First question that arise in our mind that what is Data Flow Diagrams (DFD). so in simple words DFD is Captures the flow of data in a
system.
It helps in developing an
understanding of system’s functionality. What are the different sources of
data, what different transformations take place on data and what are final
outputs generated by these transformations. It describes data origination,
transformations and consumption in a system. Information is organized and
disseminated at different levels of abstraction. Thus this technique becomes a
conduit for top down system analysis and requirements modeling.
The Notation
There are several notations of
the data flow diagrams. In the following, four different shapes are explained.
Process
What are different processes or
work to be done in the system?
Transforms of data.
External
Agent
External systems which are
outside the boundary of this system, these are represented using the squares
Data
Store
Where data is being stored for
later retrieval. Provides input to the process Outputs of the processes may be
going into these data stores.
Data
Flow
Where the data is flowing.
Represents the movement of the
data in a data flow diagram.
Adding Levels of
Abstraction to Data Flow Modeling
As we have already described that
in data flow modeling only those processes can be expressed that perform
certain processing or transformation of information. Now the question arises
how far these processes need to be expressed? if we start adding each bit of system functionality
in a single data flow diagram, it would become an enormously large diagram to
be drawn on a single piece of paper. Moreover, requirement analysis is an
ongoing activity in which knowledge expands as you dig out details of
processes. Therefore, it may not be possible for an analyst to know each bit of
all the processes of the system from the very beginning. Keeping the complexity
of systems in view, data flow modeling technique has suggested disseminating
information of a system in more then just one levels of abstraction. What are
these levels please see below for a discussion
Context Level or 0 level Data Flow Diagram
In a top-down system analysis, an
analyst is required to develop high level view of the system at first. In data
flow modeling, this high-level view is the Context level data flow diagram. In
this diagram, system’s context is clarified such that all the external agents
or entities with which the system interacts are captured. It captures the
details of what information flows between the system and these external
entities, and what outputs are generated against inputs from these external
agents and so on. So, the analyst probes out all the external agents that may
involve persons, organizations or other systems who directly interacts with
this system and their specific involvement in the system. At this level,
systems internal details are not exposed, as we want to see system behavior as
a black box.
Above diagram shown is Context level or 0 level Data Flow Diagram. in which three external agent, one Process and one Data Store is shown.
