arthas's answer is essentially correct. If "database" intimidates you, think "cache". A flat file works, though you'll be rewriting it each time you record a page hit from a user. Depending on your site load, this may or may not be an issue.

No matter how you decide to capture data, it suffices to record a username and a "last visited" timestamp. The timestamp is used to determine who to show as being online. You might, for example, say that any user who has hit the website within the last 5 minutes is "online". Depending on how you're keeping records, the timestamp can also be used to expire a user from the cache.

indeed, and if you think "cache" you should check out Cache::Cache.

my $who_here = Cache::FileCache->new(
  default_expires_in => 300,    # 5 minutes
);

# reset current user expiration time
$who_here->set($user, 1) if have_valid_user();

# if you have session data you can store it instead of
# the '1' with the side benefit of logging out users
# after inactivity
#
# my $userinfo;
# unless ($userinfo = $who_here->get($user)) {
#  ... make them login again
#  $who_here->set($user, { src_ip => 'foo', favorite_colour => 'green'
+ });
# } else {  # $userinfo has session info
#   print "your favorite colour is $userinfo->{favorite_colour}<br>";
# }

my @who_here_now = $who_here->get_keys();

print "there are ", scalar @who_here_now, " recent visitors:<br>";
print "$_<br>" for @who_here_now;
[download]

Just to add to Michele's comprehensive answer, you realise of course that the *only* thing you can tell is whether somebody's made a request to your wibstie recently...they could request one of your pages, "move off-site" by requesting another page, then request another of yours, all within your 'visitor' time and you wouldn't know about it.

This concept of being "on" or "off" a wibstie is a confusing one, perpetuated by marketing departments and stats-sellers. Think about your own browsing habits - how often do you 'visit' a whole site, look around for a while, then 'move off' to another site? I generally have 4/5 windows open - a Newest Nodes on 20 minute refresh, a google sitting waiting etc., and at least 70% of the time I leap straight from a search into a single page, absorb /reject, then go back to the search, check out the next page etc...

I'm not saying at all you shouldn't do it - I like seeing 'Other Users' - but you should be aware of the fact that it doesn't mean as much as your PHB's think it does. :)

Cheers
Ben

HTTP is a simple protocol which basically goes: make connection, do request, get response, break connection, although with HTTP/1.1, if you have a batch of requests, you don't have to create a new connection for all of them. But one doesn't stay connected, so a "who is online" feature is just as meaningful as the current temperature of the site.

Abigail

The good monk merlyn has an article doing something similar to what you want.

I did something like this using AOL Instant Messenger.

John J Reiser
newrisedesigns.com