3.1 Primitive actions and flows

Figure 4: The minimal core of the action semantics package

Action semantics defines a core package for UML, which serves as the basis of all kinds of actions. The core consists of a set of fundamental actions and their semantics, as is illustrated in Figure 4[BB01].

Two kinds of flows control the execution sequence of actions: control flow and data flow. All actions are treated as executing concurrently except those sequenced by a data flow or a control flow [AILKC⁺00].

• Control flow. An action is executed only after all its antecedents are completed. The termination of an action may activate one or more of its consequents, provided that their other antecedents are also finished executing. So, as the class diagram shows, an action is associated with a set of antecedents and a set of consequents.
• Data flow. Every action has two sets of input pins and two sets of output pins associating with it. One of the input pin sets (denoted by $A$)contains all the required input pins for the action. They can be considered as the formal input parameters. The other input pin set (denoted by $B$) contains available pin data at a certain point in time. Since this set is associated with the action and only provides information which it interests in, it is a subset of $B$. An action can be executed only after all the associating input pins are available, i.e. $A=B$. Similarly, only when the action is completed does output pin set $A'$ equal to $B'$. As the data flow carries on, data from the output pins of a preceding action become a source of the data flow, and then the confluence reaches the input pins of another action.