In a UML sequence diagram, what is an actual gate?

In a UML sequence diagram, what is an actual gate?

• A. A message that enters or leaves an interaction use
• B. A guard condition
• C. A final state
• D. A sub-activity region

Answer: (A)

In a sequence diagram, the boundary between an interaction use and everything outside it needs a way to show how messages cross in and out. An actual gate is that boundary point. It marks where a message can enter the interaction use from outside or leave the interaction use to go to something outside. Think of it as the doorway through which external stimuli trigger or receive the behavior modeled by the interaction. This helps distinguish boundary crossing from other concepts: a guard condition is about selecting which path to take, not about how messages cross into or out of the interaction; a final state indicates termination of a flow; and a sub-activity region is a grouping construct in activity diagrams. So the actual gate specifically represents the entry or exit point for messages the moment they cross into or out of the interaction use.

In a sequence diagram, the boundary between an interaction use and everything outside it needs a way to show how messages cross in and out. An actual gate is that boundary point. It marks where a message can enter the interaction use from outside or leave the interaction use to go to something outside. Think of it as the doorway through which external stimuli trigger or receive the behavior modeled by the interaction.

This helps distinguish boundary crossing from other concepts: a guard condition is about selecting which path to take, not about how messages cross into or out of the interaction; a final state indicates termination of a flow; and a sub-activity region is a grouping construct in activity diagrams. So the actual gate specifically represents the entry or exit point for messages the moment they cross into or out of the interaction use.