(Remember HTTP is stateless so there's really no such thing as
"currently", there's only "in the last 10 minutes", or whatever time
period you choose to define.)
For logged in users, yes definitely, see these threads from
Oct.
and
March
2003. I think there was also some more recent discussion of that
stuff just in the last week or two, but I can't find it right now.
For non-logged-in users, dunno, the above might track number of
user_id 0 hits, or it might not. (Try it and see.) If not, what you
want is to instead look in the AOLserver access log. It is easy to
configure AOLserver and OpenACS to log the OpenACS user_id to the
AOLserver access log, so to check unregistered users you can just grep
for only user_id 0 hits in the log.
Keep in mind that the web pages showing the "who's online in the last
10 minutes" sort of info are intended for transient human use, not for
any sort of load tracking over time. If you want to graph load or
track which users have been clicking on what over time (clickstream
data warehousing, etc.), then post-processing the AOLserver access log
is the right way to go.