Re: repeated problems with xar_session_info table

Ryan Walker wrote:
> Hi,
> 
> I manage a site where I'm repeatedly getting the "Undefined index:  
> xarSession_isNewSession" error.
> 
> I can fix it each time by emptying the sessions table.  But it has now 
> recurred three times in a few weeks.  I've never seen it occur more than 
> once during the lifetime of any other site.
> 
> Does anyone know what triggers this problem and why might it have 
> occurred three times in just a few weeks?
> Xaraya 1.1.3
> PHP 4.4.4
> MySQL 4.1.22
> 
> Thanks,
> Ryan
> 

A. Where does it go wrong
=========================
The following happens during session initialisation in xarSession_init(),
see file includes/xarSession.php :

1. specify how PHP should handle sessions in xarSession__setup()

This tells PHP (amongst other things) to call the xarSession__phpRead()
function whenever it wants to read session information.

2. start the actual session with session_start()

Here PHP will start the session and call xarSession__phpRead() to
try & read session information from the xar_session_info table. This
will set $GLOBALS['xarSession_isNewSession'] to true or false depending
on whether information is found or not.

3. if $GLOBALS['xarSession_isNewSession'] is true, call
xarSession__new(), otherwise call xarSession__current() to
finalise session initialisation for Xaraya

This is where the error message is generated - in your case, the
variable $GLOBALS['xarSession_isNewSession'] is not defined at all,
because xarSession__phpRead() never gets to the point of setting
its value :

     $result =& $dbconn->Execute($query,array($sessionId));
     if (!$result) return; // <-- yep, here's where it happens

     if (!$result->EOF) {
         $GLOBALS['xarSession_isNewSession'] = false;
         ...
     } else {
         $GLOBALS['xarSession_isNewSession'] = true;
         ...
     }

In other words, trying to select something from xar_session_info
fails altogether, which is typical when e.g. a table is corrupted.

Check the error messages you get and the MySQL error log to make
sure that's the real cause...

B. Why is this happening
========================

The xar_session_info table is heavily loaded and gets at least one
SELECT and two INSERTs/UPDATEs on each page hit, not to mention DELETEs
on garbage collection.

So if your site gets a lot of visitors and/or robots crawling around
at the same time, you get a lot of read/writes on the same table, which
isn't exactly MySQL's strong point, especially if you use the old MyISAM
table format.

C. How should I fix this
========================

1. Make sure your PHP/MySQL combination doesn't have a known issue with
table corruption

2. Switch to InnoDB as table format for the xar_session_info table

3. Have a look at sessionless page caching, e.g. for your home page

4. Add a cosmetic fix in includes/xarSession.php to initialise
$GLOBALS['xarSession_isNewSession'] before calling session_start()

5. Do a regular check/repair of your MySQL tables

HTH,

Mike.