What is a message send occurrence?

What is a message send occurrence?

• A. Exists on a lifeline at the point where the tail end of a message meets the lifeline
• B. Exists on a lifeline at the point where the arrowhead end of a message meets the lifeline
• C. Occurs at the start of the lifeline
• D. Occurs only at creation events

Answer: (A)

In a sequence diagram, a message send occurrence is the moment the sender’s lifeline begins the message, at the tail end of the message arrow. This tail marks where the message is issued from the sender and is considered sent. The receive occurrence happens where the arrowhead meets the receiver’s lifeline, marking arrival. So the statement that places the send event at the tail end on the sender’s lifeline is the correct one. The other ideas—placing the event at the arrowhead on the receiver, at the start of the lifeline, or only during creation events—don’t describe the send action.

In a sequence diagram, a message send occurrence is the moment the sender’s lifeline begins the message, at the tail end of the message arrow. This tail marks where the message is issued from the sender and is considered sent. The receive occurrence happens where the arrowhead meets the receiver’s lifeline, marking arrival. So the statement that places the send event at the tail end on the sender’s lifeline is the correct one. The other ideas—placing the event at the arrowhead on the receiver, at the start of the lifeline, or only during creation events—don’t describe the send action.