Business Process Diagrams
The notational elements in business process diagrams are divided into four
basic categories, each of which consists of a set of elements.
Flow objects are the building blocks of business processes; they include
events, activities, and gateways. The occurrence of states in the real world that are relevant for business processes are represented by events. Activities
represent work performed during business processes. Gateways are used to
represent the split and join behaviour of the flow of control between activities,
events, and gateways.
Organizational aspects are represented in business process diagrams by
swimlanes. Swimlanes are restricted to a two-level hierarchy: pools and lanes.
Pools represent organizations that participate in the interaction of multiple
business processes, each of which is enacted by one organization. Lanes represent
organizational entities such as departments within a participating organization.
By drawing flow objects in swimlanes, the organizational entity
responsible for performing the specific objects can be represented graphically.
Artefacts are used to show additional information about a business process
that is “not directly relevant for sequence flow or message flow of the process”,
as the standard mentions. Data objects, groups, and annotations are supported
as artefacts. Each artefact can be associated with flow elements. Artefacts
serve only information purposes. The process flow is not influenced by
artefacts.
Data objects are represented simply by a name. The main purpose of data
object artefacts is documentation of the process. Using directed association
arcs, the modeller can represent the fact that a data object is used (or created/
modified) by an activity in the process. Paper documents and electronic
documents, as well as information on any type of medium; can be represented
by data objects.
Text annotations are used to document certain aspects of the business
process in textual form. The text is graphically associated with the object
in the business process diagram that the text explains. Group objects are
artefacts that are used to group elements of a process. Groups do not have a
formal meaning; they just serve documentation purposes. Groups may span
lanes and even pools.
Connecting objects connect flow objects, swimlanes, or artefacts. Sequence
flow is used to specify the ordering of flow objects, while message flow describes
the flow of messages between business partners represented by pools. Association
is a specific type of connecting object that is used to link artefacts to
elements in business process diagrams. The categories of notational elements
are shown Figure 4.78.
The core element set provides a small set of concepts and their graphical
representation for modelling business processes. The idea is to provide an
easy-to-use notation by concentrating on the main aspects of business process
modelling, without introducing complexity that might not be relevant.
The business process diagram shown represents a review
process for a scientific conference. The program committee chairperson is the
central role in the process; this role is represented by the PC Chair pool. There
are two additional roles involved, namely authors who submit their research
papers and reviewers who perform peer reviews for the submitted papers. Since the goal of this example is to introduce the core elements, simplifications
are in place: the business process model provides a simplified view of how
review processes are actually conducted. In addition, there are many authors
and there are also many reviewers. For convenience, just one author and one
reviewer are shown. As will be discussed below, situations in which multiple
participants are involved in the same role cannot be covered conveniently.
The pools in this example represent roles and not concrete participants in a
business process. Each role at run time has multiple concrete participants who
are actually involved in the business process instance. The BPMN standard
indicates that “a pool represents a participant in a process. It is also acts as a
“swimlane” and a graphical container for partitioning a set of activities from
other pools, usually in the context of B2B situations.”
The process starts when the PC Chair is asked to organize the scientific
program of a conference. This is reflected by the start event of the process at
the PC Chair. An event is something that ‘happens’ during the course of a
business process. These events affect the flow of the process and usually have
a cause (trigger) or an impact (result). Events are circles with open centres
to allow internal markers to differentiate different triggers or results.”
The activity enacted first is the publication of a call for papers with detailed
information on the conference, such as name and location, and also
information regarding the topics addressed by the conference. The receipt of
a message can be an event that is relevant for the process. This concept is
used in the sample process when the published call for papers activity sends
a message that the author receives. Receiving this message is represented by
the start event of the author process. The cause of this event is receiving the message and the effect is spawning the process. The same concept is used to
start the reviewer process in a later phase of the overall process.
Activities represent units of work that are actually conducted during the
course of the business process. The sample process contains, in the PC Chair
process, a set of activities that are enacted in sequence. The standard states
that “an activity is a generic term for work that a company performs. An
activity can be atomic or non-atomic (compound). The types of activities
that are a part of a process model are: process, subprocess, and task. Tasks
and subprocesses are rounded rectangles. Processes are either unbounded or
contained within a Pool.”
Business processes contain execution constraints between activities. The
simplest form of execution constraint is execution order. “A Sequence Flow is
used to show the order that activities will be performed in a process.”
More complex execution constraints are splits and joins of branches, where
different types of splits and joins are available. These structures are represented
by gateways. A gateway is graphically represented by a diamond. Split
nodes have multiple outgoing edges and join nodes have multiple incoming
edges. The notation for a gateway is identical to that for split and join nodes.
The split and join behaviour is represented only by the edges attached to
a gateway symbol. “A Gateway is used to control the divergence and convergence
of Sequence Flow. Thus, it will determine branching, forking, merging,
and joining of paths. Internal markers will indicate the type of behaviour
control.”
In the sample business process, the author pool contains a split gateway
and a join gateway. The split gateway evaluates the received notification information
and—in case the paper is accepted—triggers the preparation of the final version of the paper that will be printed in the conference proceedings.
In case the paper is rejected, the join gateway is triggered. To clarify the behaviour
of the split gateway, the outgoing sequence flows are associated with
the respective annotations.
The participants that cooperate in the context of a business process communicate
by sending and receiving messages. In business process diagrams,
messages are represented by message flow. Typically a message flow connects
an activity of one participant to an activity or an event of another participant.
Depending on the kind of business process diagram (abstract or global),
message flows can link pairs of flow objects, pools, and events. Detailed rules
on message flow connections are discussed below. “A Message Flow is used to
show the flow of messages between two participants that are prepared to send
and receive them. In BPMN, two separate Pools in the Diagram will represent
the two participants (e.g., business entities or business roles).”
The notational elements of the BPMN regarding transactional behaviour of
business processes (transaction groups, compensation flow, and cancellation)
will not be covered, because their semantics is not laid out in sufficient detail
and precision.
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