Checking client connection status.

Question

Hello everyone,
This post is basically just to give me a starting point for a program i am GOING to write. Heres the situation, i want a server program that will basically send out messages to other client machines, originally i was using JMS but now i'm just trying something new. Okay, client machines will have lousy conections to the server, i.e. wireless links that will drop in and out of range and usability, and i want the server to recognise that this is happenning and stop sending messages to that client when is is inactive, but restart again automatically when the link returns. This will hopefully be simultaed by simply unplugging network cables.
So, all i really want to know is if java socket programming (or something similar) will let this occur WITH EASE. :-)
Well, as always, all help is greatly apprectiated, and hopefully i get enough feedback to start coding today :-P
Also, any alternate ways to do such a thing would be awesome.

Answer 1

The problem you are addressing is not an "easy" problem, so the solution may not be programmed "with ease", but it is solvable. If you have control of the definition of both the client and the server, you can build a reliable message service using UDP or a reconnection protocol with TCP. Both of these use sockets. The choice is based on whether you want to maintain a session or not. If you wish to maintain a long term session, and plan to send lots of data over it, TCP is probably better. If there is no session, or messages are infrequent UDP is probably better.
If you use TCP you need to detect loss of connection by sending a keepalive or heartbeat message on a regular basis, say every miinute and closing sockets that can't send or get eof when reading. You also need to build a reconnection process that attempts to reestablish lost connections, again using a timer.
If you use UDP and reliable transport iof messages is critical you need to sequence the messages sent and provide an acknowledgement protocol to determine if a message has been received and attempt to retransmit if one is lost. Some kinds of data don't realy need this since the next message sent will contain all the data anyway.