> b) Two different servers (or the same server) trying to access the > session data for the same SID at the same time. How can a user of SM > generate two accesses to session data at the same time?. And how do you > use SM to generate two pararell accesses from two different webservers > in the cluster?
Let's assume you've got two LB'd web servers balanced by rr. One click in left_main, say, "check mail," generates one session hit on the first server. The second click, say, selecting a message, generates a second session hit on the second server. If they're happening at the same time and your session store isn't lock-safe, you can end up in trouble. Is that now clear? > PS.- I do not use locking in my SQL session handler (only normal > transactions). I have never had (or got reports from other users) about > this problem, but I am interested in this discusion to find out if in > theory this can happen in a webcluster running PHP. I am not convinced, > ... yet ;) With SQL, you're a little more protected because you do at least have transactional isolation, but database locks are there for a reason. Just imagine a scenario where two instances of the same session write to the it the same variable based on two different values of some other session variable. I haven't looked at these SQL-based session handler functions in awhile, but I assume all you'd have to do to make them "balancesafe" would be to change the SELECT's to SELECT FOR UPDATE's. That brings up another point though: if PHP's session locking is based on flock(), what do you do to serialize multiple accesses even from the same web server? John -- John Madden UNIX Systems Engineer Ivy Tech State College [EMAIL PROTECTED] ------------------------------------------------------- This SF.Net email is sponsored by: NEC IT Guy Games. Get your fingers limbered up and give it your best shot. 4 great events, 4 opportunities to win big! Highest score wins.NEC IT Guy Games. Play to win an NEC 61 plasma display. Visit http://www.necitguy.com/?r -- squirrelmail-users mailing list Posting Guidelines: http://squirrelmail.org/wiki/wiki.php?MailingListPostingGuidelines List Address: [email protected] List Archives: http://news.gmane.org/thread.php?group=gmane.mail.squirrelmail.user List Archives: http://sourceforge.net/mailarchive/forum.php?forum_id)95 List Info: https://lists.sourceforge.net/lists/listinfo/squirrelmail-users
