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Data flow Diagram
We use the CASE tool S-Designor Process Analyst to aid in this step.
External Entities
An external entity is an external source of information or an
external sink for information. An external entity is a person or
an organization that interacts with the system being built, but
is beyond the boundaries of the system being designed. The
characteristics of an external entity are taken as given and
cannot be influenced by the design process. External entities are
named with a noun. All external entities must have at least one
data flow entering or leaving it. An external entity is
symbolized by a (shadowed) rectangle.
•External entities are not labeled with people’s names.
•A handler is not an external entity.
•Communications between
external entities is not documented.
Stores
Stores store data for use at a later time, by another process, or
in a different place. Stores usually correspond to tables or
views in a database.
Data Flows
The data flow arrow carries data in the direction of the arrow.
The name of the data flow is shown above the arrow. The arrow can
carry data from an external entity or store to a process and from
a process to a store, an external entity, or another process.
Data cannot flow directly between external entities, between
stores, from an external entity to a store, or from a store to an
external entity. The flow is labeled with a noun and sometimes an
adjective to distinguish different flows with the same name. Data
flows to and from stores are usually not labeled because the data
available in the store is already documented within the store.
Processes
The process box transforms one or more incoming data flows into
one or more outgoing data flows. A process without any incoming
flows is a random number generator or a cosmic big bang, since no
incoming data determines the outcome of the process. A process
without any outgoing data is a black hole since nothing useful
ever emerges. Each process name should be a strong verb with a
single explicit terminator.
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