Synchronize YouTube videos

Recently, I created a website to synchronize YouTube videos. The idea is that there is a master video and all other videos (called slave videos)  will be synchronized to the master one, in the sense that all playback action on the master video will be reflected on slave videos. This is useful when you want to watch a clip with friends on the other side of the Internet and want to keep your videos play synchronously, even if you pause or seek.

The master page send all playback actions to the web server for temporary storage using standard Ajax. The slave page, on the other hand, need to get these event updates from the server. This is a problem because in the HTTP paradigm, web clients are supposed to send queries to web server, not the other way around. The technology used here is called Comet, which is also called reverse Ajax. The idea of Comet is to allow server to push data to the client. There are multiple methods and framework to achieve Comet. The method I chose is called long polling. As its name suggests, the client need to poll the server continuously with high frequency to ask for update. This is normally done by send a new XMLHttpRequest at the end of the onload callback of the previous XMLHttpRequest. In my code, I add some rate limit so about 2 polling are executed per second. This seems to work but will be harder to scale as the server is being queried continuously, even if there is no event to update.