what is a data flow diagram

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What is a data flow diagram (DFD)?

Data Flow Diagrams (DFD)helps us in identifying existing business processes. It is a technique we

benefit from particularly before we go throughbusiness process re-engineering.

At its simplest, a data flow diagram looks at how data flows through a system. It concerns things like

where the data will come from and go to as well as where it will be stored. But you won't find

information about the processing timing (e.g. whether the processes happen in sequence or in

parallel).

We usually begin with drawing a context diagram, a simple representation of the whole system. To

elaborate further from that, we drill down to a level 1 diagram with additional information about the

major functions of the system. This could continue to evolve to become a level 2 diagram when

further analysis is required. Progression to level 3, 4 and so on is possible but anything beyond level

3 is not very common. Please bear in mind that the level of detail asked for depends on your process

change plan.

The following clarifies what each symbol in Process Modeling represents.

ProcessAn activity or a function that is performed for some specific reason; can be manual orcomputerized; ultimately each process should perform only one activity

Data Flowsingle piece of data or logical collection of information like a bill

Data Storecollection of data that is permanently stored

External EntityA person, organization, or system that is external to the system but interacts

with it

Diagram Notations

Now we'd like to briefly introduce to you a few diagram notations which you'll see in the tutorial

below.

External Entity

An external entity can represent a human, system or subsystem. It is where certain data comes from

or goes to. It is external to the system we study, in terms of the business process. For this reason,
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people use to draw external entities on the edge of a diagram.

Process

A process is a business activity or function where the manipulation and transformation of data takes

place. A process can be decomposed to finer level of details, for representing how data is being

processed within the process.

Data Store

A data store represents the storage of persistent data required and/or produced by the process.

Here are some examples of data stores: membership forms, database table, etc.

Data Flow

A data flow represents the flow of information, with its direction represented by an arrow head that

shows at the end(s) of flow connector.

Level 0 Diagram

Shows all the processes that comprise the overall system

Shows how information moves from and to each process

Adds data stores


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Level 1 Diagrams

Shows all the processes that comprise a single process on the

level 0 diagram

Shows how information moves from and to each of theseprocesses

Shows in more detail the content of higher level process

Level 1 diagrams may not be needed for all level 0 processes

Level 2 Diagrams

Shows all processes that comprise a single process on the level 1

diagramShows how information moves from and to each of these

processes

Level 2 diagrams may not be needed for all level 1 processes

Correctly numbering each process helps the user understand

where the process fits into the overall system

DFD Fragment Example


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Case Study 1

ABank Manager actor provides New account details to the Open Account

process which results in Customer details being persisted in the Customer

Database data store and Account details being persisted in theAccount Database

data store. Although we have used the phrase results in as part of this explanation, the

DFD implies no such cause and effect; all it shows is that the Open Account process

can read in data from the Bank Manager interface and write out data to the

Customer Database andAccount Database data stores in no particular order.

ACustomer actor using the Online Banking Login process must provide some datain the form of a set ofLogin credentials such as a user name and password.

ACustomer actor can receive a Money amount from the Withdraw process and

can supply a Money amount to the Deposit process; in either case causing (although


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this causation cannot be explicitly modeled) anAccount balance update to the

Account Database data store.

ACustomer actor can initiate the Transfer Funds process, to which he or she must

provide anAccount destination and money amount. The Transfer Funds processcan send a Money amount to another bank via the Other Bankinterface.

Just like the Customer actor, a Third Party actor can make use of the Deposit

process (but obviously not the Withdraw process) by supplying a Money amount.


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Case Study 2

1. A hospital admission system records the details of the admission, treatment and dischargeof all patients. It also provides a number of official reports to satisfy the requirements of

external authorities.

Admission consists of the following tasks:

Administration staff enter the patients personal details from a completed hospital admission form and

sometimes helping them to complete the form.

Next they allocate appropriate hospital accommodation if one or more nights stay is required.

Following this they assign a doctor to attend for an initial examination, and check (in the case that

private medical care is required) that the patient has valid medical insurance.

Doctors and nurses use the system to record medical care given to patient during their stay. (This

medical care consists of treatments given and drugs administered). Only a doctor may prescribe drugs to

a patient, and this information is also recorded in the system. Both doctors and nurses may administer

drugs and doses are similarly recorded. Only doctors have the authority to discharge a patient, and this

information is, again, recorded within the system.

what is a data flow diagram

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