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Data Flow Diagram
What it is
The Data Flow Diagram shows the
flow of data or information. It can be partitioned into single processes
or functions. Data Flow Diagrams can be grouped together or decomposed
into multiple processes. There can be physical DFD's that represent the
physical files and transactions, or they can be business DFD's (aka
logical, or conceptual).
When it's
Used
The DFD is an excellent
communication tool for analysts to model processes and functional
requirements. One of the primary tools of the structured analysis efforts
of the 1970's it was developed and enhanced by the likes of Yourdon,
McMenamin, Palmer, Gane and Sarson. It is still considered one of the best
modeling techniques for eliciting and representing the processing
requirements of a system.
Used effectively, it is a useful and easy
to understand modeling tool. It has broad application and usability across
most software development projects. It is easily integrated with data
modeling, workflow modeling tools, and textual specs. Together with these,
it provides analysts and developers with solid models and specs. Alone,
however, it has limited usability. It is simple and easy to understand by
users and can be easily extended and refined with further specification
into a physical version for the design and development teams.
The different versions are Context
Diagrams (Level 0), Partitioned Diagrams (single process only -- one
level), Functionally decomposed, leveled sets of Data Flow Diagrams.
Description
Process (aka Activity, Function)
Depending on the level of the diagram it may represent the whole system as
in a Context (level 0) diagram or a business area, process (activity),
function, etc. in lower levels.
Symbol: Circle (Yourdon notation), or a Rounded Rectangle (Gane &
Sarson notation). In the physical model, a program label is identified in
the bottom of the symbol.
External Entity(s) (aka Sink, Source, Terminator)
Symbol: rectangular box which may be shaded.
A person or group which interacts with the system. Something outside the
system. It is not a user.
e.g., Customer, Supplier, Government Agency, Accounting Department, The
Human Resources System, etc.
Usually external to the business or system but may be internal (e.g.,
Marketing Dept).
Data Store
A repository of information. In the physical model, this represents a
file, table, etc. In the logical model, a data store is an object or
entity.
Symbol: Two parallel lines (Yourdon notation), or an open ended rectangle
(G&S notation)
Data Flows
The directional movement of data to and from External Entities, the
process and Data Stores.
In the physical model, when it flows into a data store, it means a write,
update, delete, etc. Flows out of Data Stores, mean read, query, display,
select types of transaction.
Symbol: Solid line with arrow. Each data flow is identified with a
descriptive name that represents the information (data packet) on the data
flow.
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