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Data Flow Diagrams Symbols

There are some symbols that are used in the drawing of business process diagrams (data flow diagrams). These are now explained, together with the rules that apply to them.

Flow diagrams in general are usually designed using simple symbols such as a rectangle, an oval or a circle depicting a processes, data stored or an external entity, and arrows are generally used to depict the data flow from one step to another.

A DFD usually comprises of four components. These four components can be represented by four simple symbols. These symbols can be explained in detail as follows: External entities (source/destination of data) are represented by squares; Processes (input-processing-output) are represented by rectangles with rounded corners; Data Flows (physical or electronic data) are referred to by arrows; and finally, Data Stores (physical or electronic like XML files) are presented by open-ended rectangles.

Data Flow Diagram Shapes

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Data flow diagrams present the logical flow of information through a system in graphical or pictorial form. Data flow diagrams have only four symbols, which makes it useful for communication between analysts and users. Data flow diagrams (DFDs) show the data used and provided by processes within a system. DFDs make use of four basic symbols.

Create structured analysis, information flow, process-oriented, data-oriented, and data process diagrams as well as data flowcharts.

External Entity

An external entity is a source or destination of a data flow which is outside the area of study. Only those entities which originate or receive data are represented on a business process diagram. The symbol used is an oval containing a meaningful and unique identifier.

Process

A process shows a transformation or manipulation of data flows within the system. The symbol used is a rectangular box which contains 3 descriptive elements:

Firstly an identification number appears in the upper left hand corner. This is allocated arbitrarily at the top level and serves as a unique reference.

Secondly, a location appears to the right of the identifier and describes where in the system the process takes place. This may, for example, be a department or a piece of hardware. Finally, a descriptive title is placed in the centre of the box. This should be a simple imperative sentence with a specific verb, for example 'maintain customer records' or 'find driver'.

Data Flow

A data flow shows the flow of information from its source to its destination. A data flow is represented by a line, with arrowheads showing the direction of flow. Information always flows to or from a process and may be written, verbal or electronic. Each data flow may be referenced by the processes or data stores at its head and tail, or by a description of its contents.

Data Store

A data store is a holding place for information within the system:

It is represented by an open ended narrow rectangle. Data stores may be long-term files such as sales ledgers, or may be short-term accumulations: for example batches of documents that are waiting to be processed. Each data store should be given a reference followed by an arbitrary number.

Resource Flow

A resource flow shows the flow of any physical material from its source to its destination. For this reason they are sometimes referred to as physical flows.

The physical material in question should be given a meaningful name. Resource flows are usually restricted to early, high-level diagrams and are used when a description of the physical flow of materials is considered to be important to help the analysis.

External Entities

It is normal for all the information represented within a system to have been obtained from, and/or to be passed onto, an external source or recipient. These external entities may be duplicated on a diagram, to avoid crossing data flow lines. Where they are duplicated a stripe is drawn across the left hand corner, like this.

The addition of a lowercase letter to each entity on the diagram is a good way to uniquely identify them.

Processes

When naming processes, avoid glossing over them, without really understanding their role. Indications that this has been done are the use of vague terms in the descriptive title area - like 'process' or 'update'.

The most important thing to remember is that the description must be meaningful to whoever will be using the diagram.

Data Flows

Double headed arrows can be used (to show two-way flows) on all but bottom level diagrams. Furthermore, in common with most of the other symbols used, a data flow at a particular level of a diagram may be decomposed to multiple data flows at lower levels.

Data Stores

Each store should be given a reference letter, followed by an arbitrary number. These reference letters are allocated as follows:

'D' - indicates a permanent computer file
'M' - indicates a manual file
'T' - indicates a transient store, one that is deleted after processing.

In order to avoid complex flows, the same data store may be drawn several times on a diagram. Multiple instances of the same data store are indicated by a double vertical bar on their left hand edge.

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