Data-flow Diagrams

Chapter 17

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.

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

Data transforms

• Data transforms labeled circles with one or more incoming and outgoing information flows:

CalculateTax

Taxable Income

Tax Tables

Tax owed

Information sources and sinks

• Information sources and sinks are information that come into the system, or leave the system, and are represented by squares:

Rate

Hours

Pay Check

Pay Stub

Payroll System

Information stores

• Information Stores represent locations where information can be store for the duration of the system activity:

Total Payroll Expense

Refining DFDs

• DFDs are “refined” (decomposed) to the point that they are 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!)

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.

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.

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’).

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!

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.