(where "the book" = Philip and Alex's Guide to Web Publishing)
In many areas of a community site, we will want to distinguish "new
since your last visit" content from "the stuff that you've already seen"
content. The obvious implementation of storing a single
last_visit column is inadequate. Suppose that a user
arrives at the site and the ACS sets the last_visit column
to the current date and time. HTTP is
a stateless protocol. If the user clicks to visit a discussion forum,
the ACS queries the users table and finds that the last
visit was 3 seconds ago. Consequently, none of the content will be
highlighted as new.
The ACS stores last_visit and
second_to_last_visit columns. We take advantage of the
AOLserver filter facility to specify a Tcl program that runs before
every request is served. The program does the following:
IF a request comes with a user_id cookie, but the last_visit cookie is either not present or more than one day old, THEN the filter proc augments the AOLserver output headers with a persistent (expires in year 2010) set-cookie of last_visit to the current time (HTTP format). It also grabs an Oracle connection, and setsWe do something similar for non-registered users, using pure browser cookies rather than the database.We set a persistent second_to_last_visit cookie with thelast_visit = sysdate, second_to_last_visit = last_visitlast_visittime, either from the last_visit cookie or, if that wasn't present, with the value we just put into thesecond_to_last_visitcolumn of the database.
n_sessions column in the users table.
create table session_statistics ( session_count integer default 0 not null, repeat_count integer default 0 not null, entry_date date not null );
| last_visit cookie present? | log a session | log repeat session | update last_visit cookie | update second_to_last_visit_cookie | |
| Yes | Yes if date - last_visit > LastVisitExpiration | Yes if date - last_visit > LastVisitExpiration | Yes if date - last_visit > LastVisitUpdateInterval | Yes if date - last_visit > LastVisitExpiration | |
| No | Yes if the IP address has not been seen in the LastVisitCacheUpdateInterval seconds | No | Yes if the IP address has not been seen in the LastVisitCacheUpdateInterval seconds | No |
Upon login, a repeat session (but not an extra session) is logged
if the second_to_last_visit is not present.
Logic: The user is a repeat user since they are logging in
instead of registering.
He either lost his cookies or is using a different
browser. On the first page load, the last_visit
cookie is set and a session is recorded. When the user logs in,
we learn that he is a repeat visiter
and log the repeat session. (If the user was only missing a
user_id cookie, both the last_visit
and second_to_last_visit cookies would been updated on the
initial hit.)
LastVisitUpdateInterval - The last_visit cookie represents the date of the most recent visit, inclusive of the current visit. If the user remains on the site longer than the LastVisitUpdateInterval, the last_visit cookie is updated. The database stores the last_visit date as well for using tracking and to display "who's online now".
LastVisitExpiration - The minimum time interval separating 2 sessions.
LastVisitCacheUpdateInterval - The period of time non-cookied hits from an individual IP is considered the same user for the purpose of session tracking. (IP tracking and caching is necessary to not overcount browsers that do not take cookies.)