DFD levels: context diagram, Level 1 & Level 2

What are DFD levels?

A single diagram that showed every detail of a system would be impossible to read, so data flow diagrams are drawn in levels. Each level zooms into the level above it:

• Context diagram (Level 0): the whole system as one process.
• Level 1 DFD: that one process split into its main sub-processes.
• Level 2 DFD: one Level 1 process split into detailed steps.

This top-down breakdown is called functional decomposition or levelling. Most college projects need a context diagram and a Level 1 DFD, and larger systems also need Level 2 for their most complex processes.

Context diagram (Level 0)

The context diagram shows the system boundary: what is inside the system and who it talks to.

• Exactly one process, numbered 0, named after the whole system.
• All external entities that send data to or get data from the system.
• The main data flows between them.
• Usually no data stores, because those are internal details.

Open this context diagram in DFD Maker →

Level 1 DFD

The Level 1 DFD opens process 0 and shows the main functions inside it.

• Usually 3 to 7 processes, numbered 1.0, 2.0, 3.0…
• Data stores appear here (D1, D2…), showing where each process reads and writes data.
• The same external entities and outside data flows as the context diagram. This is called balancing (see below).
• Processes can pass data directly to each other.

Open a Level 1 example in DFD Maker →

Level 2 DFD

A Level 2 DFD takes one process from Level 1 and shows how it works in detail. If process 1.0 is “Place Order”, its Level 2 might contain:

• 1.1 Browse Menu
• 1.2 Add Items to Cart
• 1.3 Validate Order
• 1.4 Save Order

Only draw Level 2 for processes that are complex enough to need it. In DFD Maker you can frame a Level 2 diagram with a System boundary shape labelled with the parent process, like “1.0 Place Order”.

How to number processes

The number tells you exactly where a process sits in the hierarchy. DFD Maker numbers new processes and data stores automatically, and you can edit any number in the side panel.

Balancing DFD levels

A lower-level DFD must be balanced with its parent. The data coming in and going out at the child level must match the data flows connected to the parent process.

• If the context diagram shows “Order details” coming from Customer, the Level 1 DFD must also show “Order details” coming from Customer.
• Don't add new external entities at a lower level, and don't drop existing ones.
• New data stores and internal flows are allowed at lower levels.

When to stop breaking processes down

Stop when a process does one clear job that you could describe in a few sentences, or when splitting it further would only show single steps like “read record” or “add numbers”. For most projects that's Level 1 or Level 2.

Logical vs physical DFD

• Logical DFD: shows what the system does, the business activities and data, without saying how.
• Physical DFD: shows how it's done: the actual software, files, people and devices (for example, “MySQL orders table” or “Razorpay API”).

You draw both with the same symbols. Only the names change.