mikesigs:

Using NServiceBus 6's Callback feature I have found no way to alert the client that the Request Handler failed. The request handler will go through all the recoverability steps, and eventually put the message into the error queue. Meanwhile, the client just sits there waiting for its reply.

// Client code (e.g. in an MVC Controller)
var message = new FooRequest();
var response = await endpoint.Request(message);

// Handler code
public class FooRequestHandler : IHandleMessages
{
    Task Handle(FooRequest message, IMessageHandlerContext context)
    {
        throw new Exception("Fails before the reply");
        return context.Reply(new FooResponse());
    }
}

In the above situation, how can I let the MVC controller/calling code know that the handler has permanently failed?