Behavioral Constructs: Activity Modelling

Activity modeling highlights the inputs, outputs, sequences and conditions for coordinating other behaviors of a system. SysML extensions to UML 2.1 activity diagrams are summarized below.

1.1 Control as data

Control is extended by SysML in activity diagrams:

• In UML 2.1 activities, control can only enable actions to start. Whereas in SysML, control can support both enabling and disabling of activities.
• A control value is an input or output of a control operator. Control operator transforms its inputs into an output through complex logical operations. This output controls other actions.

1.2 Continuous systems

SysML provides extensions for continuous behaviors; however, these are applicable to any other distributed flow of information and physical items through a system. These extensions are:

• Restrictions on the rate of flow of an entity along edges in an activity, or in and out of behavior parameters. SysML allows both continuous and discrete flows of material, energy, and information.
• These two flows are unified under rate of flow, similar to that in traditional mathematical models of continuous change, where the discrete increment of time approaches zero.
• Extension of object nodes, including pins, with the option of new values replacing values existing in the object nodes and also discarding values if they do not immediately flow downstream. These extensions help ensure that the most recent information is available to actions. They also help prevent fast or continuously flowing values from accumulating in an object node and also modeling transient values (e.g., electrical signals).