Chapter 11. Modeling the State Structure of a SystemChapter 12. Modeling the Dynamics of a System (19/35)12.1. Action Types and Action Steps

Chapter 12. Modeling the Dynamics of a System

In general, the dynamics of a system can be captured by modeling the types of events and processes (including actions and activities), occurring in the system. However, SysML does neither include an explicit concept of events (and event types), nor of processes (and process types). Rather, SysML's dynamics modeling is based on the concepts of actions and states, while the term "event" is only used in a very rudimentary form in the technical keyword "event occurrence" (in the rather confused Section 7.9.5 of the spec), but not as a modeling concept defined in the metamodel.

Ontologically, events may be (1) instantaneous or have a duration, (2) atomic or composite, while activities are special processes, and we have to distinguish between ongoing and completed processes, see (Guarino and Guizzardi 2024) where only completed activities and processes correspond to events, since events are required to have happened in the past. However, in systems modeling, we may choose using a concept of events that allows for ongoing and current events, such that both completed and ongoing processes and actions/activities are subsumed as events.

In SysML, the term "event" is only used in a technical sense for defining occurrence-valued features, typically in the context of modeling messaging interactions (as in UML sequence diagrams). The spec contains only the following statements using the term "event":

  1. In 7.9.5, it is stated that "An event occurrence usage [...] is related to another occurrence usage, representing the occurring event, by a reference subsetting relationship [...]. Or, if the event occurrence usage has no such reference subsetting, then the referenced event occurrence is the event occurrence usage itself."
  2. In 7.16.6, it is stated that "A perform action usage is a kind of event occurrence usage (see 7.9.5) for which the event occurrence is an action usage, known as the performed action".
  3. In 8.3.9.2, it is stated that "An EventOccurrenceUsage is an OccurrenceUsage that represents another OccurrenceUsage occurring as a suboccurrence of the containing occurrence of the EventOccurrenceUsage".
  4. In 8.4.5.3, it is stated that "An EventOccurrenceUsage is a kind of OccurrenceUsage that is required to always be referential by the validateEventOccurrenceUsageIsReference".

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