How to call a client callback on the server?

Question

RPC follow these steps:

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 method is not flexible, because I have to define all the functions on the server. My thing is that I want to support a client-side custom callback function, so how can I call a client callback function on the server?

thanks