Online dating sites such as Yahoo Personals and Match.com allow
members to send instant messages to other members that are listed as
"Online Now". Since these sites don't use an outside downloadable
client that would instantly inform the system of their presence,
exactly how do these sites determine somebody's presence? I understand
that they probably determine it based on some function of listing
someone online if they have clicked on a page within the last X number
of minutes. If this is indeed the case, what is that X number? 

I'm interested in knowing the general approach/technology these sites
use to calculate presence, as well as the specific policy (i.e., the X
number of minutes).

Thanks.

Hi, 

Typically this is handled by using PHP sessions of similar technology.
http://us2.php.net/manual/en/ref.session.php

The idea is that there is a memory log on the server (most of the time
in a database table) which keeps track of when the last time you
clicked on a page was. It does this by setting the session variable
when you log into the website. The time frame for inactivity log-out
is usually around 20 minutes for casual meeting and social websites
like the ones you have listed. Websites like these tend to want as
little "hassle" associated with them as possible. Whereas a bank
website (like Washington Mutual for instance) will log you out in 2
minutes of inactivity, stressing security rather than relaxed hassle
free surfing.

You can test the actual time, or at least get a close figure by
testing the log out period on the site. Sometimes you can see the
information in a cookie in your browser. If you have Firefox for
example, you can use it to check the page information looking for the
expire-time. Go to Tools, Page Information, Cookies to see this in
Firefox.

Get Firefox Browser
http://www.mozilla.org/products/firefox/

Also, if you logout of course then the setting is switched instantly. 

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