Question

I’m building a basic little AJAX shoutbox/chat for my website, but I’m not sure exactly how to implement the server polling.

Here’s the basic program flow I’m thinking of:

1. User comes to page and is shown the last 10 messages
2. To get messages sent by others, the client javascript would request a URL with a timestamp parameter (set to the value of the last message the client received)
3. The server returns all messages (up to a max of 10) since that timestamp.

The only issue is how often to poll the server. Obviously it should poll each time a new message is added, but when you’re just reading others’ messages it needs to automatically update.

Should it be a set time limit? eg: every 10 seconds. Or, should it vary depending on usage? eg: Check after 5 seconds. If there’s no messages, don’t check for another 10 seconds. If there’s still no new messages, check in 15 seconds, then 20, up to maybe once every 30 seconds max. Each time there’s a new message detected reset your timer back down to 5 seconds and start again.

I’m just concerned about putting unnecessary stress on the server, considering that we could have hundreds of users concurrently online.

…or have I got the whole thing wrong? Is there a better way to implement a basic javascript chat?

Answer 1

Comet, described by Brian is a nice technique, but requires session support on the server, which is probably more advanced than you care to implement for a simple chat box.

The best way to implement polling intervals is to imagine you having a chat window which you can minimize to do other stuff, or open to see if you have new messages. When you are in the middle of a conversation, you’ll switch to it (poll) frequently. If you don’t get any messages for a while, you will start looking rarer and rarer until you only check it occasionally.

Assuming you don’t need to do real-time typing, you can probably poll every 3 seconds or so when at peak activity, and if nothing shows up for 5-10 polls, start to crank the interval up (perhaps doubling it every time) until it hits 30-60 seconds. Getting a message back should reset the poll interval back to a few seconds, while sending a message should poll instantly, but probably doesn’t need to effect the frequency of polling otherwise.