Data Flow Diagram: The Top-Down Decomposition Technique

As we know, the top-down approach is easier to solve the problem, that is, start with the overview and then develop the details. Similarly, hierarchical DFDs are easier to understand than a single large detailed DFD.

Hierarchical DFDS are drawn using a top-down approach, that is, at the top level DFD or context diagram, and then this level 0 DFD is expanded from (extended and divided) the main process bubble of level 0 DFDS to level 1 DFDs.

In other words, the data flow diagram uses a layered form to describe the data flow of the system. Each level represents an abstract level of the data flow of the system. The higher the level, the more abstract the data flow. The processing of high-level DFD can be further decomposed into low-level, more detailed DFDs as shown in the Figure below.

According to the hierarchical data flow diagram, it is divided into top-level data flow diagram, middle-level data flow diagram and bottom-level data flow diagram. Except for the top-level data flow diagram, the other data flow diagrams are numbered 1, 1.1, 1.1.1 and etc.

Context Diagram: The Top Level DFD

The top-level data flow diagram contains only one processing to represent the entire system; the output data flow and the input data flow are the input data and output data of the system, indicating the scope of the system and the data exchange relationship with the external environment.

The middle-level data flow diagram is to refine a certain process in the parent-level data flow diagram, and a certain process of it can also be refined again to form a sub-diagrams; the number of middle levels generally depends on the complexity of the system.

At the bottom level, if the underlying data flow diagram process can no longer be decomposed, they are considered "atomic processes".

Consistency between multiple levels of DFDs

When performing top-down decomposition to a DFD to lower level DFDs, the inputs and outputs must be conserved between levels of DFDs. For example, level n & n+1 must have the same inputs and outputs

Data Flow Diagram Examples

Data Flow Diagram for a securities trading platform. Edit this DFD online.

A DFD for a food ordering system. Edit this DFD with our DFD maker online.

A data flow diagram example for a customer service system. Open and edit this DFD with our online DFD creator.