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MB0047 : What are dataflow diagrams? Construct a DFD using the different conventions.

Posted on: September 29, 2011


Answer: – A Data Flow Diagram (DFD) is a diagrammatic representation of the information flows within a system, showing:

  • how information enters and leaves the system,
  • what changes the information,
  • Where information is stored.

In SSADM a DFD model includes supporting documentation describing the information shown in the diagram. DFDs are used not only in structured system analysis and design, but also as a general process modeling tool. There are a number of commercial tools in the market today which are based on DFD modeling.

SSADM uses DFDs in three stages of the development process:

    • Current Physical DFDs. These record the results of conventional fact finding.
    • Current Logical DFDs. The logical information processing of the current system
    • Required Logical DFDs. The logical information processing requirements of the proposed system.

The DFD conventions:-

DFDs show the passage of data through the system by using 5 basic constructs: Data flows, Processes, Data Stores, External Entities, and Physical Resources.

  1. 1.      Data Flows

A data flow shows the flow of data from a source to a destination. The flow is shown as an arrowed line with the arrowhead showing the direction of flow. Each data flow should be uniquely identified by a meaningful descriptive name (caption).

Flow may move from an external entity to a process, from a process to another process, into and out of a store from a process, and from a process to an external entity. Flows are not permitted to move directly from an external entity to a store or from a store directly to an external entity.

  1. 2.      Processes

Processes are transformations, changing incoming data flows into outgoing data flows. Processes are drawn as rectangular boxes with a descriptive name occupying the middle of the box. The box has a top stripe that contains an identification number in the left, and the location (or the role carrying out the work) on the right (this is optional and used only in the current physical DFD).

The numbering generally follows a left to right convention. This does not indicate priority or sequence. The identification number is purely an identifier. It also helps to associate a high level process with its decomposed sub processes.

The name of the process should describe what happens to the data as it passes through it. An active verb (verify, compute, extract, create, retrieve, store, determine, etc.) followed by an object or object clause is a suggested notation.

  1. 3.      Data Stores

A store is a repository of data; it may be a card index, a database file, a temporary pile of sales orders awaiting processing, or a folder in a filing cabinet. The store may contain permanent data or temporary accumulations (pending documents, daily movements).

A store is represented by an open-ended box and is given a meaningful descriptive name. Each store is also given a reference number prefixed by a letter. In logical and required system DFD, data stores are regarded as computerized and hence only a ‘D’ will be used. Some transient stores may remain and retain the ‘T’.

To prevent a DFD becoming ‘spider’s web’ of crossing lines, the same data store may be included more than once on a DFD. Such duplication is shown by an additional vertical line within the store symbol.

  1. 4.      External Entities (Source or Sink)

The external entity represents a person or a part of an organization which sends or receives data from the system but considered to be outside the system boundary (scope of the project). As with the data stores these may be duplicated on a DFD to simplify presentation. External entities may be further referenced by the use of an alpha character, and this is particularly recommended if at a lower level the entity is being decomposed.

Sometimes external entities are referred to as sources and sinks. An External entity either supplies data to the system, which makes it a source and /or receives data from the system, which makes it a sink.

  1. 5.      Physical Resources

 A physical flow represents the flow of material (as opposed to data flows representing the flow of information), the movement of some resources or goods which are relevant to the information system, from source to destination. They are included to aid communication. A physical flow is represented by a broad arrow. The resource store is represented by a closed rectangle.


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