A data flow diagram is a picture of the movement of

A data flow diagram is a picture of the movement of data between external entities and the
processes and data stores within a system.
Systems analysts use data flow diagrams to help them model the processes internal to an
information system as well as how data from the system’s environment enter the system, are
used by the system, and are returned to the environment. DFDs help analysts understand
how the organization handles information and what its information needs are or might be.
Analysts also use DFDs to study alternative information handling procedures during the
process of designing new information services.
2. Explain the rules for drawing good DFDs.

• Data cannot move directly to an outside sink from a data store. Data must be moved by
processes.
• A data store has a noun phrase label.
Source/Sink:
• Data cannot move directly from a source to a sink. Data must be moved by a process. If
the data are of any concern to our system. Otherwise, the data flow is not shown on the
DFD.
• A source/sink has a noun phrase label
Data Flow:
• A data flow has only one direction of flow between symbols. It may flow in both
directions between a process and a data store to show a read before an update. The latter
is usually indicated; however, by two separate arrows because these happen at different
times.
• A fork in a data flow means that exactly the same data goes from a common location to
two or more different processes, data stores, or sources/sinks (this usually indicates
different copies of the same data going to different locations).
• A join in a data flow means that exactly the same data come from any of two or more
different processes, data stores, or sources/sinks to a common location.
• A data flow cannot go directly back to the same process it leaves. There must be at least
one other process that handles the data flow, produces some other data flow, and returns
the original data flow to the beginning process.
• A data flow to a data store means update (delete or change).
• A data flow from a data store means retrieve or use.
• A data flow has a noun phrase label. More than one data flow noun phrase can appear on
a single arrow as long as all of the flows on the same arrow move together as one
package.