05-01-2013, 03:09 PM
Understanding Data Flow Diagrams
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Data flow diagrams (DFDs) reveal relationships among
and between the various components in a program or
system. DFDs are an important technique for modeling a
system’s high-level detail by showing how input data is
transformed to output results through a sequence of
functional transformations. DFDs consist of four major
components: entities, processes, data stores, and data
flows. The symbols used to depict how these components
interact in a system are simple and easy to understand;
however, there are several DFD models to work from,
each having its own symbology. DFD syntax does remain
constant by using simple verb and noun constructs. Such
a syntactical relationship of DFDs makes them ideal for
object-oriented analysis and parsing functional
specifications into precise DFDs for the systems analyst.
DEFINING DATA FLOW
DIAGRAMS (DFDs)
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 components1
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
Why They Aren’t Called “Rules”
The most important thing to remember is that there are
no hard and fast rules when it comes to producing DFDs,
but there are when it comes to valid data flows. For the
most accurate DFDs, you need to become intimate with
the details of the use case study and functional
specification. This isn’t a cakewalk necessarily, because
not all of the information you need may be present. Keep
in mind that if your DFD looks like a Picasso, it could be
an accurate representation of your current physical
system. DFDs don’t have to be art; they just have to
accurately represent the actual physical system for data
flow.