Called in a client connection (on an event boundary) when the client connections's request method is invoked. The return value for this method is returned as the result of the client's request method, so the client blocks until the server returns a value from on-request.
The on-request method is invoked with the topic used by the client to connect to the server. The remaining arguments are as they were passed to the client connection's request method, but the data size argument represents the actual number of characters sent.
Returns null.