Data flow diagram is used to represent the flow of data in the ssytem that are flow with some modification,data flow diagram

show that flow of data where transformation of data take place, it helps in understanding function of system that transforms data from surce to final activity in the system.









Data flow diagram has following components

Processes

Process or processes are the action or work done in the ssytem which perform transformation in the ssytem.It is represented by rounded square.

External agent

External agent are the external element that interact with system provide information or request information to the system.It is represented by square

Data store

Datastore are those places where data is store after input is provided by the external agent and transformed by the processes.It is represented by rectangle with small square at left side.

Data flow

It is used to represent the flow of data from one component to another component.It is represented by arrow line.

Processes that show in data flow diagram

Following are different process that are allowed to represent in dfd.

Process that computes

Process that perform calculation on data and produces new data or perform computation are allowed to show in data flow diagram.

For ex in calculating salary in payroll system taking input of how many days work is don and calculate salary.

Processes of decision making

Process that make decision in the sytem are show in the Data flow diagram.For ex make decision to perform a transaction or not.

Process that rigger other processes.

Processes that trigger other process or function are showed in dara fkiw diagram. For ex in telecommunication company system a process each month triggers billing process that calculate one month internet usage and generate a print of bill.

Processes that action on stored daat

Process that perform on stored data either on files or in daatbase , In database these all operation is called CRUD i.e. create,read,update and delete means creation of data,reading data from database,updating data and deleting data.

Level of abstraction or level of data flow diagram

Level of data flow diagram means creating data flow diagram with different level like zero level dfd ,first level dfd ,third level dfd or can create as many level s depends upon complexity of ssytem.

In zero level dfd there is a simple context diagram with processes ,external agent,data store and daat flow.

At sexond level dfd more details of the processes are show to define flow of data.

Example of dara flow diagram

In the following example we discuss a patient monitoring system in the hospital.

Zero level data flow diagram

Here is context diagram (zero level dara flow diagram) of the patient montiroing system

Processes- Patient monitoring ssytem

External agent- Patient,Nutse

Data store- Patient log

From following context diagram of patient monitoring ssytem first patient(external agent) vital signs (heart beat, blood pressure) is given to the patient monitoring system (processes) it porcess to it to standard bound if if crosses to the limit a warraning message is sent to nurse (external agent) and handles patient or if it does not cross the limits then patient monitoring system generate report to the ssytem and also nurse can request for report in future, patient monitoring system also send patient log data is send to store in patient log(data store) and receive daya from patient log.

In the following zero level dfd patient monitoring system(process ) is explain in broad aspect, no detail is showed how report is generated, how vital signs are matched, so zero level dfd show overal data flow of the system.

First level data flow diagram

From previous zero level dfd of patient monitoring ssytem the process patient monitoring system is describe as broad level , first level data flow diagram describes this processes in detail , show internal sub processes of this big process.

From the following frist level dfd of patient monitoring system, process patient monitoring system describes in detail as follow

First patient(external agent) give its vital signs to local monitoring system (sub process of patient monitroing system) then pass data to central monitoring system (sub process of patient monitroing system) which has access to patient data(data store), it takes vital signs bound from data store and match with current patient vital sigms and if it cross the limit of vital sign then central monitoring system generate warraning message to nurse(external agent).

Common mistake in data flow diagram

If in dfd process has only inputs but not any output then it is an mistake.

If in data flow diagram processes has only output but mot nay input then it is an mistake.

If in dfd any output is generated without any sufficient input then it is am mistake. for ex If in banking system system an employee statement is generated with input of its address , then it is insufficient as without amount it is not possible to generate an statement.

Illegal things in data flow diagram

if two external agnet passes daat to each other it is illegal, there must be aprocess between them.

if two data store passes data between them it is illegal , there must be a process between them.

if a external agent and a data store has flow of data it is illegal m there must be a process between them.