Real example of a data flow diagram!

Data flow diagrams come up a lot when people are trying to map out a system, so I wanted to share a real example most of you can actually relate to instead of the complex stuff.

An online shopping checkout is a good one because everyone has been through it.

At the highest level (Level 0), a customer (external entity) sends order details into the checkout process. That process talks to the payment gateway, writes to the order database, sends a confirmation back to the customer, and updates inventory. It involves one bubble for the process, a couple of data stores, an external entity, plus arrows for the data moving between them. That’s what a Level 0 DFD looks like in its simplest form.

Go down a layer to Level 1 and that single checkout bubble breaks into smaller processes. Validate cart, calculate tax and shipping, authorize payment, write the order to the database, send confirmation. It’s the same data flowing through, just zoomed in to show the inner mechanics.

Level 2 zooms further into any one of those. Take payment authorization, you’d show the back and forth with the gateway, the retry logic, the error path, the fraud check. The deeper you go, the more implementation detail shows up.

A couple things worth knowing before you start drawing:

Logical vs physical DFDs.

Logical shows what the system does. Physical shows how it does it, including the actual software, hardware, files, and people involved in the flow. Most folks start logical, then move to physical when implementation gets closer.

The notation conventions you’ll run into.

Yourdon-Coad and Yourdon-DeMarco both use circles for processes. Gane-Sarson uses rounded rectangles. The shapes differ but the logic is identical. Pick one and stay consistent across your diagrams.

A few habits that help:
- Number your processes (1.0, 1.1, 1.2) so levels stay traceable
- Every data flow needs a label, no unnamed arrows
- Data stores never talk directly to each other or to external entities, always through a process
- If a Level 1 diagram has more than seven processes, you’re probably trying to do too much on one canvas

If you want the full walkthrough with the shopping checkout diagrammed out and the symbol reference for the different conventions, here’s the full tutorial.