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302 PART THREE CONVENTIONAL METHODS FOR SOFTWARE ENGINEERING

modeling activity. The attributes of each data object noted in the ERD can be described
using a data object description.
The data flow diagram (DFD) serves two purposes: (1) to provide an indication of
how data are transformed as they move through the system and (2) to depict the func-
tions (and subfunctions) that transform the data flow. The DFD provides additional
information that is used during the analysis of the information domain and serves as
a basis for the modeling of function. A description of each function presented in the
DFD is contained in a process specification (PSPEC).
The state transition diagram (STD) indicates how the system behaves as a conse-
quence of external events. To accomplish this, the STD represents the various modes
of behavior (called states) of the system and the manner in which transitions are made
from state to state. The STD serves as the basis for behavioral modeling. Additional
information about the control aspects of the software is contained in the control spec-
ification (CSPEC).
The analysis model encompasses each of the diagrams, specifications, descrip-
tions, and the dictionary noted in Figure 12.1. A more detailed discussion of these
elements of the analysis model is presented in the sections that follow.

12.3 DATA MODELING

Data modeling answers a set of specific questions that are relevant to any data pro-
cessing application. What are the primary data objects to be processed by the sys-
tem? What is the composition of each data object and what attributes describe the
? What object? Where do the the objects currently reside? What are the relationships between
questions
does data each object and other objects? What are the relationships between the objects and
modeling answer? the processes that transform them?
To answer these questions, data modeling methods make use of the entity rela-
tionship diagram. The ERD, described in detail later in this section, enables a soft-
ware engineer to identify data objects and their relationships using a graphical notation.
In the context of structured analysis, the ERD defines all data that are entered, stored,
“The power of the ER transformed, and produced within an application.
approach is its ability The entity relationship diagram focuses solely on data (and therefore satisfies the
to describe entities
in the real world of first operational analysis principles), representing a "data network" that exists for a
the business and the given system. The ERD is especially useful for applications in which data and the rela-
relationships tionships that govern data are complex. Unlike the data flow diagram (discussed in
between them.”
Section 12.4 and used to represent how data are transformed), data modeling con-
Martin Modell
siders data independent of the processing that transforms the data.
12.3.1 Data Objects, Attributes, and Relationships

The data model consists of three interrelated pieces of information: the data object,
the attributes that describe the data object, and the relationships that connect data
objects to one another.