.
- A flow into a store means writing or updating it. A flow out of a store means reading it.
- Context diagrams usually don't show data stores. They first appear on Level 1.
4. Data flow
A data flow is an arrow showing data moving from one symbol to another. The arrow's direction shows which way the data travels.
- Always label it with the data it carries, using a noun: “Payment details”, “Login credentials”, “Invoice”.
- Show data, not actions or triggers. “Click submit” is not a data flow.
- Every data flow must start or end at a process.
- If the same data goes both ways, you can use a two-way arrow, but two separate labelled arrows are usually clearer.
Extra DFD symbols
These appear in some textbooks and larger diagrams. All of them are in DFD Maker's shape panel.
- Duplicate entity / duplicate store: the same entity or store drawn a second time to avoid crossing lines. It's marked with a slash in the corner (entity) or an extra line (store).
- Multi-process: a process drawn with a double outline to show it's broken down further in a lower-level DFD.
- Control process: drawn dashed. Used in real-time DFDs (the Ward–Mellor extension), where processes are triggered by control signals rather than data.
- System boundary: a dashed box around the processes that belong to the system. It helps show what's inside and outside.
- Trust boundary: a dashed line used in security and threat-model DFDs to mark where data crosses between different levels of trust (for example, from the internet into your server).
Rules for connecting DFD symbols
| From → To | Allowed? | Why |
|---|---|---|
| Entity → Process | ✅ Yes | Data enters the system |
| Process → Entity | ✅ Yes | Data leaves the system |
| Process → Process | ✅ Yes | One step passes data to the next |
| Process ↔ Data store | ✅ Yes | Reading and writing stored data |
| Entity → Entity | ❌ No | Outside the system, so not part of the DFD |
| Entity ↔ Data store | ❌ No | Data must pass through a process first |
| Data store → Data store | ❌ No | Moving data needs a process |
DFD Maker's Check DFD rules button finds all of these mistakes automatically. See DFD levels explained for how the symbols fit together level by level.
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