In a sequence diagram, when is the behavior execution start occurrence generally implicit?

In a sequence diagram, when is the behavior execution start occurrence generally implicit?

• A. At the point where a lifeline receives a synchronous or asynchronous message
• B. At the point where a lifeline sends a reply message
• C. When the lifeline is created
• D. At the end of the interaction

Answer: (A)

In a sequence diagram, the behavior on a lifeline starts when that lifeline receives a message. The moment the message arrives—whether it’s synchronous (the caller waits for a response) or asynchronous (the caller continues)—an activation on the lifeline begins, signaling the start of its execution. This is why the initiation is considered implicit: the diagram conveys that the response-driven flow starts from the arrival of the message, not from creating the lifeline, sending a reply, or the interaction ending. Creating a lifeline merely establishes the object; a reply comes after work is done, and the end of the interaction marks termination, not the start of execution.

In a sequence diagram, the behavior on a lifeline starts when that lifeline receives a message. The moment the message arrives—whether it’s synchronous (the caller waits for a response) or asynchronous (the caller continues)—an activation on the lifeline begins, signaling the start of its execution.

This is why the initiation is considered implicit: the diagram conveys that the response-driven flow starts from the arrival of the message, not from creating the lifeline, sending a reply, or the interaction ending. Creating a lifeline merely establishes the object; a reply comes after work is done, and the end of the interaction marks termination, not the start of execution.