Rules for Data Flow Diagram

Following are the rules which are needed to keep in mind while drawing a DFD(Data Flow Diagram).

• Data can not flow between two entities. –
  Data flow must be from entity to a process or a process to an entity. There can be multiple data flows between one entity and a process.
  
  
• Data can not flow between two data stores
  Data flow must be from data store to a process or a process to an data store. Data flow can occur from one data store to many process.
  
  
• Data can not flow directly from an entity to data store –
  Data Flow from entity must be processed by a process before going to data store and vice versa.
• A process must have atleast one input data flow and one output data flow –
  Every process must have input data flow to process the data and an output data flow for the processed data.
• A data store must have atleast one input data flow and one output data flow –
  Every data store must have input data flow to store the data and an output data flow for the retrieved data.
• Two data flows can not cross each other.
• All the process in the system must be linked to minimum one data store or any other process.