What is the shape of the notation for a synchronous message?

The notation for a synchronous message is represented by a solid line with a filled-in arrowhead. This design choice reflects the nature of synchronous communication, where the sending and receiving entities are expected to engage in a direct interaction. The filled-in arrowhead indicates a return or response that is crucial in a synchronous context, as it signifies that the sender is awaiting a reply before proceeding with further actions.

In modelling terms, this type of message conveys that the initiating process is blocked and cannot continue until the receiving process completes its operation and sends a response back. The solid line further emphasizes the real-time connection and immediacy of this interaction, differentiating it from asynchronous messages, which utilize different graphical representations to illustrate their non-blocking nature.