Diagram Desk

Data flow diagram example

Where data moves: people, processes, and stores.

StartActionYes?End
Flowchart: start, one action, a yes or no, then an end.

Data moves. Control flow does not.

A data flow diagram (DFD) shows how information moves. External entities, drawn as rectangles, are people or systems outside the piece you are describing. Processes transform data. Data stores hold it. Arrows are data flows, and each arrow should carry a noun phrase: borrow request, loan record, availability. A sample data flow diagram is not a flowchart. It does not show yes or no, and it does not show the order of a clerk's afternoon.

A context diagram is the smallest sample: one process in the middle and the external entities around it. Later levels split that process. Start at context. Most arguments about a system can be settled there.

Example: library checkout

External entity: patron. Process: check out a book. Data store: loans. Data store: catalog. Flows: patron sends a borrow request (card number and book id) into the process. The process reads availability from the catalog and writes a loan record to loans. The process returns a due date to the patron. No diamond asks whether the patron was polite. That fact is not data this process stores.

If you later need servers and apps, move to a software architecture diagram. The DFD has done its job when every arrow is a thing you could write down.

Steps

  1. Name one process. A verb phrase: check out a book. 'Library system' is too big for a first diagram.
  2. Place external entities. Who sends data in, and who receives data out.
  3. Name every arrow. Nouns. An unnamed arrow is a sign the flow is unclear.
  4. Add a store only if data rests. If nothing is kept after the process ends, you do not need a store.

Questions

What does a data flow diagram show?
How data moves among external entities, processes, and data stores. Arrows are named pieces of data, not decisions.
What is a sample data flow diagram for beginners?
One process, one or two outside people, and one store. The library checkout above is that size: a patron, a checkout process, a catalog, and a loans store.
How is a DFD different from a flowchart?
A flowchart shows the order of actions and decisions. A data flow diagram shows what information moves and where it is kept. It omits yes or no branches.

Keep reading