diagrams for each of the functions. The data flow diagram starts with the input to the
module and shows how the input data will be processed to achieve the output data. The
three major components of a data flow diagram are an identification of the input, a
description of the logic that will occur in the module, and a description of the output.
If the functional decomposition is like a map of the United States, the data flow diagram
is like a detailed map of a state. The data flow diagram tells you how to get across Texas.
You start at El Paso, you head east, past McKittrick Canyon, go to Van Horn and Sierra
Blanca, go through Pecos, then on to Midland and Odessa, and so forth. The map of
Texas shows details that the map of the United States cannot show. By the same token,
the map of Texas does not show you how to get from Los Angeles to San Jose or from
Chicago to Naperville.
The nature of functional decomposition and data flow diagrams are such that process and
data are intimately intertwined. Both process and data are needed in order to build a
functional decomposition and data flow diagrams.
Fig. 14.1.3 shows the tight interrelationship of data and process in the functional
decomposition.
Chapter 14.1: Data Models Across the End-State Architecture