How would you implement a "who is online" feature?

Question

I have been meaning to implement a "who is online" feature on my site.

Was wondering how would you decide if a user is online or not?

Some options are:

1. Last seen, less than N minutes ago (what is N?)
2. A comet server with long polling
3. Something else

Answer 1

If you are using session variables then the user is online if last_activity + session_expiry > current_date. Else the session has already expired and they are not online.

Now, it depends on what people will be able to do with this "who is online" feature. You might prefer a more conservative measure to have a higher confidence the user's active.

But, given the nature of the web, there's no sure fire way to ensure the user's really online and active in your site, short of requiring user interaction every once in a while, but that would be annoying.

Answer 2

I would go with option 1 and allow N to be set from a configuration file. Presumably user activity is being logged with a timestamp in some datastore, so calculating whether a user is considered online (seen less than N time ago) should be pretty straightforward. You may consider using a periodic AJAX request to update the online collection of users at regular intervals.

Answer 3

You could also use a ping method. Send a light ajax request from the client to server approximately every 30-60 seconds. Keep the request and response as small as possible to reduce bandwidth and this should perform almost as well as the Comet method.