2. Elements of data-flow diagrams
There are four basic elements of data flow diagrams:
Data Transforms: where data is filtered, formatted or
synthesized.
Information sources and sinks: essentially inputs and
outputs to the system.
Information stores: where information is kept for the
duration of system activity.
Information Flows: data that is passed into and out of
transforms, information stores.
3. Information flows
Information flows represent the information
being passed into or out of a transform,
source, sink or store. It is represented as a
labeled arrow:
Taxable Income
4. Data transforms
Data transforms labeled circles with one or
more incoming and outgoing information
flows:
Taxable
Income
Calculate Tax owed
Tax Tables Tax
5. Information sources and sinks
Information sources and sinks are information
that come into the system, or leave the
system, and are represented by squares:
Pay
Hours Stub
Payroll
System Pay
Rate Check
6. Information stores
Information Stores represent locations where
information can be store for the duration of
the system activity:
Total Payroll Expense
7. Refining DFDs
DFDs are “refined” (decomposed) to the point that
they contain only “primitive” transforms. They are
refined by drawing another DFD as if they were a
system in themselves!
A “primitive” transform has only one input flow or
only one output flow (note that it can have multiple
input flows or multiple output flows, but not both!)
8. Heritage numbers
Refinement of DFDs leads to a problem in larger
systems: we have a large number of diagrams!
When we look at a diagram by itself the only way
we can tell what it represents is by looking at the
parent diagram.
Heritage numbers allow this. A data transform
numbered 2.4.3 is the child of DFD number 2.4
which is the child of DFD 2 in the level 1 DFD
diagram.
9. The data dictionary
As the data is defined in the DFD (the flows),
definitions should be entered in the ‘data dictionary’.
The ‘data dictionary’ is a reference that allows us to
determine the type of the named data (int, float,
struct, whatever.)
This will be essential during the programming phase,
when knowing what variable names to use.
10. Checking DFDs - CASE tools
Since DFDs are a formal method, tools have
been developed that allow automated drawing
of the DFDs, and management of refined
DFDs.
Since this can be automated, checking DFDs
can also be automated (such as ‘Process
Analyst’).
11. Structure charts
Translation of a DFD to a ‘structure’ chart is a
mechanical procedure (see the book.)
The procedure involves ‘isolating’ the central
transform area, which is independent of the format
transforms and the validation transforms.
Structure charts also provide a schematic showing
the flow of the data in a control diagram!
Now we have a complete picture: DFDs show how
the data flows, and structure charts show how
control is passed!
12. Payroll System
Format
Paycheck
Employee
Records Paychecks
and Stubs
Calculate
Withholding
Format
Accounting
Calculate
Gross Pay Accounting
Reports
Validate
Timecard
Employee Input/output stream
Timecards At highest abstraction
Central transform
13. Payroll System
Validate Timecard Calculate Net Pay Format Paycheck Format Accounting
Get Imployee Info Calculate Gross Pay Calculate Withholding
14. Implementing a DFD
Once we have a structure chart, the actual
implementation also becomes mechanical.
An essential pre-defined form becomes
apparent for implementing structure charts.
An example using C is available in the book.
