Data Flow Diagram DFD
A data flow diagram DFD is a graphical representation of the flow of data through an information system. They are also known as bubble charts.
- Shows how information is input to and output from the system, the sources and destinations of that information, and where that information is stored.
- Maps out the flow of information for any process or system.
- Uses defined symbols like rectangles, circles and arrows, plus short text labels, to show data inputs, outputs, storage points and the routes between each destination.
Symbols and Notations Used in Data Flow Diagram DFD
Two common systems of symbols are named after their creators:
- Yourdon and Coad
- Gane and Sarson
Rules of DFD
- Each process should have at least one input and an output.
- Each data store should have at least one data flow in and one data flow out.
- Data stored in a system must go through a process.
- All processes in a DFD go to another process or a data store.
Basic Terminologies of DFD
Black hole: The situation where the processing step may have input flows but no output flows is called a black hole.
Miracle: The situation where the processing step may have output flows but now input flows is called a miracle.
Grey hole: The situation where the processing step may have outputs that are greater than the sum of its inputs – e.g., its inputs could not produce the output shown is referred to as a grey hole.
Major components of Data Flow Diagram DFD
DFDs are constructed using four major components:
- External entities
- Data stores
- Processes and
- Data flows
- External entities represent the source of data as input to the system. They are also the destination of system data. External entities can be called data stores outside the system. These are represented by squares.
- Data stores represent stores of data within the system, for example, computer files or databases. An open-ended box represents a data, which implies store data at rest or a temporary repository of data.
- Processes represent activities in which data is manipulated by being stored or retrieved or transferred in some way. In other words, we can say that process transforms the input data into output data. Circles stand for a process that converts data into information.
- Data flow represents the movement of data from one component to the other. An arrow (→) identifies data flow, i.e. data in motion. It is a pipeline through which information flows. Data flows are generally shown as one-way only. Data flows between external entities are shown as dotted lines (——›).
Different levels of Data Flow Diagram DFD
DFD levels are numbered 0, 1 or 2, and occasionally go to even Level 3 or beyond. The necessary level of detail depends on the scope of what you are trying to accomplish.
0 level DFD
It is also called a Context Diagram. It shows a glance as if you are looking into a system through a helicopter. It’s a basic overview of the whole system or process being analyzed or modeled. It’s designed to be an at-a-glance view, showing the system as a single high-level process, with its relationship to external entities. It should be easily understood by a wide audience, including stakeholders, business analysts, data analysts and developers.
1 level DFD
DFD Level 1 provides a more detailed breakout of pieces of the Context Level Diagram. Here the main functions carried out by the system are highlighted as we break down the high-level process of the Context Diagram into its sub-processes.
2 level DFD
DFD Level 2 then goes one step deeper into parts of Level 1. It may require more text to reach the necessary level of detail about the system’s functioning.
3 level Data Flow Diagram DFD
and so on
Progression to Levels 3, 4 and beyond is possible, but going beyond Level 3 is uncommon. Doing so can create complexity that makes it difficult to communicate, compare or model effectively.
Logical vs Physical Data Flow Diagram DFD
|Logical DFD||Physical DFD|
|1. Logical DFD depicts how the business operates.||1. Physical DFD depicts how the system will be implemented (or how the current system operates).|
|2. The processes represent the business activities.||2. The processes represent the programs, program modules, and manual procedures.|
|3. The data stores represent the collection of data regardless of how the data are stored.||3. The data stores represent the physical files and databases, manual files.|
|4. It s how business controls.||4. It show controls for validating input data, for obtaining a record, for ensuring successful completion of a process, and for system security.|
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A Soon-to-be Computer Engineer by profession, he has profound devotion to add Data Scientist as a prefix to his name. He’s been to the nooks of the web (at least of the surface web) and planning to add some real figures in his pocket!