Asynchronous calls on the GWT Server Side

I'm looking for some general guidance on implementing asynchronous calls to a third party on the GWT Server side.  The scenario is the GWT client side sends an RPC request to the server side, the server side does some processing then needs to invoke a service in a remote third party.  The call to the third party is asynchronous, the call is made and control is return immediately.  Sometime later a callback is received with the actual result from the service.  The challenge I have is what to do when i've made the asynchronous call on the server side.  I don't want to "return" a response back to the client side, as I really can't until the 3rd party has called back.  How do I put myself into a "suspend" state until the callback comes without unnecessarily tying up threads on the server?

Many thanks for any suggestions.

Simon

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