Process Description Capture Method
The IDEF3 Process Description Capture Method provides
a mechanism for collecting and documenting processes. IDEF3 captures
precedence and causality relations between situations and events
in a form natural to domain experts by providing a structured method
for expressing knowledge about how a system, process, or organization
works. IDEF3 descriptions can:
IDEF3 captures the behavioral aspects of an existing
or proposed system. Captured process knowledge is structured within
the context of a scenario, making IDEF3 an intuitive knowledge acquisition
device for describing a system. IDEF3 captures all temporal information,
including precedence and causality relationships associated with
enterprise processes. The resulting IDEF3 descriptions provide a
structured knowledge base for constructing analytical and design
models. Unlike simulation languages (e.g., SIMAN, SLAM, GPSS, WITNESS)
that build predictive mathematical models, IDEF3 builds structured
descriptions. These descriptions capture information about what
a system actually does or will do and also provide for the organization
and expression of different user views of the system.
There are two IDEF3 description modes, process flow
and object state transition network. A process flow description
captures knowledge of "how things work" in an organization,
e.g., the description of what happens to a part as it flows through
a sequence of manufacturing processes. The object state transition
network description summarizes the allowable transitions an object
may undergo throughout a particular process. Both the Process Flow
Description and Object State Transition Description contain units
of information that make up the system description. These model
entities, as they are called, form the basic units of an IDEF3 description.
The resulting diagrams and text comprise what is termed a "description"
as opposed to the focus of what is produced by the other IDEF methods
whose product is a "model."
An IDEF3 Process Flow Description captures a description
of a process and the network of relations that exists between processes
within the context of the overall scenario in which they occur.
The intent of this description is to show how things work in a particular
organization when viewed as being part of a particular problem solving
or recurring situation. The development of an IDEF3 Process Flow
Description consists of expressing facts, collected from domain
experts, in terms of five basic descriptive building blocks.
The following example illustrates how the building
blocks of the IDEF3 method can describe a scenario typically found
in a manufacturing environment. The situation to be described is
a painting and inspection process associated with applying primer
paint to a part that will become an element of a subassembly for
a piece of heavy construction equipment. The example IDEF3 description
shown in Figure 1 is the graphical representation of the scenario
(story) told by a paint shop supervisor when asked to describe:
"What goes on in the primer shop?"
The story the example describes follows:
Parts enter the shop ready for the primer coat
to be applied. We apply one very heavy coat of primer paint at
a very high temperature. The paint is allowed to dry in a bake
oven after which a paint coverage test is performed on the part.
If the test reveals that not enough primer paint has been sprayed
on the surface of the part, the part is re-routed through the
paint shop again. If the part passes the inspection, it is routed
to the next stop in the process.Note the activities described
in the scenario are clearly identified and appear as labeled boxes
in Figure 1 and that the labeled boxes can describe activities,
processes, events, etc. The IDEF3 term for elements represented
by boxes is a Unit Of Behavior (UOB). The arrows (links) tie the
boxes (activities) together and define the logical flows. The
smaller boxes define junctions that provide a mechanism for introducing
logic to the flows.
Not directly represented in Figure 1 is the Decomposition
and Elaboration component of IDEF3. Each UOB can have associated
with it both "descriptions in terms of other UOBs" and
a "description in terms of a set of participating objects and
their relations." We refer to the former as decompositions
of a UOB and the latter as an elaboration of a UOB. Intuitively,
a decomposition is a closer look at some given UOB within a larger
diagram. A decomposition is a diagram, and may be a decomposition
of some UOB in the scenario (top level) diagram or it may be the
decomposition of a UOB in a decomposition. More precisely, a decomposition
of a given UOB is a more fine grained IDEF3 representation of that
UOB. Multiple views (decompositions) are allowed in IDEF3 primarily
because it is meant to be used as a description capture method.
Experience with related modeling methods has demonstrated the need
to capture different views of the same activity. IDEF3 provides
this capacity by allowing multiple decomposition of the same UOB.
1: IDEF3 Process Description Diagram
An elaboration is an element of the IDEF3 description
that captures the objects that participate in a particular activity
and the facts and constraints that are defined on these objects
and on instances of that activity. Each element of an IDEF3 description
can have an elaboration. It is in the elaboration that resource
requirements of systems will be captured.
Object state transition network (OSTN) diagrams capture
object-centered views of processes which cut across the process
diagrams and summarize the allowable transitions. Figure 2 shows
a sample OSTN diagram.
Figure 2: IDEF3 Object State Transition Network Diagram
Object states and state transition arcs are the key
elements of an OSTN diagram. In OSTN diagrams, object states are
represented by circles and state transition arcs are represented
by the lines connecting the circles. An object state is defined
in terms of the facts and constraints that need to be true for the
continued existence of the object in that state and is characterized
by entry and exit conditions. The entry conditions specify the requirements
that need to be met before an object can transition into a state.
The exit conditions characterize the conditions under which an object
can transition out of a state. The constraints are specified by
a simple list of property/value pairs or by a constraint statement.
The values of the attributes must match the specified values for
the requirements to be met.
State transition arcs represent the allowable transitions
between the focus object states. It is often convenient to highlight
the participation of a process in a state transition. The importance
of such a process constraint between two object states can be represented
in IDEF3 by attaching a UOB referent to the transition arc between
the two object states.
KBSI has developed two automated Process Modeling
tools, PROSIM® and PROCAP®,
to support the IDEF3 method.