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Rules For Creating Data Flow Diagrams

25 Feb

When creating data flow diagrams (DFD’s), there are certain rules which must be followed. This rules allow for the DFD to be make sense and also to be easily understood. In this blog I will go through the rules which must be followed and show practical examples of these rules.

1. All data flows must flow to or from a process

All flows of data must be either coming from or going to a process. External entities can not flow directly to each other. A data flow can not link a data store to an external entity. Data can not move between data stores without first being processed.


2. A Process must have at least one input flow and one output flow.

When a process has input flow but no output flow, it is called a “black hole”. When a process has output flows but no input flows, it is called a “miracle”.


A process must have at least one input flow and one outflow flow.

3. The inputs to a process must be sufficient to produce output flows.

A “grey hole” is when the outputs of a process are greater then the sum of its inputs. For example, if a customers name and address is an input, their bank details can not be an output, as the process doesn’t have enough information to produce it.

4. Processes must transform data.

When naming data flows, adjectives should be used which show how processing has changed the data flow.


5. Data Flows can not cross each other.

The flows of data can not cross each other. To overcome this problem, data stores and entities can be duplicated. However, processes can not be deplicated. Data flows must be unidirectional.

6. Entities must be labelled in lower case.



Cathal Doyle Lecture 17:


Data Flow Diagrams: A Clearer Insight

25 Feb

As mentioned in my previous blogs, Data Flow Diagrams (DFD’s) are graphical representations of the flow of data in an information system. A form of process modelling, they are made up of four main components:


Processes are jobs that are there to be done within the data. They are rectangular in shape. They transfer incoming data flows into outgoing data flows.

External Entities

These are any class of people, an organization or another system that exists outside the system that interacts with the system. These are the places that provide the organisation with data sent to them by the organisation. (eg. customers, partners) These are generally oval in shape.

Data Flows

These mark movement in a system and can be described as a pipeline that will carry the data. These arrows connect the processes, external entities and data stores. Without data flows the DFD would have no substance.

Data Stores

The best description of these is data at rest. It represents holding areas for collection of data. Processes add or retrieve data from these stores. Only processes are connected to data stores.

DFD’s fit in at the Analysis and Design phase of the Software Development Life Cycle. In the analysis stage it is mainly fact finding. They investigate business processes and the current system. They help with modelling the current systems. In the Design phase, it describes how the system will fulfill the user requirements. It also looks after the modelling of the current systems.

In my next blog I will focus on the advantages of both flowcharts and data flow diagrams.

References:– Lecture 17

The image attatched shows what a complex DFD may look like.


Flowcharts for dummies

23 Feb


A flowchart is used for analysing, processes, programs or systems. The system or process is divided into events or activities, and the logical relationship between them is depicted.
These are the following steps involved in the process of constructing a flowchart:
Identify the process or system to be depicted on the flowchart.
  1. Identify the process or system to be depicted on the flowchart. (what is the product(result) of the flowchart
  2. Highlight the various stages involved in the process/system.
  3.  Identify any decisions to be made. (make it obvious to the user that a decision must be made)
  4.  Construct the flowchart bearing in mind that it must begin and end with an oval-shaped terminator. Every flow chart must start with an oval shaped terminator. (It’s like the keys to your house, you need them to open and close the door.)
  5.  If the flowchart does not fit on one page, an off-page connector should be used. (this is like when you were in secondary school and when your exam question followed on to a new page you put P.T.O(please turn over) on the bottom of the sheet)
  6.  Use comments at stages of the flowchart which you feel are significant. (To avoid confusion)


Flow Charts- A Closer Look

23 Feb

Flowcharts are important tools used for analyzing processes and systems. As mentioned, flowcharts use many symbols to portray the information given and to give sense to the chart itself. Flowcharts use a terminal, flow line and a document. They use further shapes to give information such as an input/output, a process and a decision.

An input/output is a slanted rectangular shape. It refers to a point at which information is entered into the flowchart, or displayed in the flowchart. A process is a regular rectangle shape. It denotes a process to be carried out by the system and indicates that an operation is due to be carried out on the data. A decision is diamond in shape. It indicates a possible decision to be made and there are two possible alternatives. In programming terms it indicates the program should continue along two branches. A predefined process is another rectangular shape that is used. This refers to a process that is better defined in a separate flowchart portion.

There are four different types of flowcharts. A systems flowchart, deployment flowchart, top-down flowchart and logic flowchart. A systems flowchart is generally the most well known and used flowchart.

Flowcharts can be extremely effective in that they successfully allow a quick view of an entire chart on one page.

References: Lecture 15- Flowcharts


The difference between Flowcharts and Data Flow Diagrams

23 Feb

A common question asked throughout the use of information systems is that of the difference between Flowcharts and Data Flow Diagrams.

A flowchart is a tool for analyzing processes, programs or systems. It can be said that it represents a process. It shows the steps as boxes of various kinds and then gives them their order by connecting the boxes with arrows. Flowcharts are implied by the sequencing of operations. Flowcharts are used by documenting, analyzing and managing processes in various fields.

Flow charts consist of many different symbols to represent the data being portrayed. Some examples include a terminal, flowline and a document. A terminal is oval in shape and denotes the beginning or end of the flow chart. A flowline is an arrow shape that indicates the logical flow of control in a flow chart. A document denotes a hard copy of a document. These different shapes help to show what is happening in a Flow chart.

A data flow diagram is similar to a flowchart yet they are not the same. A DFD is a graphical representation of the flow of data in an information system. It can be defined as a form of process modelling and is made up of four different areas: Processes, External Entities, Data Flows and Data Stores.

DFD’s provide a pictorial non technical representation of the information given. They are easy to produce and quick to understand.

My next blog will look at both DFD’s and Flow Charts in further detail.ImageImage

How Data Flow Diagrams (DFD’s) operate…

20 Feb

1. Imagine you work in a small stock control environment where goods are bought and sold

2. There are two job descriptions in our imaginary system, stock clerks and cashiers

3. Stock Clerks ‘order’ and ‘receive’ goods, and Cashiers ‘sell’ goods

4. An analyst has observed you and come up with the following diagram…


5. The Data Flow Diagram (DFD) is the visible part of the Data Flow Modelling (DFM) technique

6. The DFD is drawn at the very beginning of the analysis where, in various guises, it helps define the context of the system under consideration

7. It then becomes, with the LDS, the main place for recording the analysts’ understanding of how the current system functions

8. When a good understanding of the data movements of the current system has been achieved, the logic of the system is distilled from the DFD and a new ‘logical’ DFD may be produced

9. This DFD contains the essence of the system’s functionality, free from technical and physical constraints that may exist in the current system

10. With the logical view of the system in hand, the analysts propose alternative options for a new system

11. The users choose one of these options and a final DFD is drawn for the, by now, ‘required’ system

Advantages Vs Disadvantages of DFD’s…

20 Feb

Last week I had a discussion on the key characteristics of Data Flow Diagrams. This week I will discuss the advantages of DFD’s and also the drawbacks of DFD’s to try and get a full understanding of the system.


 1. Advantages of data flow diagram:

  • It aids in describing the boundaries of the system.
  • It is beneficial for communicating existing system knowledge to the users.
  • A straightforward graphical technique which is easy to recognise.
  • DFDs can provide a detailed representation of system components.
  • It is used as the part of system documentation file.
  • DFDs are easier to understand by technical and nontechnical audiences
  • It supports the logic behind the data flow within the system.

                     2. Disadvantages of data flow diagram:

  • It make the programmers little confusing concerning the system.
  • The biggest drawback of the DFD is that it simply takes a long time to create, so long that the analyst may not receive support from management to complete it.
  • Physical considerations are left out.
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