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I am trying to model a call where a synchronous call is made to a server which is handled in a single thread with a mailbox. It fires an asynchronous call off, and eventually receives another asynchronous message with the response, then the synchronous call returns. I'm not sure if this is possible to model directly, an asynchronous message to an existing life line. Showing the mail box as a discrete actor is not an appropriate detail.
Product: Astah Professional
I may not have an exact same picture with you, but here's the diagram I drew from my understanding:
You can depict that the asynchronous call is made to the existing execution specification on diagram, but is really a separate one (colored). But as a model, asynchronous messages should not be made between the synchronous message and its reply message.
What we'd suggest is to create another Server lifeline (e.g. Sever #2) and create Asynchronous message to the #2.
My answer may not be sufficient, so if you could paste a visual of how you'd like to model with a mark where you have a doubt for, it'd be very helpful.