Process Model (Data Flow Diagrams)

The process model is a core diagram in structured analysis and design. Also called a data flow diagram (DFD), it shows the flow of information through a system. Each process transforms inputs into outputs.
The model generally starts with a context diagram showing the system as a single process connected to external entities outside of the system boundary. This process explodes to a lower level DFD that divides the system into smaller parts and balances the flow of information between parent and child diagrams. Many diagram levels may be needed to express a complex system.
Primitive processes, those that don't explode to a child diagram, are usually described in a connected textual specification. This text is sometimes referred to as a mini-spec. It textually describes how the outputs are generated from the inputs.
When drawing data flow diagrams, the designer adds an entry for each data flow or store into a data dictionary. The data dictionary integrates the stack of diagrams into a cohesive model by defining all the names and data composition.
The balancing process ensures that data is conserved between diagram levels. If flow A enters a parent process, its child diagram should have flow A coming into that diagram. Likewise flow B leaving the child diagram should balance with flow B leaving the parent process. Data decomposition can occur within the data dictionary so flow A into the parent process will balance if flows X and Y enter the child diagram and A = X + Y in the data dictionary.

Yourdon/DeMarco DFD

A Yourdon/DeMarco style DFD is shown below. It includes both data flow (solid lines) and control flow (dotted lines) as specified in the Hatley/Pirbhai method. This DFD style is typically used in real-time system analysis and design.

Gane & Sarson DFD

The Gane & Sarson style DFD shown below is typically used for information systems. Here we show the flow of information in a small software company.