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A Data Flow Diagram

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A Data Flow Diagram


A data flow diagram is a graphic representation of a system or portion
of system. It consists of data flows, processes, sources,
destinations, and stores - all described through the use of easily
understood symbols. An entire system can be described from the
viewpoint of the data it processes with only four symbols. At the same
time, data flow diagrams are powerful enough to show parallel
activities. When standard symbols limit communication, a presentation
graph, which uses symbols of people, files, terminals, and documents,
can be used to discuss a system with users.



DFD Principles
==============

v The general principle in Data Flow Diagramming is that a system can
be decomposed into subsystems, and subsystems can be decomposed into
lower level subsystems, and so on.

v Each subsystem represents a process or activity in which data is
processed. At the lowest level, processes can no longer be decomposed.

v Each 'process' (and from now on, by 'process' we mean subsystem and
activity) in a DFD has the characteristics of a system.

v Just as a system must have input and output (if it is not dead), so
a process must have input and output.

v Data enters the system from the environment; data flows between
processes within the system; and data is produced as output from the
system.

An example:

The 'Context Diagram ' is an overall, simplified, view of the target
system, which contains only one process box, and the primary inputs
and outputs.

Context diagram 1

[IMAGE]

Context diagram 2

[IMAGE]

Both the above diagrams say the same thing. The second makes use of
the possibility in SSADM of including duplicate objects. (In context
diagram 2 the duplication of the Customer object is shown by the line
at the left hand side. Drawing the diagram in this way emphasizes the
Input-Output properties of a system.

The Context diagram above, and the decomposition which follows, are a
first attempt at describing part of a 'Home Catalogue' sales system.
In the modeling process it is likely that diagrams will be reworked
and amended many times - until all parties are satisfied with the
resulting model. A model can usefully be described as a co-ordinated
set of diagrams.



The Top (1st level) DFD
=======================

[IMAGE]

The Top or 1st level DFD, describes the whole of the target system. It
'bounds' the system under consideration.
(To simplify the diagram some notation has been left)

Data Flow Diagrams show:

v The processes within the system

v The data stores (files) supporting the system's operation

v The information flows within the system

v The system boundary

v Interactions with external entities

(SSADM) DFD Notations

DFDs are used in most system analysis methodologies.
Processes, in other methodologies, may also be called 'Activities',
'Actions', 'Procedures', 'Subsystems' etc.

They may be shown as a circle, an oval, or (typically) a rectangular
box.
Data are generally shown as arrows coming to, or going from the edge
of a process box.

SSADM (see figure) uses 4 diagramming notations (symbols) in DFDs
including 2 other diagrammatic notations, 'external entities' and
'data stores', which are useful when using DFDs to describe physical
or actual systems - as opposed to logical, conceptual systems.

(Note that there is no 'Decision' symbol. A decision is a Process.

[IMAGE]

The Process Symbol


Processes transform or manipulate data. Each box has a unique number
as identifier (top left) and a unique name (an imperative - e.g. 'do
this' - statement in the main box area) the top line is used for the
location of, or the people responsible for, the process.

Processes are 'black boxes' - we don't know what is in them until they
are decomposed.

Processestransform or manipulate input data to produce output data.
Except in rare cases, you can't have one without the other.

[IMAGE]



Data process
============

A data process transforms data values.

[IMAGE]

You can make a distinction between the following types of processes:

Process Type

Indicates

High-level

Process containing non-functional components such as data stores or
external objects that cause side effects

Low-level

Pure function without side effects, such as the sum of two numbers

Leaf or atomic processes

Process that is not further decomposed

Data Flows depict data/information flowing to or from a process. The
arrows must either start and/or end at a process box. It is impossible
for data to flow from data store to data store except via a process,
and external entities are not allowed to access data stores directly.
Arrows must be named.
Double ended arrows may be used with care.

[IMAGE]

External Entities, also known as 'External sources/recipients, are
things (eg: people, machines, organisations etc.) which contribute
data or information to the system or which receive data/information
from it.
The name given to an external entity represents a Type not a specific
instance of the type.
When modelling complex systems, each external entity in a DFD will be
given a unique identifier.
It is common practice to have duplicates of external entities in order
to avoid crossing lines, or just to make a diagram more readable.

[IMAGE]

Data Stores are some location where data is held temporarily or
permanently.
In physical DFDs there can be 4 types.
D = computerised Data
M = Manual, e.g. filing cabinet.
T = Transient data file, e.g. temporary program file
T(M) = Transient Manual, e.g. in-tray, mail box.
As with external entities, it is common practice to have duplicates of
data stores to make a diagram less cluttered.

[IMAGE]

DFD Levels

The Context and Top Level diagrams in the example start to describe
'Home Catalogue' type sales system. The two diagrams are just the
first steps in creating a model of the system. (By model we mean a
co-ordinated set of diagrams which describe the target system and
provide answers to questions we need to ask about that system).As
suggested the diagrams presented in the example will be reworked and
amended many times - until all parties are satisfied. But the two
diagrams by themselves are not enough; they only provide a high level
description. On the other hand, the initial diagrams do start to break
down, decompose, what might be quite a complex system into manageable
parts.

A revision of the example Top Level DFD

[IMAGE]

The next step - the Next Level(s)

Each Process box in the Top Level diagram will itself be made up of a
number of processes, and will need to be decomposed as a second level
diagram.

[IMAGE]

Each box in a diagram has an identification number derived from the
parent - in the top left corner. (The Context level is seen as box 0)

Any box in the second level decomposition may be decomposed to a third
and then a fourth level. Very complex systems may possibly require
decomposition of some boxes to further levels.

Decomposition stops when a process box can be described with an
Elementary Process Description using ordinary English, later on the
process will be described more formally as a Function Description
using, for example, pseudo code.

[IMAGE]

Notes

[IMAGE]

v Redrawing the diagram makes it clear that Process 3, 'Maintain
Credit Rating' requires some input - if it is to produce output.

v Note that 'Goods', while it is in reality a physical thing, is seen
here as data. This is because this is a model. We will represent
'Goods' in our model by some description. In the model, 'Goods'
becomes a set of data items. In the real world, there will be some
physical objects, but in our model we only have an astract
description.

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MLA Citation:
"A Data Flow Diagram." 123HelpMe.com. 19 Apr 2014
    <http://www.123HelpMe.com/view.asp?id=121686>.




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