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Analysis and Design
Elements of Data Flow Diagrams
Data Flow Diagrams are composed of the four basic symbols shown below.
The External Entity symbol represents sources of data to the system or destinations
of data from the system.
The Data Flow symbol represents movement of data.
The Data Store symbol represents data that is not moving (delayed data at rest).
The Process symbol represents an activity that transforms or manipulates the
data (combines, reorders, converts, etc.).
Any system can be represented at any level of detail by these four symbols.
External entities determine the system boundary. They are external to the system
being studied. They are often beyond the area of influence of the developer.
These can represent another system or subsystem. These go on margins/edges
of data flow diagram. External entities are named with appropriate name.
Processes are work or actions performed on incoming data flows to produce outgoing
data flows. These show data transformation or change. Data coming into a process
must be "worked on" or transformed in some way. Thus, all processes
must have inputs and outputs. In some (rare) cases, data inputs or outputs will
only be shown at more detailed levels of the diagrams. Each process in always
"running" and ready to accept data.
Major functions of processes are computations and making decisions. Each process
may have dramatically different timing: yearly, weekly, daily.
Processes are named with one carefully chosen verb and an object of the verb.
There is no subject. Name is not to include the word "process". Each
process should represent one function or action. If there is an "and"
in the name, you likely have more than one function (and process). For example,
get invoice ,update customer and create Order Processes are numbered within the
diagram as convenient. Levels of detail are shown by decimal notation. For example,
top level process would be Process 14, next level of detail Processes 14.1-14.4,
and next level with Processes 14.3.1-14.3.6. Processes should generally move from
top to bottom and left to right.
Data flow represents the input (or output) of data to (or from) a process ("data
in motion"). Data flows only data, not control. Represent the minimum essential
data the process needs. Using only the minimum essential data reduces the dependence
between processes. Data flows must begin and/or end at a process.
Data flows are always named. Name is not to include the word "data".
Should be given unique names. Names should be some identifying noun. For example,
order, payment, complaint.
Data Stores are repository for data that are temporarily or permanently recorded
within the system. It is an "inventory" of data. These are common link
between data and process models. Only processes may connect with data stores.
There can be two or more systems that share a data store. This can occur in
the case of one system updating the data store, while the other system only accesses
Data stores are named with an appropriate name, not to include the word "file",
Names should consist of plural nouns describing the collection of data. Like customers,
orders, and products. These may be duplicated. These are detailed in the data
dictionary or with data description diagrams.
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