When you send a message that is a request, the request takes a round-trip from sender to handler and back; copies of the message take a one-way trip to interested observers. Figure 8–2 illustrates the request routing procedure.
The ToolTalk service delivers a request to only one handler. The handler adds results to the message and sends it back. Other processes can observe a request before or after it is handled, or at both times; observers absorb a request without sending it back.