Symbols used in Data Flow Diagrams (DFDs)
In previous article, we discussed about the concept of Data Flow Diagrams, now it’s time to learn some symbols used in it. Designing of model using concept of DFDs is an easy way because it uses not more than 4 simple notations. Using less number of notations makes it look simple and easily understandable. DFD consists of four notations for four different functions. Functions are processes, data flows (inputs or outputs), external entities and definitely data store. So, let’s explore the symbols used in Data Flow Diagrams.
Processes in Data Flow Diagram:
A circle is used to represent a process. Processes are directed by inputs and outputs with the notation of line with arrow heads. Processes are named and when are in two or more numbers they are also numbered with numbering technique. The term bubble is also sometimes used instead of circle. Somewhere we can also use a notation for Process that is rectangle. As these notations are not in-bounded by any universal standard so, you can find either of these two notations in two different books you refer.
Data flows in Data Flow Diagram:
Data flows are represented by a line with an arrow head to show the direction of flow of data. These data flows are labeled with the name of data. A data flow shows both inputs and outputs. When data flow is directed to process then the data is taken as inputs and when data flow is directed to external entities or data store then the data is taken as output. If data flow consists of arrow head in both directions then it indicates that the data is taken as both input and output. For symbol of data flow refer Figure1-2.
External entities in Data Flow Diagram:
External entities are represented by rectangles. External entities can be vendors, accountant, customers, and managers and so on. Logically they are outside the system; it means system designer has no control over them. They either supply or consume data. Entities supplying data are known as sources and those that consume data are called sinks.
Data store in Data Flow Diagram:
Data store are represented by two parallel lines. Simply, they are used store data outgoing form processes. Processes can also retrieve data from data store. The storing and retrieving of data from data store is indicated by data flow lines and there arrow heads. If both arrows are there, it indicates that one may write in or read from the store. An alternate notation is also used for data store (rectangle with one open breadth) as shown is figure.
Data flow can take place between
- two different processes
- a data store and a process
- a process and a data store
- an external entity and a process
- a process and external entities.
But data flows cannot take place between two data stores or between two external entities.