How to pull new post and comments

I am currently developing a website where many users post topics and the associated topics' comments are shown in the page. Currently I'm developing using cake php.

The very first time a user clicks the website, all the topics and comments are displayed. But when other users add new topic or comment to a topic, I need to show the update in the same page. I am confused as in how am I able to retrieve new contents and update accordingly in a page. For instance how facebook does it where when your friends adds status or comments on your status, it updates without refreshing the page.

I know that AJAX technology is used but how is it done. Any source that I can refer? Hope someone can help as I have been doing research for the past one week but no answers so far.

You can go two routes in this.

Server Push

http://en.wikipedia.org/wiki/Push_technology

This technique is perhaps the most efficient as the server notifies the client of any updates. However this technique usually requires a bit more work than a simple polling system. You can use something like nodejs or Comet to push updates. If you're using nodejs, I highly recommend using SocketIO to handle the client side. With Socket.io you can have the client side listening to the server on a channel so that the server can notify the client whenever an update happens.

Client polls server

In this version, the client (new visitors browser) constantly polls the server for updates. You can set whatever gap you want, but keep in mind that if you make the polling gap too small your server might take a performance hit as each new user will create many requests. This method is as simple as setting up a setInterval() call in JS coupled with an AJAX call.