Symbols used in DFDs


Symbols used in DFDs


Processes:
  • must have at least one input and at least one output
  • at the primitive level (see below for primitive DFD) are labeled with verb + object (e.g. “print invoice” or “add customer”) (e.g. in the hierarchy below, none of the processes are primitive)
  • at the non-primitive level, are labeled more generally (e.g. “customer maintenance” or “warehouse reports”)
Data stores:

  • Data stores can be online or “hard copy” (see notes on logical VS physical DFDs)
  • Data stores will labeled with a noun (e.g. the label “customer” indicates that information about customers is kept in that data store)
  •  Data is stored whenever there are more than one process that needs it and these processes don’t always run one after the other (if the data is ever needed in the future it must be stored.)


Entities/Sources:

  • can be people, departments, other companies, other systems…
  • are called sources if they are external to the system and provide data to the system, and sinks if they are external to the system and receive information from the system

Data flows:
  • must originate from and/or lead to a process (this means that entities and data stores cannot communicate with anything except processes –remember that it takes a process to make the data flow)
  • can go from process to process, but that does imply that no data is stored at that point
  • can have one arrowhead indicating the direction in which the data is flowing
  • can have 2 arrowheads when a process is altering (updating) existing records in a data stores.


Common mistakes while creating DFDs

Types & Conversion of Data Flow Diagrams

Physical and Logical DFDs:

There are mainly two types of Data Flow Diagrams namely Physical DFD and Logical DFD.Here is some information which might help you to distingush between two of them:

Physical DFD:

An Implementation dependent view of your system which shows what tasks are to be carried out and how are they they carried out.It may include:

  • Names of people,procedure,departments
  • Form and document names or numbers
  • Master and transaction files
  • Equipment and device used
  • Locations

Logical DFD:

An Implementation independent view of your system which will just focus on the flow of the data between the processes regardless to their storage devices,storage locations or people in the system.None of the characteristics defined above for physical DFD will be included in Logical DFDs.


Conversion from Physical DFD -> Logical DFD

A logical DFD will provide you the basics for examining the combination of process,data flows,data stores,input and outputs involved in the system without concerning with the physical devices,people or any other controlling factor that characterise the implementation part.

Steps for converting Physical DFD into Logical DFD:
  1. Directly show the actual data needed in a process,not the documents that contain them.
  2. Only show the flows between the processes. Remove all other routing informations such as flow between the people,office etc.
  3. Remove reference to physical devices.
  4. Remove reference to controlling factors.
  5. Remove unnecessary processes i.e. the process which do not affect on data or data flows.

Data Flow Diagrms(DFD)

What is Data Flow Diagram(DFD)?


data-flow diagram (DFD) is a graphical representation of the "flow" of data through an information system. DFDs can also be used for the visualization of data processing (structured design).

On a DFD, data items flow from an external data source or an internal data store to an internal data store or an external data sink, via an internal process.
A DFD provides no information about the timing or ordering of processes, or about whether processes will operate in sequence or in parallel. It is therefore quite different from a flowchart, which shows the flow of control through an algorithm, allowing a reader to determine what operations will be performed, in what order, and under what circumstances, but not what kinds of data will be input to and output from the system, nor where the data will come from and go to, nor where the data will be stored (all of which are shown on a DFD).

When it comes to conveying how information data flows through systems (and how that data is transformed in the process), data flow diagrams (DFDs) are the method of choice over technical descriptions for three principal reasons.

(1) DFDs are easier to understand by technical and nontechnical audiences.
(2) DFDs can provide a high level system overview, complete with boundaries and connections to other systems.
(3) DFDs can provide a detailed representation of system components.

DFDs help system designers and others during initial analysis stages visualize a current system or one that may be necessary to meet new requirements. Systems analysts prefer working with DFDs, particularly when they require a clear understanding of the boundary between existing systems and postulated systems. DFDs represent the following:

1. External devices sending and receiving data
2. Processes that change that data
3. Data flows themselves
4. Data storage locations

The hierarchical DFD typically consists of a top-level diagram (Level 0) underlain by cascading lower level diagrams (Level 1, Level 2…) that represent different parts of the system.