bpmn is a de-facto standard with more than 70 commercial tools that currently support its use. However, its the semantic ambiguities jeopardize its implementation. We perform an ontological analysis of important constructs of bpmn like activities and events to highlight their implicit commitments. State of the art The literature about bpmn focuses on three types of analysis: (i) the syntactic analysis, (ii) the behavioral analysis, and (iii) the ontological analysis. The syntactic analysis aims at de ning the structural constraints that bpmn-models 3This work is part of a larger study [11] that will be presented at the International Conference on Formal Ontology in Information Systems (FOIS 2014). In this version we assume some familiarity with bpmn.
Business process (BP) modelling concerns the analysis and representation of the
activities by which companies coordinate their organisation and work, produce
goods, interact with each others and customers. The goal is a common
conceptual modelling language that can be easily understood to facilitate business
communication. The Business Process Model and Notation (bpmn) [
In this paper we are interested in the ontological analysis of bpmn with the
aim of clarifying how one can understand the notions of activity and event. We
carry out our study in two ways: rst by ontologically analyzing the information
provided by the bpmn standard, and then by characterizing our ndings on these
concepts with the Dolce ontology [
The bpmn diagram in Fig. 1 represents a process with four participants: Purchaser, Service provider A, Service provider B and Service provider C. The process starts when the event type None in the Purchaser pool happens. This is followed by the execution of task Request Quotes which ends with the sending of a message to each service provider participant in the process. Once a service provider receives the message, it starts its own process consisting in sending a quote to the Purchaser. After this, the service provider process ends. When the Purchaser receives at least two quotes, it executes the Assess the Quotes task after which the process ends for the Purchaser provided the condition Su cient reserve amount? is satis ed. Otherwise, the process is back to the Request Quotes and ows again.
The reason why some parts of the process are marked as activities and others
as events is not immediately clear. In the diagram below, messages are exchanged
between the process participants by using tasks of type Message send. However,
in bpmn one could also use a Message as throw event (not showed in Figure
1), which models the sending of a message as well. The meanings of these two
di erent constructs { that seem to model the same thing { is not immediately
clear. The bpmn speci cation [
Events and activities in bpmn are connected to other events and activities in the same (in a di erent) pool by solid (dashed) arrows; these arrows mark execution precedence and thus temporal dependences. This reveals the temporal nature of events and activities in bpmn. From the ontological viewpoint, we can classify them as (some type of) perdurants or occurrents, in the sense of entities that extend in time possibly by accumulating temporal parts. In the following we use the Dolce taxonomy of perdurants|mainly the concepts of Achievement, Accomplishment, State, and Event|to discuss and ontologically characterize the di erence between Activities and Events, and between Catch- and Throw-events. While we nd helpful to use a foundational ontology like Dolce, we remark that the analysis could be based on other ontological systems provided they include a taxonomy of perdurants.
Activities and tasks: We have seen that bpmn activities are not instantaneous, thus they take time to execute. In addition, bpmn distinguishes between two types of activities: tasks, i.e., atomic activities and sub-processes, i.e., non-atomic activities. The relationship between being instantaneous and being atomic is not trivial given that a task can have a positive temporal extension.
In some ontological theory [
We observe also that the mereological sum of two instances of a task like Send quote is not an entity of the same type. This is consistent with the assumption that bpmn activities represent units of work with the purpose of achieving given business goals; they thus culminate with the achievement of the goal.
We can then conclude that by considering a strict ontological modality activities are anti-atomic and anti-cumulative, i.e., they can be mapped to Dolce accomplishments. Vice versa, by assuming a conceptual modality, only subprocesses may be mapped to accomplishments. More generally tasks would be mapped to Dolce events, i.e., as anti-cumulative types of perdurants with no commitment on atomicity and homeomericity.
Catch events: We saw that events are instantaneous, consequently they are temporally atomic, that is, they cannot extend over or throughout a temporal stretch. Catch events like the reception of a message, are in general exogenous, i.e., their happening is outside of the control of the pool they belong to or, at least, of the branch of the pool process at stake. In this perspective None start events could be understood as `the system is turned-on'. In addition, being culminating perdurants the catch events are anti-cumulative. Anti-cumulativity and atomicity characterize the subcategory of achievements in Dolce.
The process of Service Provider A in Figure 1 cannot proceed unless a trigger is received, i.e., a message is received. Accordingly, if the system of this service provider is `turned-o ', the message will never be received. Thus, behind a catch event there is the assumption that the process is waiting to be triggered, i.e., the system is on a receiving mode.
Di erently from activities, these kinds of perdurants (e.g., waiting) are homeomeric|i.e., the temporal slices of waiting-instances (if any) are themselves waiting instances|and cumulative|i.e., the mereological sum of two (immediately consecutive) waiting-instances is still a waiting-instance. Homeomeric and cumulative perdurants are called states in Dolce. For example, Figure 1 indicates that Service providers A, B and C are (by default) in a waiting status for receiving messages. Thus, a catch event identi es (perhaps implicitly) a state and it further indicates that the pool is committed towards a speci c trigger to occur.
However, on the same line of [
Throw events: Similarly to catch events, throw events are instantaneous, then temporally atomic, and anti-cumulative, i.e., in Dolce they are classi ed as achievements. Di erently from catch events, throw events tend to be endogenous: actions under the control of the pool they belong to. Note that di erently from tasks, which can be conceived as structured perdurants, although atomic under a certain granularity, throw events are punctual, thus intrinsically unstructured.
Throw None end events can be understood as the achievement of the whole process, and, in a fashion similar to start events, they can be interpreted either as an ontologically neutral placeholder in the model, as a logical constraint, or as an (ontologically committed) achievement. Note that the throw end events marked with a speci c trigger icon, like Message, Terminate, Signal and Error, indicate an achievement as well but now the culmination point is quali ed: the triggers that are speci ed in these cases (message, signal, termination and error) are amongst the participating entities of the achievement.
5The analysis of the causal dependencies among triggers, events and tasks could be very informative.
We focused on the ontological analysis of the bpmn notions of activity (task) and event, and classi ed them within the Dolce account of perdurant entities. The results are still preliminary and the hope is that it can help to reach a deeper understanding of the system; and to develop sound bpmn-driven ontologies. In the future, we shall expand this rst analysis and develop a formalization capturing our results on bpmn.
Acknowledgements: This research is in part supported by the Gecko and Pro2Evo projects of the \Fabbrica del Futuro" (funding: MIUR).