Is there an alternative of ajax that does not require polling without server side modifications?
I'm trying to create a small and basic "ajax" based multiplayer game. Coordinates of objects are being given by a PHP "handler". This handler.php file is being polled every 200MS, by using ajax.
Since there is no need to poll when nothing happens, I wonder, is there something that could do the same thing without frequent polling? Eg. Comet, though I heard that you need to configure server side applications for Comet. It's a shared webserver, so I can't do that.
Maybe prevent the handler.php file from even returning a response if nothing has to be changed at the client, is that possible? Then again you'd still have the client uselessly asking for a response even though something hasn't changed yet. Basically, it should only use bandwidth and sever resources if something needs to be told to the client, eg. the change of an object's coordinates.
Answer
Comet is generally used for this kind of thing, and it can be a fragile setup as it's not a particularly common technology so it can be easy not to "get it right." That said, there are more resources available now than when I last tried it ~2 years ago.
I don't think you can do what you're thinking and have handler.php simply not return anything and stop execution: The web server will keep the connection open and prevent any further polling until handler.php does something (terminates or provides output). When it does, you're still handling a response.
You can try a long polling technique, where your AJAX allows a very large timeout (e.g. 30 seconds), and handler.php spins without responding until it has something to report, then returns. (You'll want to make sure the spinning is not resource-intensive). If handler.php "expires" and nothing happens, have it exit and let AJAX poll again. Since it only happens every 30 seconds, it will be a huge improvement over ~5 times a second. That would keep your polling to a minimum.
But that's the sort of thing Comet is designed for.