Excerpt from Enterprise Architecture Fundamentals:
As defined by the Business process model and notation (BPMN), business pro- cesses are built from “elementary activities (or tasks) representing single units of work that cannot be broken down to a further level of business process detail.” Such a cursory definition falls short of addressing architectural concerns, because it ignores the critical architectural distinction between the business content of activities and the way it should be executed at the system level.
Something more thorough is clearly needed for the definition of processes’ execution units, and Aristotle’s classical three unities of drama fits the bill:
Unity of action (roles): execution units should have one main course of action initiated by a single primary role (e.g., Customer #), with all resources identified (e.g., Book Tour, Reservation). Subplots, if any, can summon the collaboration of secondary actors, providing a return to the main plot once completed; that condition guarantees that all possible execution dependencies are identified.
Unity of place (locations): execution units must be run in a single location (e.g., Cloud Services #), where all resources (e.g., Book Tour, Reservation) can be obtained directly and immediately, so as not to depend on communication capabilities between locations. That is a necessity if functional architectures are to be defined and managed independently of technical ones.
Unity of time (events): execution units must be timed in relation to a single event (e.g., Customer Request #) and, consequently, to a single time frame, within which accesses to resources can be synchronized. That is necessary if the innards of business logic and the synchronization of business processes are to be managed independently of communication channels and mechanisms.