The two largest outstanding issues. One is what to use for a key to identify the callback handlers (both on server and client side). Currently, the code still allows the caller to specify a session id, which is used as the key (and the only part of the key). If the session id passed is null, the Client class will generate is UID.
The other is the format of the data to be passed back and forth. For example, what should the object passed to the callback handler look like? Currently, it is an InvocationRequest. Also, what data should be contained within the callback handler to let the server invocation handler know what the callback handler wants to get callback on? Currently, there is no method to get this.