# Functional Modelling in object oriented analysis and design

• Difficulty Level : Medium
• Last Updated : 22 Apr, 2020

Functional Modelling provides the outline that what the system is supposed to do.It does not describes what is the need of evaluation of data, when they are evaluated and how they are evaluated apart from all it only represent origin of data values.It describes the function of internal processes with the help of DFD (Data Flow Diagram).

Data Flow Diagrams: Function modelling is represented with the help of DFDs. DFD is the graphically representation of data.It shows the input, output and processing of the system .When we are trying to create our own business, website, system, project then there is need to find out how information passes from one process to another so all are done by DFD. There are number of levels in DFD but upto third level DFD is sufficient for understanding of any system.

The basic components of the DFD are:

1. External Entity :
External entity is the entity that takes information and gives information to the system. It is represented with rectangle.
2. Data Flow :
The data passes from one place to another is shown by data flow. Data flow is represented with arrow and some information written over it.
3. Process :
It is also called function symbol.It is used to process all the information.If there are calculations so all are done in the process part.It is represented with circle and name of the process and level of DFD written inside it.
4. Data Store :
It is used to store the information and retrieve the stored information.It is represented with double parallel lines.

Some Guidelines for creating a DFD:

1. Every process must have meaningful name and number.
2. Level 0 DFD must have only one process.
3. Every data flow and arrow has given the name.
4. DFD should be logical consistent.
5. DFD should be organised in such a way that it is easy to understand.
6. There should be no loop in the DFD.
7. Each DFD should not have more than 6 processes.
8. The process can only connected with process, external entity and data store.
9. External entity cannot be directly connected with external entity.
10. The direction of DFD is left to right and top to bottom representation.
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