Multicast multithreaded mayhem

Javagroups is cool. Multicast peer to peer messaging and remote procedure calls in a nice easy package. The documentation could be better – figuring out how to make it do stuff invariably involves a short stroll through the source. Good thing its open source, really.

One thing that did give me trouble was the supplied RpcDispatcher. This handy utility class makes it really easy to do RPC over multicast, upon which can be built such things as grid computing, or distributed failover. The flaw being that if you happen to make an RPC call that blocks, all subsequent RPC calls to that node will also block, and probably time out. Javagroups handles client-side timeout quite well, but the server side can potentially block indefinitely.

Fixing this meant I got to play with the deep joys of inter-thread communication and synchronisation. Even deeper joy was found when trying to test it. The only means of doing this I’ve found so far is to liberally sprinkle log statements all over the code, and watch the order they appear in, and when they stopped appearing as my code proceeded to block in all the wrong places. Maybe a log4j JUnitAppender class is required, that can assert that log calls were made with the right messages in the right order? Hmm.