Help in Asychronous return message??

Hi all,

I am trying to programming a server to do some function
calls in a P2P. One of this functions return a String but
in asynchronous way.Can you give some code guidelines of
how to cope with asynchronous logic? I mean how to catch
this string(message) that returned from the function.

My protocol:
The client asks the server for some data,server don't know
and ask P2P,server "take" that reply and forward it to client.

Thank you,
Adrian.