I'm wondering what's the right way to code a chat application for thousands of users.
I'm just confused how will I be able to ping the server using AJAX every second or maybe less and check if there are new records in MySQL, etc with an acceptable amount of server(s) load.
I'm currently thinking about coding this using jQuery, PHP and MySQL.
Please advice. Your help would be greatly appreciated.
For any program that needs to poll the server I would recommend WebSockets.
I wrote an extremely basic WebSocket tutorial. I also used the web-socket-js code to implement a FlashSocket that will make it work across Firefox, IE 8+, and Chrome, as well as any browser that supports WebSockets.
I don't believe that polling would be a good choice for a chat application. While it would work, the request overhead would be much higher then using a WebSocket. The tradeoff (benefit) is that more browsers support it.
Also, hitting a MySQL database to see if there are messages is going to incur a good deal of DB overhead. I would recommend using a MySQL database for chat logs, and only keep a limited number of "back" messages on hand for new connections. Then simply broadcast new messages to all connected clients. The frontend application would then take the message and append it to chat window.
Node.js is an evented server-side JavaScript framework. While it is still young, several very interesting applications have been coded in it. The Node.js people setup a chat program (not WebSockets) the source of which has been made available. That would be a very good place to start if not wanting to write it from scratch.
There is a PHP WebSocket implementation. Depending on your requirements it could be used just fine. Having coded in both Node.js and PHP I would say I think Node.js is a better fit for this.