How to invoke client's callback at server end?

问题

RPC do things like this:

server define a function foo_max(para)
client call foo_max, send 'max+para'
server get 'max' and call foo_max(para)
server send return val
client get result and return

But this way is not flexible because I must define all functions at server end. My case is that I want to support user-defined callback function at client end, so how can I call the client's callback funciton at server end?

Thanks

来源：https://stackoverflow.com/questions/19740380/how-to-invoke-clients-callback-at-server-end